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Guido GOLDONI

Professore Ordinario
Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica

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Pubblicazioni

2024 - Enhancement and anisotropy of electron Landé factor due to spin-orbit interaction in semiconductor nanowires [Articolo su rivista]
Czarnecki, Julian; Bertoni, Andrea; Goldoni, Guido; Wójcik, Paweł
abstract

2023 - Spin-Resolved Magneto-Tunneling and Giant Anisotropic g-Factor in Broken Gap InAs-GaSb Core-Shell Nanowires [Articolo su rivista]
Clericò, V.; Wójcik, P.; Vezzosi, A.; Rocci, M.; Demontis, V.; Zannier, V.; Díaz-Fernández, A.; Díaz, E.; Bellani, V.; Domínguez-Adame, F.; Diez, E.; Sorba, L.; Bertoni, A.; Goldoni, G.; Rossella, F.
abstract

: We experimentally and computationally investigate the magneto-conductance across the radial heterojunction of InAs-GaSb core-shell nanowires under a magnetic field, B, up to 30 T and at temperatures in the range 4.2-200 K. The observed double-peak negative differential conductance markedly blue-shifts with increasing B. The doublet accounts for spin-polarized currents through the Zeeman split channels of the InAs (GaSb) conduction (valence) band and exhibits strong anisotropy with respect to B orientation and marked temperature dependence. Envelope function approximation and a semiclassical (WKB) approach allow to compute the magnetic quantum states of InAs and GaSb sections of the nanowire and to estimate the B-dependent tunneling current across the broken-gap interface. Disentangling different magneto-transport channels and a thermally activated valence-to-valence band transport current, we extract the g-factor from the spin-up and spin-down dI/dV branch dispersion, revealing a giant, strongly anisotropic g-factor in excess of 60 (100) for the radial (tilted) field configurations.

2022 - Band structure of n - and p -doped core-shell nanowires [Articolo su rivista]
Vezzosi, A.; Bertoni, A.; Goldoni, G.
abstract

We investigate the electronic band structure of modulation-doped GaAs/AlGaAs core-shell nanowires for both n- and p-doping. We developed an 8-band Burt-Foreman k.p Hamiltonian approach to describe coupled conduction and valence bands in heterostructured nanowires of arbitrary composition, growth directions, and doping. Coulomb interactions with the electron/hole gas are taken into account within a mean-field self-consistent approach. We map the ensuing multi-band envelope function and Poisson equations to optimized, non-uniform real-space grids by the finite element method. Self-consistent charge density, single-particle subbands, density of states and absorption spectra are obtained at different doping regimes. For n-doped samples, the large restructuring of the electron gas for increasing doping results in the formation of quasi-1D electron channels at the core-shell interface. Strong heavy-hole/light-hole coupling of hole states leads to non parabolic dispersions with mass inversion, similarly to planar structures, which persist at large dopings, giving rise to direct LH and indirect HH gaps. In p-doped samples the hole gas forms an almost isotropic, ring-like cloud for a large range of doping. Here, as a result of the increasing localization, HH and LH states uncouple, and mass inversion takes place at a threshold density. A similar evolution is obtained at fixed doping as a function of temperature. We show that signatures of the evolution of the band structure can be singled out in the anisotropy of linearly polarized optical absorption.

2022 - Disclosing the Microscopic Picture: An Interdisciplinary Investigation of Friction and Wetting Using a Gecko-Inspired Tape [Articolo su rivista]
Scorzoni, C.; Goldoni, G.; Rota, A.; De Renzi, V.
abstract

We propose an innovative approach to teaching friction. Our approach aims to educate students on its microscopic nature by highlighting its origin in intermolecular interactions. We have designed a teaching sequence (TS) based on a set of experimental investigations of the properties of a gecko-inspired tape at different length scales. The TS has been conceived to unravel the peculiar behavior of this man-made, commercially available biomimetic material and to train students to identify the connection between the micrometer-scale patterning and the peculiar tribological properties. Specifically, our approach compares friction (and adhesion) to wetting, occurring at solid/solid and solid/liquid interfaces, respectively. The aim is to scaffold a correct mental model of real interfaces and disclose the common origin of both phenomena in intermolecular interactions. The TS has been devised according to the design-based research scheme and it was inspired by the Investigative Science Learning Environment (ISLE) and the 5E paradigms. It has been tested and tuned with students at level 3 in the International Standard Classification of Education (ISCED), during several on-campus stages. We report here the details and results of pre- and post-tests, which demonstrate the effectiveness of our method. Specifically, we measure success in terms of the students' comprehension of the link between contact area and friction and of the role of intermolecular forces. We are confident that the learning experience with our TS will lead students to recognize the enormous potential impact of surface patterning in technological applications, in a curiosity-driven manner that will likely result in students' interest in quantitative studies of science and technology.

2021 - Anisotropy of the spin-orbit coupling driven by a magnetic field in InAs nanowires [Articolo su rivista]
Wojcik, P.; Bertoni, A.; Goldoni, G.
abstract

We use the k·p theory and the envelope function approach to evaluate the Rashba spin-orbit coupling induced in a semiconductor nanowire by a magnetic field at different orientations, taking explicitly into account the prismatic symmetry of typical nanocrystals. We make the case for the strongly spin-orbit-coupled InAs semiconductor nanowires and investigate the anisotropy of the spin-orbit constant with respect to the field direction. At sufficiently high magnetic fields perpendicular to the nanowire, a sixfold anisotropy results from the interplay between the orbital effect of field and the prismatic symmetry of the nanowire. A backgate potential, breaking the native symmetry of the nanocrystal, couples to the magnetic field inducing a twofold anisotropy, with the spin-orbit coupling being maximized or minimized depending on the relative orientation of the two fields. We also investigate in-wire field configurations, which shows a trivial twofold symmetry when the field is rotated off the axis. However, isotropic spin-orbit coupling is restored if a sufficiently high gate potential is applied. Our calculations are shown to agree with recent experimental analysis of the vectorial character of the spin-orbit coupling for the same nanomaterial, providing a microscopic interpretation of the latter.

2021 - The Gecko® Approach to Friction: A Novel Teaching Learning Sequence [Capitolo/Saggio]
Scorzoni, C.; Goldoni, G.; De Renzi, V.
abstract

We here propose a novel teaching–learning sequence on tribology, based on the experimental investigation of Gecko® Nanoplast®, a bio-inspired micro-structured synthetic material with peculiar tribological properties. Measurements of static and dynamic friction show that Gecko® does not obey to Leonardo-Amontons’ law, thus providing a way to introduce the key concept of real—as opposed to nominal—contact area. As in ‘real’ material-science research, investigation of the Gecko® macroscopic behavior is combined with its structural determination, based on both optical microscopy observations and on diffraction measurements. The sequence has been tested on a group of honors high-school students with encouraging results.

2019 - Anisotropies of the g-factor tensor and diamagnetic coefficient in crystal-phase quantum dots in InP nanowires [Articolo su rivista]
Wu, S.; Peng, K.; Battiato, S.; Zannier, V.; Bertoni, A.; Goldoni, G.; Xie, X.; Yang, J.; Xiao, S.; Qian, C.; Song, F.; Sun, S.; Dang, J.; Yu, Y.; Beltram, F.; Sorba, L.; Li, A.; Li, B. -B.; Rossella, F.; Xu, X.
abstract

Crystal-phase low-dimensional structures offer great potential for the implementation of photonic devices of interest for quantum information processing. In this context, unveiling the fundamental parameters of the crystal phase structure is of much relevance for several applications. Here, we report on the anisotropy of the g-factor tensor and diamagnetic coefficient in wurtzite/zincblende (WZ/ZB) crystal-phase quantum dots (QDs) realized in single InP nanowires. The WZ and ZB alternating axial sections in the NWs are identified by high-angle annular dark-field scanning transmission electron microscopy. The electron (hole) g-factor tensor and the exciton diamagnetic coefficients in WZ/ZB crystal-phase QDs are determined through micro-photoluminescence measurements at low temperature (4.2 K) with different magnetic field configurations, and rationalized by invoking the spin-correlated orbital current model. Our work provides key parameters for band gap engineering and spin states control in crystal-phase low-dimensional structures in nanowires. [Figure not available: see fulltext.].

2019 - Enhanced Rashba spin-orbit coupling in core-shell nanowires by the interfacial effect [Articolo su rivista]
Wójcik, Paweł; Bertoni, Andrea; Goldoni, Guido
abstract

We report on k → · p → calculations of Rashba spin-orbit coupling controlled by external gates in InAs/InAsP core-shell nanowires. We show that charge spilling in the barrier material allows for a stronger symmetry breaking than in homogenous nano-materials, inducing a specific interface-related contribution to spin-orbit coupling. Our results suggest additional wavefunction engineering strategies to enhance and control spin-orbit coupling.

2019 - Polychromatic emission in a wide energy range from InP-InAs-InP multi-shell nanowires [Articolo su rivista]
Battiato, S.; Wu, S.; Zannier, V.; Bertoni, A.; Goldoni, G.; Li, A.; Xiao, S.; Han, X. D.; Beltram, F.; Sorba, L.; Xu, X.; Rossella, F.
abstract

InP-InAs-InP multi-shell nanowires (NWs) were grown in the wurtzite (WZ) or zincblende (ZB) crystal phase and their photoluminescence (PL) properties were investigated at low temperature (≈6 K) for different measurement geometries. PL emissions from the NWs were carefully studied in a wide energy range from 0.7 to 1.6 eV. The different features observed in the PL spectra for increasing energies are attributed to four distinct emitting domains of these nano-heterostructures: the InAs island (axially grown), the thin InAs capping shell (radially grown), the crystal-phase quantum disks arising from the coexistence of InP ZB and WZ segments in the same NW, and the InP portions of the NW. These results provide a useful frame for the rational implementation of InP-InAs-InP multi-shell NWs containing various quantum confined domains as polychromatic optically active components in nanodevices for quantum information and communication technologies.

2018 - Classical and quantum dynamics of indirect excitons driven by surface acoustic waves [Articolo su rivista]
Grasselli, Federico; Bertoni, Andrea; Goldoni, Guido
abstract

We perform explicit time-dependent classical and quantum propagation of a spatially indirect exciton (SIX) driven by surface acoustic waves (SAWs) in a semiconductor heterostructure device.We model the SIX dynamics at different levels of description, from the Euler-Lagrange propagation of structureless classical particles to unitary Schrödinger propagation of an electron-hole wave packet in a mean field and to the full quantum propagation of the two-particle complex. A recently proposed beyond mean-field self-energy approach, adding internal virtual transitions to the c.m. dynamics, has been generalized to time-dependent potentials and turns out to describe very well full quantum calculations, while being orders of magnitude numerically less demanding. We show that SAW-driven SIXs are a sensitive probe of scattering potentials in the devices originating, for example, from single impurities or metallic gates, due to competing length and energy scales between the SAW elastic potential, the scattering potential, and the internal electron-hole dynamic of the SIX. Comparison between different approximations allow us to show that internal correlation of the electron-hole pair is crucial in scattering from shallow impurities, where tunneling plays a major role. On the other hand, scattering from broad potentials, i.e., with length scales exceeding the SIX Bohr radius, is well described as the classical dynamics of a pointlike SIX. Recent experiments are discussed in light of our calculations

2018 - Enhanced light-harvesting of protein-pigment complexes assisted by a quantum dot antenna [Relazione in Atti di Convegno]
Gil, G; Goldoni, G; Corni, S
abstract

We predict the enhanced light harvesting of a protein-pigment complex when assembled to a quantum dot (QD) antenna. Our prototypical nanoassembly setup is composed of a Fenna-Mattews-Olson system hosting 8 Bacteriochlorophyll (BChl) a dyes, and a near-infrared emitting CdSexTe(1-x)/ZnS alloy-core/shell nanocrystal. BChl a has two wide windows of poor absorption in the green and orange-red bands, precisely where most of the sunlight energy lies. The selected QD is able to collect sunlight efficiently in a broader band and funnel its energy by a (non-radiative) Forster resonance energy transfer mechanism to the dyes embedded in the protein. By virtue of the coupling between the QD and the dyes, the nanoassembly absorption is dramatically improved in the poor absorption window of the BChl a.

2018 - Tuning Rashba spin-orbit coupling in homogeneous semiconductor nanowires [Articolo su rivista]
Wojcik, Pawel; Bertoni, Andrea; Goldoni, Guido)
abstract

We use k·p theory to estimate the Rashba spin-orbit coupling (SOC) in large semiconductor nanowires. We specifically investigate GaAs- and InSb-based devices with different gate configurations to control symmetry and localization of the electron charge density. We explore gate-controlled SOC for wires of different size and doping, and we show that in high carrier density SOC has a nonlinear electric field susceptibility, due to large reshaping of the quantum states. We analyze recent experiments with InSb nanowires in light of our calculations. Good agreement is found with the SOC coefficients reported in Phys. Rev. B 91, 201413(R) (2015)PRBMDO1098-012110.1103/PhysRevB.91.201413, but not with the much larger values reported in Nat. Commun. 8, 478 (2017)2041-172310.1038/s41467-017-00315-y. We discuss possible origins of this discrepancy.

2017 - Exact long-wavelength plasmon dispersion on a ring with soft Coulomb interactions [Articolo su rivista]
Grasselli, Federico; Bertoni, Andrea; Goldoni, Guido
abstract

We obtain the analytical dispersion of 1D plasmonic modes on a ring from the exact solution of the hydrodynamical model with soft Coulomb potential. We compare our results with the exact plasmon dispersion in straight 1D systems and find a set of formal correspondences between the two. In light of our results, we discuss recent experiments (Schmidt et al 2014 Nat. Commun. 5 3604) where ring-confined modes in nanodiscs are found to almost coincide with plasmonic excitations in 1D metallic nanostructures. We trace the similarity to the scaling properties of the plasmonic dispersion.

2017 - The role of internal dynamics in the coherent evolution of indirect excitons [Articolo su rivista]
Grasselli, Federico; Bertoni, Andrea; Goldoni, Guido
abstract

We study the time-dependent quantum scattering of a spatially indirect exciton by an external potential, taking fully into account the relative quantum dynamics of the electron-hole (e-h) pair. Exact calculations for an e-h wave packet show that transfer of energy between centre-of-mass (c.m.) and relative degrees of freedom may result in a genuine correction to the evolution during the scattering and eventually at asymptotic times. We show in experimentally relevant regimes and device configurations, that transmission resonances, tunnelling probabilities, diffraction patterns and wave packet fragmentation of indirect excitons are largely determined by the internal dynamics, and could not be reproduced by point-like dipole models or mean-field calculations. We show that a properly-designed local self-energy potential to be added to the c.m. Hamiltonian embeds the effects of the c.m.-internal motion correlation at a small fraction of the computation load needed for full-propagation calculations. The explicit form of this self-energy emphasises the dominant role of internal virtual transitions in determining scattering coefficients of indirect excitons.

2016 - Erratum: Excitation energy-transfer in functionalized nanoparticles: Going beyond the Förster approach (The Journal of Chemical Physics (2016) 144 (074101) DOI: 10.1063/1.4941565) [Articolo su rivista]
Gil, G; Corni, Stefano; Delgado, A; Bertoni, Andrea; Goldoni, Guido
abstract

Ref. 1 contains an error in the expression linking 〈NP0|E(-)(R)|NPv〉 and 〈NP0|E(+)(R)|NPv〉 transition electric fields in terms of the transformation T, given in Sec. II D 1. In particular, the last two sentences of the last paragraph of Sec. II D 1 should be replaced by "The transition electric field 〈NP0|E(-)(R)|NPv〉 can be readily obtained in terms of a rotation T of the spatial coordinates R around the x axis, defined by its applications Tex = ex and Tey = ez. Hence 〈NP0|E(-)(R)|NPv〉 = -iT-1〈NP0|E(+)(TR)|NPv〉." As a consequence, Eq. (42) and the numerical results reported in Figs. 4 and 5 should be amended. Eq. (42) should read kavgRET = 2/3 πJ|〈M|d|M∗〉|2|〈NP0|E(+)(R)|NPv〉|2 + |〈NP0|E(+)(TR)|NPv〉|2. (42) We include in this erratum the new version of Figs. 4 and 5 reporting the correct results (captions unchanged). The trends are the same and only quantitative differences are present. The discussion and the conclusions of the article are not affected by this change. (Figure Presented). In the following, we take the opportunity to correct a few typing mistakes present in the manuscript. These errors do not affect the results.We list them in order of appearance: • In the last sentence of the first paragraph of Sec. II B, we inverted the order of the involved spectra. The actual sentence reads "J is the spectral overlap between the emission and absorption spectra of the donor and the acceptor, respectively, accounting for the vibronic coupling within the separated segments.19" • In Eqs. (12), (15), and (17), the dielectric constant is missing. They shall be re-written as Ṽ = - 3(dD · R)(dA · R)/R2 - dD · dA/ϵR3, (12) E(r;R) = 1/ϵ∇' [1/|r r'|]|r'=R' (15), E(r;R) = 1/ϵ ∑ l,m 4π/2l + 1 rl Ylm(ω) Glm(R). (17) • In Eqs. (25) and (26), the sign and the p/i order are inverted. The corrected expressions are 〈NP0|Ẽ (R) |NPv〉 = ∑i,p Cipv 〈p|E|i〉, (25) 〈NP0|d|NPv〉 = ∑i,p Cipv 〈p|r|i〉. (26) • In the first sentence of the last paragraph of Sec. II C should be written 〈i|e · pcv|p〉 instead of i|e · pcv|p〉. · In the last sentence of the first paragraph of Sec. II D 2, we mistyped "1/3" instead of the correct "2/3." This is not connected with the amendment of Eq. (42). The phrase with the corrected inline expression is "Using ∫π0 d(cos(α))cos2(α) = 2/3, we get" • In the third sentence of the penultimate paragraph of Sec. IV, there is a misplaced "not." The actual sentence should read "Additionally, RET rate is usually integrated over absorption peaks corresponding to nearly degenerate transitions.".

2016 - Exact two-body quantum dynamics of an electron-hole pair in semiconductor coupled quantum wells: A time-dependent approach [Articolo su rivista]
Grasselli, Federico; Bertoni, Andrea; Goldoni, Guido
abstract

We simulate the time-dependent coherent dynamics of a spatially indirect exciton - an electron-hole pair with the two particles confined in different layers - in a GaAs coupled quantum well system. We use a unitary wave-packet propagation method taking into account in full the four degrees of freedom of the two particles in a two-dimensional system, including both the long-range Coulomb attraction and arbitrary two-dimensional electrostatic potentials affecting the electron and/or the hole separately. The method has been implemented for massively parallel architectures to cope with the huge numerical problem, showing good scaling properties and allowing evolution for tens of picoseconds. We have investigated both transient time phenomena and asymptotic time transmission and reflection coefficients for potential profiles consisting of (i) extended barriers and wells and (ii) a single-slit geometry. We found clear signatures of the internal two-body dynamics, with transient phenomena in the picosecond time scale which might be revealed by optical spectroscopy. Exact results have been compared with mean-field approaches which, neglecting dynamical correlations by construction, turn out to be inadequate to describe the electron-hole pair evolution in realistic experimental conditions.

2016 - Excitation energy-transfer in functionalized nanoparticles: Going beyond the Förster approach [Articolo su rivista]
Gil, G.; Corni, Stefano; Delgado, A.; Bertoni, Andrea; Goldoni, Guido
abstract

We develop a novel approach to treat excitation energy transfer in hybrid nanosystems composed by an organic molecule attached to a semiconductor nanoparticle. Our approach extends the customary Förster theory by considering interaction between transition multipole moments of the nanoparticle at all orders and a point-like transition dipole moment representing the molecule. Optical excitations of the nanoparticle are described through an envelope-function configuration interaction method for a single electron-hole pair. We applied the method to the prototypical case of a core/shell CdSe/ZnS semiconductor quantum dot which shows a complete suppression of the energy transfer for specific transitions which could not be captured by Förster theory.

2016 - Predicting signatures of anisotropic resonance energy transfer in dye-functionalized nanoparticles [Articolo su rivista]
Gil Pérez, Gabriel José; Corni, Stefano; Delgado, Alain; Bertoni, Andrea; Goldoni, Guido
abstract

Resonance energy transfer (RET) is an inherently anisotropic process. Even the simplest, well-known Förster theory, based on the transition dipole-dipole coupling, implicitly incorporates the anisotropic character of RET. In this theoretical work, we study possible signatures of the fundamental anisotropic character of RET in hybrid nanomaterials composed of a semiconductor nanoparticle (NP) decorated with molecular dyes. In particular, by means of a realistic kinetic model, we show that the analysis of the dye photoluminescence difference for orthogonal input polarizations reveals the anisotropic character of the dye-NP RET which arises from the intrinsic anisotropy of the NP lattice. In a prototypical core/shell wurtzite CdSe/ZnS NP functionalized with cyanine dyes (Cy3B), this difference is predicted to be as large as 75% and it is strongly dependent in amplitude and sign on the dye-NP distance. We account for all the possible RET processes within the system, together with competing decay pathways in the separate segments. In addition, we show that the anisotropic signature of RET is persistent up to a large number of dyes per NP.

2016 - Tailoring the core electron density in modulation-doped core-multi-shell nanowires [Articolo su rivista]
Buscemi, Fabrizio; Royo, Miquel; Goldoni, Guido; Bertoni, Andrea
abstract

We show how a proper radial modulation of the composition of core-multi-shell nanowires (NWs) critically enhances the control of the free-carrier density in the high-mobility core with respect to core-single-shell structures, thus overcoming the technological difficulty of fine tuning the remote doping density. We calculate the electron population of the different NW layers as a function of the doping density and of several geometrical parameters by means of a self-consistent Schrödinger-Poisson approach: free carriers tend to localize in the outer shell and screen the core from the electric field of the dopants.

2016 - Time-dependent scattering of a composite particle: A local self-energy approach for internal excitations [Articolo su rivista]
Grasselli, Federico; Bertoni, Andrea; Goldoni, Guido
abstract

When composite particles - such as small molecules, nuclei, or photogenerated excitons in semiconductors - are scattered by an external potential, energy may be transferred between the c.m. and the internal degrees of freedom. An accurate dynamical modeling of this effect is pivotal in predicting diverse scattering quantities and reaction cross sections, and allows us to rationalize time-resolved energy and localization spectra. Here, we show that time-dependent scattering of a quantum composite particle with an arbitrary, nonperturbative external potential can be obtained by propagating the c.m. degrees of freedom with a properly designed local self-energy potential. The latter embeds the effect of internal virtual transitions and can be obtained by the knowledge of the stationary internal states. The case is made by simulating Wannier-Mott excitons in one- and two-dimensional semiconductor heterostructures. The self-energy approach shows very good agreement with numerically exact Schrödinger propagation for scattering potentials where a mean-field model cannot be applied, at a dramatically reduced computational cost.

2016 - Two-body quantum propagation in arbitrary potentials [Relazione in Atti di Convegno]
Grasselli, Federico; Bertoni, Andrea; Goldoni, Guido
abstract

We have implemented a unitary, numerically exact, Fourier split step method, based on a proper Suzuki-Trotter factorization of the quantum evolution operator, to propagate a two-body complex in arbitrary external potential landscapes taking into account exactly the internal structure. We have simulated spatially indirect Wannier-Mott excitons - optically excited electron-hole pairs with the two charges confined to different layers of a semiconductor heterostructure with prototypical 1D and 2D potentials emphasizing the effects of the internal dynamics and the insufficiency of mean-field methods in this context.

2015 - Aharonov-Bohm oscillations and electron gas transitions in hexagonal core-shell nanowires with an axial magnetic field [Articolo su rivista]
Royo, Miquel; Segarra, Carlos; Bertoni, Andrea; Goldoni, Guido; Planelles, Josep
abstract

We use spin-density-functional theory within an envelope function approach to calculate electronic states in a GaAs/InAs core-shell nanowire pierced by an axial magnetic field. Our fully three-dimensional quantum modeling includes explicitly a description of the realistic cross section and composition of the sample, and the electrostatic field induced by external gates in two different device geometries: gate-all-around and back-gate. At low magnetic fields, we investigate Aharonov-Bohm oscillations and signatures therein of the discrete symmetry of the electronic system, and we critically analyze recent magnetoconductance observations. At high magnetic fields, we find that several charge and spin transitions occur.We discuss the origin of these transitions in terms of different localization and Coulomb regimes, and we predict their signatures in magnetoconductance experiments.

2015 - Magnetophotoluminescence in GaAs/AlAs core-multishell nanowires: A theoretical investigation [Articolo su rivista]
Buscemi, Fabrizio; Royo, Miquel; Bertoni, Andrea; Goldoni, Guido
abstract

The magnetophotoluminescence in modulation doped core-multishell nanowires is predicted as a function of photoexcitation intensity in nonperturbative transverse magnetic fields. We use a self-consistent field approach within the effective mass approximation to determine the photoexcited electron and hole populations, including the complex composition and anisotropic geometry of the nanomaterial. The evolution of the photoluminescence is analyzed as a function of (i) photoexcitation power, (ii) magnetic field intensity, (iii) type of doping, and (iv) anisotropy with respect to field orientation.

2015 - Prediction of inelastic light scattering spectra from electronic collective excitations in GaAs/AlGaAs core-multishell nanowires [Articolo su rivista]
Royo, Miquel; Bertoni, Andrea; Goldoni, Guido
abstract

We predict inelastic light scattering spectra from electron collective excitations in a coaxial quantum well embedded in a core-multishell GaAs/AlGaAs nanowire. The complex composition, the hexagonal cross section, and the remote doping of typical samples are explicitly included, and the free electron gas is obtained by a density functional theory (DFT) approach. Inelastic light scattering cross sections due to charge and spin collective excitations belonging to quasi-one-dimensional (1D) and quasi-2D states, which coexist in such radial heterostructures, are predicted in the nonresonant approximation from a fully three-dimensional multisubband time-dependent DFT (TDDFT) formalism. We show that collective excitations can be classified in azimuthal, radial, and longitudinal excitations, according to the associated density fluctuations, and we suggest that their character can be exposed by specific spectral dispersion of inelastic light scattering along different planes of the heterostructure.

2015 - Space- and time-dependent quantum dynamics of spatially indirect excitons in semiconductor heterostructures [Articolo su rivista]
Grasselli, Federico; Bertoni, Andrea; Goldoni, Guido
abstract

We study the unitary propagation of a two-particle one-dimensional Schrödinger equation by means of the Split-Step Fourier method, to study the coherent evolution of a spatially indirect exciton (IX) in semiconductor heterostructures. The mutual Coulomb interaction of the electron-hole pair and the electrostatic potentials generated by external gates and acting on the two particles separately are taken into account exactly in the two-particle dynamics. As relevant examples, step/downhill and barrier/well potential profiles are considered. The space- and time-dependent evolutions during the scattering event as well as the asymptotic time behavior are analyzed. For typical parameters of GaAsbased devices, the transmission or reflection of the pair turns out to be a complex two-particle process, due to comparable and competing Coulomb, electrostatic, and kinetic energy scales. Depending on the intensity and anisotropy of the scattering potentials, the quantum evolution may result in excitation of the IX internal degrees of freedom, dissociation of the pair, or transmission in small periodic IX wavepackets due to dwelling of one particle in the barrier region. We discuss the occurrence of each process in the full parameter space of the scattering potentials and the relevance of our results for current excitronic technologies

2014 - Alla scoperta delle nanotecnologie [Monografia/Trattato scientifico]
DE RENZI, Valentina; Goldoni, Guido; Annamaria, Lisotti
abstract

La fisica alla nanoscala

2014 - Symmetries in the collective excitations of an electron gas in core-shell nanowires [Articolo su rivista]
ROYO VALLS, Miguel; Bertoni, Andrea; Goldoni, Guido
abstract

We study the collective excitations and inelastic light-scattering cross section of an electron gas confined in a GaAs/AlGaAs coaxial quantum well. These systems can be engineered in a core-multishell nanowire and inherit the hexagonal symmetry of the underlying nanowire substrate. As a result, the electron gas forms both quasi-one-dimensional channels and quasi-two-dimensional channels at the quantum-well bents and facets, respectively. Calculations are performed within the random-phase approximation and time-dependent density functional theory approaches. We derive symmetry arguments which allow one to enumerate and classify charge and spin excitations and determine whether excitations may survive to Landau damping. We also derive inelastic light-scattering selection rules for different scattering geometries. Computational issues stemming from the need to use a symmetry-compliant grid are also investigated systematically

2014 - Unintentional High-Density p‑Type Modulation Doping of a GaAs/ AlAs Core−Multishell Nanowire [Articolo su rivista]
J., Jadczak; P., Plochocka; A., Mitioglu; I., Breslavetz; M., Royo; A., Bertoni; Goldoni, Guido; T., Smolenski; P., Kossacki; A., Kretinin; Hadas, Shtrikman; D. K., Maude
abstract

Achieving significant doping in GaAs/AlAs core/ shell nanowires (NWs) is of considerable technological importance but remains a challenge due to the amphoteric behavior of the dopant atoms. Here we show that placing a narrow GaAs quantum well in the AlAs shell effectively getters residual carbon acceptors leading to an unintentional p-type doping. Magneto-optical studies of such a GaAs/AlAs core−multishell NW reveal quantum confined emission. Theoretical calculations of NW electronic structure confirm quantum confinement of carriers at the core/ shell interface due to the presence of ionized carbon acceptors in the 1 nm GaAs layer in the shell. Microphotoluminescence in high magnetic field shows a clear signature of avoided crossings of the n = 0 Landau level emission line with the n = 2 Landau level TO phonon replica. The coupling is caused by the resonant hole-phonon interaction, which points to a large two-dimensional hole density in the structure

2013 - Educational pathways through nanoscience: nitinol as a paradigmatic smart material [Articolo su rivista]
Lisotti, Annamaria; DE RENZI, Valentina; Carlo Andrea, Rozzi; Elena, Villa; Franca, Albertini; Goldoni, Guido
abstract

We developed an educational path based on nitinol, a shape memory alloy which conveniently exemplifies the smart material concept, i.e., a material that performs a predetermined, reversible action in response to a change in the environment. Nitinol recovers a given shape, changes its resistivity drastically and modifies its elastic properties if subjected to a temperature change in a convenient range. Here, the properties are verified with laboratory protocols appropriate to a high-school environment. Use of mobile electronic devices is also suggested. The collected electrical and mechanical properties are analysed within a didactic path which emphasizes their common physical origin, i.e., the martensitic transition. Moreover, the peculiarities of this solid-to-solid transformation are put in correspondence with the apparently unrelated but more familiar liquid–vapour transition. The relationship with possible applications is emphasized by measuring the efficiency of using a nitinol spring as an actuator.

2013 - High Mobility One- and Two-Dimensional Electron Systems in Nanowire-Based Quantum Heterostructures [Articolo su rivista]
Stefan, Funk; Miguel, Royo; Ilaria, Zardo; Daniel, Rudolph; Stefanie, Morkötter; Benedikt, Mayer; Jonathan, Becker; Alexander, Bechtold; Sonja, Matich; Markus, Döblinger; Max, Bichler; Gregor, Koblmüller; Jonathan J., Finley; Andrea, Bertoni; Goldoni, Guido; Gerhard, Abstreiter
abstract

Free-standing semiconductor nanowires in combination with advanced gate-architectures hold an exceptional promise as miniaturized building blocks in future integrated circuits. However, semiconductor nanowires are often corrupted by an increased number of close-by surface states, which are detrimental with respect to their optical and electronic properties. This conceptual challenge hampers their potentials in high-speed electronics and therefore new concepts are needed in order to enhance carrier mobilities. We have introduced a novel type of core−shell nanowire heterostructures that incorporate modulation or remote doping and hence may lead to high-mobility electrons. We demonstrate the validity of such concepts using inelastic light scattering to study single modulation-doped GaAs/Al0.16Ga0.84As core-multishell nanowires grown on silicon. We conclude from a detailed experimental study and theoretical analysis of the observed spin and charge density fluctuations that one- and two-dimensional electron channels are formed in a GaAs coaxial quantum well spatially separated from the donor ions. A total carrier density of about 3 × 107 cm−1 and an electron mobility in the order of 50 000 cm2/ (V s) are estimated. Spatial mappings of individual GaAs/Al0.16Ga0.84As core−multishell nanowires show inhomogeneous properties along the wires probably related to structural defects. The first demonstration of such unambiguous 1D- and 2Delectron channels and the respective charge carrier properties in these advanced nanowire-based quantum heterostructures is the basis for various novel nanoelectronic and photonic devices.

2013 - Landau levels, edge states, and magnetoconductance in GaAs/AlGaAs core-shell nanowires [Articolo su rivista]
ROYO VALLS, Miguel; Bertoni, Andrea; Goldoni, Guido
abstract

Magnetic states of the electron gas confined in modulation-doped core-shell nanowires are calculated for a transverse field of arbitrary strength and orientation. Magnetoconductance is predicted within the Landauer approach. The modeling takes fully into account the radial material modulation, the prismatic symmetry, and the doping profile of realistic GaAs/AlGaAs devices within an envelope-function approach, and electron-electron interaction is included in a mean-field self-consistent approach. Calculations show that in the low free-carrier density regime, magnetic states can be described in terms of Landau levels and edge states, similar to planar two-dimensional electron gases in a Hall bar. However, at higher carrier density, the dominating electron-electron interaction leads to a strongly inhomogeneous localization at the prismatic heterointerface. This gives rise to a complex band dispersion, with local minima at finite values of the longitudinal wave vector, and a region of negative magnetoresistance. The predicted marked anisotropy of the magnetoconductance with field direction is a direct probe of the inhomogeneous electron gas localization of the conductive channel induced by the prismatic geometry.

2013 - Modeling opto-electronic properties of a dye molecule in proximity of a semiconductor nanoparticle [Articolo su rivista]
DELGADO GRAN, Alain; Corni, Stefano; Goldoni, Guido
abstract

A general methodology is presented to model the opto-electronic properties of a dye molecule in the presence of a semiconductor nanoparticle (NP), a model system for the architecture of dye-sensitized solar cells. The method is applied to the L0 organic dye solvated with acetonitrile in the neighborhood of a TiO2 NP. The total reaction potential due to the polarization of the solvent and the metal oxide is calculated by extending the polarizable continuum model integral equation formalism. The ground state energy is computed by using density functional theory (DFT) while the vertical electronic excitations are obtained by time-dependent DFT in a state-specific corrected linear response scheme. We calculate the excited state oxidation potential (ESOP) for the protonated and deprotonated forms of the L0 dye at different distances and configurations with respect to the NP surface. The stronger renormalizations of the ESOP values due to the presence of the TiO2 nanostructure are found for the protonated dye, reaching a maximum of about −0.15 eV. The role of protonation effect is discussed in terms of the atomic Löwdin charges of the oxidized and reduced species. On the other hand, we observed a weak effect on the L0 optical excitation gap due to the polarization response of the NP.

2012 - Low-lying electronic excitations and optical absorption spectra of the black dye sensitizer: a first-principles study [Articolo su rivista]
DELGADO GRAN, Alain; Corni, Stefano; Goldoni, Guido
abstract

We report on ab-initio calculations of theelectronic structure and optical absorption response of theblack dye sensitizer in gas phase. We show that, despitethe large size of this molecule, the second-order multiconfigurationquasi-degenerate perturbation theory (MCQDPT)can be used to calculate vertical excitationenergies, oscillator strengths and optical absorption spectra.The zeroth-order reference states entering perturbationcalculations are complete active space (CAS) configurationinteraction (CI) wave functions computed for 12 activeelectrons distributed in 12 active orbitals. We found thatthe CI approach is not enough for taking into account thestrong dynamical correlation effects in this system. In fact,the excitation energies of the CAS-CI target states arestrongly renormalized by the MC-QDPT calculations. Inthe calculated absorption spectra, the analysis of the perturbedwavefunctions revealed that the stronger absorptionbands correspond to metal-to-ligand and ligand-to-ligandcharge transfer processes. Comparison with independenttime-dependent extension (TDDFT) calculationsperformed with different functionals shows that correctionsto the long-range behavior of the functional is pivotal toachieve agreement with the MC-QDPT results.

2011 - A new quasiparticle in carbon nanotubes [Articolo su rivista]
Hohenester, Ulrich; Goldoni, Guido
abstract

Trions—one electron bound to two holes via Coulomb forces—can be observed in the optical spectra of doped carbon nanotubes

2011 - Absorption properties of metal-semiconductor hybrid nanoparticles [Articolo su rivista]
Shaviv, E.; Schubert, O.; Alves Santos, M.; Goldoni, Guido; Di Felice, R.; Vallée, F.; Del Fatti, N.; Banin, U.; Soennichsen, C.
abstract

The optical response of hybrid metal semiconductor nanoparticles exhibits different behaviors due to the proximity between the disparate materials. For some hybrid systems, such as CdS-Au matchstick-shaped hybrids, the particles essentially retain the optical properties of their original components, with minor changes. Other systems, such as CdSe-Au dumbbell-shaped nanoparticles, exhibit significant change in the optical properties due to strong coupling between the two materials. Here, we study the absorption of these hybrids by comparing experimental results with simulations using the discrete dipole approximation method (DDA) employing dielectric functions of the bare components as inputs. For CdS-Au nanoparticles, the DDA simulation provides insights on the gold tip shape and its interface with the semiconductor, information that is difficult to acquire by experimental means alone. Furthermore, the qualitative agreement between DDA simulations and experimental data for CdS-Au implies that most effects influencing the absorption of this hybrid system are well described by local dielectric functions obtained separately for bare gold and CdS nanoparticles. For dumbbell shaped CdSe-Au, we find a shortcoming of the electrodynamic model, as it does not predict the "washing out" of the optical features of the semiconductor and the metal observed experimentally. The difference between experiment and theory is ascribed to strong interaction of the metal and semiconductor excitations, which spectrally overlap In the CdSe case. The present study exemplifies the employment of theoretical approaches used to describe the optical properties of semiconductors and metal nanoparticles, to achieve better understanding of the behavior of metal-semiconductor hybrid nanoparticles.

2011 - Electron and hole gas in modulation doped GaAs/AlGaAs radialheterojunctions [Articolo su rivista]
A., Bertoni; M., Royo; F., Mahawish; Goldoni, Guido
abstract

We perform self-consistent Schr¨ odinger-Poisson calculations with exchange and correlation corrections to determine the electron and hole gas in a radial heterojunction formed in a GaAs/AlGaAs core-multi-shell nanowire, which is either n- or p-doped. We show that the electron and hole gases can be tuned to different localizations and symmetries inside the core as a function of the doping density/gate potential. Contrary to planar heterojunctions, conduction electrons do not form a uniform 2D electron gas (2DEG) localized at the GaAs/AlGaAs interface, but rather show a transition between an isotropic, cylindrical distribution deep in the GaAs core (low doping) and a set of six tunnel-coupled quasi-1D channels at the edges of the interface (high doping). Holes, on the other hand, are much more localized at the GaAs/AlGaAs interface. At low doping, they present an additional localization pattern with six separated 2DEGs strips. The field generated by a back-gate may easily deform the electron or hole gas, breaking the sixfold symmetry. Single 2DEGs at one interface or multiple quasi-1D channels are shown to form as a function of voltage intensity, polarity, and carrier type.

2011 - Excitonic Resonance in Semiconductor-Metal Nanohybrids [Articolo su rivista]
J. I., Climente; J. L., Movilla; Goldoni, Guido; J., Planelles
abstract

We use a configuration interaction approach within the envelope function approximation to study the nature of the excitonic resonance in nanohybrids, composite nanoparticles (NPs) combining a semiconducting and a metallic segment in contact. With reference to recent experimental reports, we specifically study CdS-based nanorods with metallic NPs deposited at the tips (matchstick) or metallic coatings (core–shell). The excitonic states are computed taking into account both the renormalization of the electron–hole interaction and self-energy effects induced by the metallic segment on the electron–hole pair, as well as by the dielectric environment, through an induced charge numerical approach. In neutral matchstick structures, the metal NP has only a minor influence (1 meV) on the excitonic states. When the metallic NP is charged, the exciton becomes rapidly red-shifted and spatially indirect. In contrast, in neutral core–shell structures the exciton energy red-shifts by tens of millielectronvolts.

2010 - Dielectric Functions of Semiconductor Nanoparticles from the Optical Absorption Spectrum: The Case of CdSe and CdS [Articolo su rivista]
M., Alves Santos; R., Di Felice; Goldoni, Guido
abstract

We propose a new method to extract the complete size-dependent complex dielectric function ε(E) ofsemiconductor nanoparticles (NPs) from spectroscopic data. Typical experimental absorption spectra areavailable in a limited energy range near the absorption edge. We show that this limitation can be overcomeby matching the NP dielectric function with the bulk dielectric function at high energies beyond the experimentalrange. Thus, using the Kramers-Kro¨nig relations that link the real and imaginary parts, it is possible toextract the complete ε(E) of the NP. This is achieved through an iterative procedure that systematically improvesthe trial ε(E) until the absorption spectrum of the NP is accurately reproduced in the experimental range.Here we describe the methodology and we obtain the ε(E) of CdSe and CdS NPs of selected sizes frompublished data sets.

2010 - Semiconductor quantum tubes: Dielectric modulation and excitonic response [Articolo su rivista]
D., Kammerlander; F., Troiani; Goldoni, Guido
abstract

We study theoretically the optical properties of quantum tubes, one-dimensional semiconductor nanostructureswhere electrons and holes are confined to a cylindrical shell. In these structures, which bridge betweentwo-dimensional and one-dimensional systems, the electron-hole interaction may be modulated by a dielectricsubstance outside the quantum tube and possibly inside its core. We use the exact Green’s function for theappropriate dielectric configuration and exact diagonalization of the electron-hole interaction within aneffective-mass description to predict the evolution of the exciton binding energy and oscillator strength.Contrary to the homogeneous case, in dielectrically modulated tubes, the exciton binding is a function of thetube diameter and can be tuned to a large extent by structure design and proper choice of the dielectric media.

2009 - Conductance crossovers in coherent surface transport on y nanojunctions [Relazione in Atti di Convegno]
Bertoni, Andrea; Cuoghi, Giampaolo; Ferrari, Giulio; Goldoni, Guido
abstract

Conductance characteristics of a nonplanar two-dimensional electron gas (2DEG) can expose the role of its bending on the 2DEG electronic states. In particular, the presence of an effective geometric potential can be revealed. Here, we present a numerical study of the coherent electron transport on Y nanojunctions of three cylindrical 2DEGs, including a proposal for the experimental detection of the geometric potential. We describe the analytical approach leading to the reduction of the problem dimensionality from 3D to 2D and sketch our simulation scheme. © 2009 IOP Publishing Ltd.

2009 - Electronic and magnetic states in core multishell nanowires: Edge localization, Landau levels and Aharonov-Bohm oscillations [Relazione in Atti di Convegno]
Ferrari, Giulio; Cuoghi, Gianpaolo; Bertoni, Andrea; Goldoni, Guido; Molinari, Elisa
abstract

We study the electronic states of hexagonal core multishell semiconductor nanowires, including the effect of magnetic fields. We find that the two dimensional electron states formed at the interface between different layers are mostly localized at the six edges of the hexagonal prism, and behave as a set of quasi-1D quantum channels. They can be manipulated by magnetic fields either parallel or perpendicular to the wire axis. These results can be rationalized in terms of Aharonov-Bohm oscillations or Landau level formation. We also show that inter-channel coupling and magnetic behavior is influenced by the geometric details of the nanowires. © 2009 IOP Publishing Ltd.

2009 - Magnetic States in Prismatic Core Multishell Nanowires [Articolo su rivista]
Ferrari, Giulio; Goldoni, Guido; Bertoni, Andrea; G., Cuoghi; Molinari, Elisa
abstract

We study the electronic states of core multishell semiconductor nanowires, including the effect of strong magnetic fields. We show that the multishell overgrowth of a free-standing nanowire, together with the prismatic symmetry of the substrate, may induce quantum confinement of carriers in a set of quasi-1D quantum channels corresponding to the nanowire edges. Localization and interchannel tunnel coupling are controlled by the curvature at the edges and the diameter of the underlying nanowire. We also show that a magnetic field may induce either Aharonov-Bohm oscillations of the energy levels in the axial configuration, or a dimensional transition of the quantum states from quasi-1D to Landau levels for fields normal to the axis. Explicit predictions are given for nanostructures based on GaAs, InAs, and InGaN with different symmetries.

2009 - Optimal generation of indistinguishable photons from non-identical artificial molecules [Articolo su rivista]
E., Cancellieri; F., Troiani; Goldoni, Guido
abstract

2009 - Probing collective modes of correlated states of few electrons in semiconductor quantum dots [Articolo su rivista]
S., Kalliakos; Rontani, Massimo; V., Pellegrini; A., Pinczuk; A., Shinga; C. P., Garcia; Goldoni, Guido; Molinari, Elisa; L. N., Pfeiffer; K. W., West
abstract

Low-lying collective excitations above highly correlated ground states of few interacting electrons confined in GaAs semiconductor quantum dots are probed by resonant inelastic light scattering. We highlight that separate studies of the changes in the spin and charge degrees of freedom offer unique access to the fundamental interactions. The case of quantum dots with four electrons is found to be determined by a competition between triplet and singlet ground states that is uncovered in the rich light scattering spectra of spin excitations. These light scattering results are described within a configuration-interaction framework that captures the role of electron correlation with quantitative accuracy. Recent light scattering results that reveal the impact of anisotropic confining potentials in laterally coupled quantum dots are also reviewed. In these studies, inelastic light scattering methods emerge as powerful probes of collective phenomena and spin configurations in quantum dots with few electrons. (C) 2009 Elsevier Ltd. All rights reserved.

2009 - The role of exciton-exciton interaction on nonlinearities in GaN microdisks [Relazione in Atti di Convegno]
Shojaei, S.; Troiani, F.; Asgari, A.; Kalafi, M.; Goldoni, G.
abstract

Large built-in piezoelectric fields in nitride nanostructures, because of their wurtzite structure, induce a spatial seperation between confined electrons and holes and lead to formation of electric dipoles. This paper investigates the effects of exciton-exciton interaction as a dipolar interaction in a GaN/AlxGa1-xN microdisk. We show how this interaction result in biexciton binding energies in the meV energy range. Also we study the effect of disk radius on exciton binding energy. Results show that the exciton binding energy in smaller disks, is larger than the bigger one. © 2009 SPIE.

2008 - A molecular state of correlated electrons in a quantum dot [Articolo su rivista]
S., Kalliakos; M., Rontani; V., Pellegrini; C. P., Garcia; A., Pinczuk; Goldoni, Guido; Molinari, Elisa; L. N., Pfeiffer; AND K. W., West
abstract

Correlation among particles in finite quantum systems leads to complex behaviour and novel states of matter. One remarkable example is predicted to occur in a semiconductor quantum dot(1-3), where at vanishing electron density the Coulomb interaction between electrons rigidly fixes their relative positions as those of the nuclei in a molecule(4-14). In this limit, the neutral few-body excitations are roto-vibrations, which have either rigid-rotor or relative-motion character(15). In the weak correlation regime, on the contrary, the Coriolis force mixes rotational and vibrational motions. Here, we report evidence for roto-vibrationalmodes of an electron molecular state at densities for which electron localization is not yet fully achieved. We probe these collective modes by using inelastic light scattering(16-18) in quantum dots containing four electrons(19). Spectra of low-lying excitations associated with changes of the relative-motion wavefunction-the analogues of the vibration modes of a conventional molecule-do not depend on the rotational state represented by the total angular momentum. Theoretical simulations by the configuration-interaction method(20) are in agreement with the observed roto-vibrational modes and indicate that such molecular excitations develop at the onset of short-range correlation.

2008 - Carrier states on cylindrical 2DEGs in a magnetic field [Articolo su rivista]
G., Ferrari; Bertoni, Andrea; Goldoni, Guido; Molinari, Elisa
abstract

We compute carrier states on a cylindrical 2DEG under the influence of a magnetic field perpendicular to the tube axis. The field and the topology of the cylindrical surface have been included in the Schrodinger equation, that has been solved exactly. The results show that carrier states can be driven from a 2D regime, to a quasi-ID regime by transverse magnetic field. In the case of a spatially modulated magnetic field, the carriers can localise to quasi-OD states. (c) 2007 Elsevier B.V. All rights reserved.

2008 - Correlated states and spin transitions in nanofabricated AlGaAs/GaAs few-electron quantum dots probed by inelastic light scattering [Articolo su rivista]
S., Kalliacos; C. P., Garcia; V., Pellegrini; A., Pinczuk; B. S., Dennis; L. N., Pfeiffer; K. W., West; Rontani, Massimo; Goldoni, Guido; Molinari, Elisa
abstract

Spin transitions and correlated few-electron states are investigated by resonant inelastic light scattering in dilute arrays of GaAs/AlGaAs modulation-doped quantum dots (QDs) fabricated by electron-beam lithography and low impact reactive-ion etching. We focus on QDs with four electrons. We show that at moderate magnetic fields, the ground state is a singlet with total spin S = 0. A rich spectrum of distinct spin and charge inter-shell excitations is found, which cannot be described by a mean-field Hartree-Fock framework based on the quantum description of Fock-Darwin energy levels. Instead, the experimental results are well modeled by numerical evaluations within a full configuration interaction approach that highlights the impact of correlation effects in this configuration. This work demonstrates that the sensitivity reached by resonant inelastic light scattering enables the study of few-electron effects in QDs formed by state-of-the-art nanofabrication processes under the extreme conditions of low temperatures and high magnetic fields. (C) 2007 Elsevier B.V. All rights reserved.

2008 - Coulomb-induced nonlinearities in GaN microdisks [Articolo su rivista]
S., Shojaei; F., Troiani; A., Asgari; M., Kalafi; Goldoni, Guido
abstract

Due to their wurzite structure, GaN quantum wells and dots arecharacterized by large built-in piezoelectric fields.These induce a spatial separation between the confined electron andholes, thus favouring the formation of electric dipoles.Here, we theoretically investigate the effects of the long-range,dipole-dipole interaction between two excitons in a GaN/Al(x)Ga(1-x)Nmicrodisk.These Coulomb interactions are shown to strongly affect the biexcitonground state. In particular, they induce strong spatial correlationsbetween the two excitons and result in biexciton binding energiesof the order of 1 meV.

2008 - Cylindrical two-dimensional electron gas in a transverse magnetic field [Articolo su rivista]
G., Ferrari; Bertoni, Andrea; Goldoni, Guido; Molinari, Elisa
abstract

We compute the single-particle states of a two-dimensional (2D) electron gas confined to the surface of a cylinder immersed in a magnetic field. The envelope-function equation is solved exactly for both a homogeneous and a periodically modulated magnetic field perpendicular to the cylinder axis. The nature and energy dispersion of the quantum states reflects the interplay between different length scales, namely, the cylinder diameter, the magnetic length, and, possibly, the wavelength of the field modulation. We show that a transverse homogeneous magnetic field drives carrier states from a quasi-2D (cylindrical) regime to a quasi-one-dimensional regime where carriers form channels along the cylinder surface. Furthermore, a magnetic field which is periodically modulated along the cylinder axis may confine the carriers to tunnel-coupled stripes, rings, and dots on the cylinder surface depending on the ratio between the field periodicity and the cylinder radius. Results in different regimes are traced to either incipient Landau-level formation or Aharonov-Bohm behavior.

2008 - Exact biexciton binding energy in carbon nanotubes using a quantum Monte Carlo approach [Articolo su rivista]
Kammerlander, David; Prezzi, Deborah; Goldoni, Guido; Molinari, Elisa; U., Hohenester
abstract

We performed quantum Monte Carlo (QMC) calculations for a model system of excitons and biexcitons in carbon nanotubes (CNTs) and compared the results with those of a variational approach [T.G. Pedersen, K. Pedersen, H.D. Cornean, P. Duclos, Nano Lett. 5 (2005) 291]. Due to their geometric properties, the shape of a hollow cylinder, CNTs can be treated as 2D objects. With decreasing diameter one expects them even to exhibit quasi-1D properties. In the present study the biexciton in its ground state is found to be more strongly bound than estimated before. Biexcitonic complexes are predicted to remain stable for all diameters even at room temperature. The binding energy grows significantly with decreasing diameter, showing indeed a transition from a quasi-2D system to a quasi-ID system. (c) 2007 Elsevier B.V. All rights reserved.

2008 - Few-particle Electron Dynamics in Coupled Quantum Dots with Phonon Interaction [Relazione in Atti di Convegno]
A., Bertoni; J. I., Climente; M., Rontani; Goldoni, Guido; U., Hohenester
abstract

The time evolution of a correlated multi-electron state ina semiconductor coupled quantum dot, interacting withthe acoustic phonon bath, is calculated within the masterequation formalism, including electron-electron interactionexactly through the configuration-interactionapproach. The system is evolved after an initializationphase, with the system under strong bias, leading to thecharge density mainly localized in one of the dots. Whenthe system is evolved (in unbiased condition), dampedmulti-frequency charge oscillations are found, whichstrongly depend on the 3D system geometry and initialization.We describe the approach used to obtain themulti-electron states and to include the phonon-inducedtransitions between them in the time evolution. Furthermore,the effect of a magnetic field applied in the axialdirection of the vertical double-dot cylindrical structureis discussed.

2008 - Influence of valence band spin-orbit coupling on the entanglement of excitons in coupled quantum dots [Articolo su rivista]
J. I., Climente Plasencia; M., Korkusinski; Goldoni, Guido; P., Hawrylak
abstract

We study the effect of valence band spin-orbit interaction on the exciton entanglement in vertically stacked quantum dots, using a multi-band k . p theory for holes. It is shown that the spin-orbit interaction reduces the exciton entanglement, establishing an intrinsic upper limit. For usual InGaAs nanostructures, however, this effect is generally small and does not pose a challenge for current attempts for the development of quantum information. (c) 2007 Elsevier B.V. All rights reserved.

2008 - MSS-13 [Articolo su rivista]
Goldoni, G.; Sorba, L.
abstract

2008 - Photoluminescence spectroscopy of trions in quantum dots: A theoretical description [Articolo su rivista]
J. I., Climente; A., Bertoni; Goldoni, Guido
abstract

We present a full configuration-interaction study of the spontaneous recombination of neutral and singly charged excitons (trions) in semiconductor quantum dots from weak- to strong-coupling regimes. We find that the enhancement of the recombination rate of neutral excitons with increasing dot size is suppressed for negative trions and even reversed for positive trions. Our findings agree with recent comprehensive photoluminescence experiments in self-assembled quantum dots [P. Dalgarno , Phys. Rev. B 77, 245311 (2008)] and confirm the major role played by correlations in the valence band. The effect of the temperature on the photoluminescence spectrum and that of the ratio between the electron and hole wave-function length scales are also described.

2008 - Preface [17th International Conference on Electronic Properties of Two-Dimensional Systems] [Articolo su rivista]
Goldoni, Guido; Sorba, Lucia
abstract

N/A

2008 - Proceedings of the13th International Conference on Modulated Semiconductor Structures (MSS-13) [Relazione in Atti di Convegno]
Goldoni, G.; Sorba, L.
abstract

2008 - Spectral diffusion and line broadening in single self-assembled GaAs/AlGaAs quantum dot photoluminescence [Articolo su rivista]
M., Abbarchi; F., Troiani; C., Mastrandrea; Goldoni, Guido; T., Kuroda; T., Mano; K., Sakoda; N., Koguchi; S., Sanguinetti; A., Vinattieri; M., Gurioli
abstract

We experimentally and theoretically investigate the photoluminescence broadening of different excitonic complexes in single self-assembled GaAs/AlGaAs quantum dots. We demonstrate that the excitonic fine-structure splitting leads to a sizable line broadening whenever the detection is not resolved in polarization. The residual broadening in polarized measurements is systematically larger for the exciton with respect to both the trion and the biexciton recombination. The experimental data agree with calculations of the quantum confined Stark effect induced by charge defects in the quantum dot (QD) environment, denoting the role of the QD spectator carrier rearrangement in reducing the perturbation of the fluctuating environment. (C) 2008 American Institute of Physics.

2008 - Spin relaxation due to spin-orbit coupling in multi-electron quantum dots [Articolo su rivista]
J. I., Climente; A., Bertoni; Goldoni, Guido; M., Rontani; Molinari, Elisa
abstract

We show that the number of electrons confined in a semiconductor quantum dot has a strong influence over the Rashba and Dresselhaus spin-orbit (SO) admixture. This can be exploited to improve the lifetime of spin excitations, as compared to the usual one- and two-electron devices. The physical mechanisms reducing SO admixture are discussed, and numerical results for realistic weakly confined GaAs/AlGaAs dots are reported. (c) 2007 Elsevier B.V. AD rights reserved.

2008 - Theory of valence-band holes as Luttinger spinors in vertically coupled quantum dots [Articolo su rivista]
J. I., Climente; M., Korkusinski; Goldoni, Guido; P., Hawrylak
abstract

We describe the valence-band holes of quantum dot molecules formed by two vertically coupled disks, using a four-band k . p Hamiltonian. It is shown that the strong spin-orbit coupling of the valence band introduces characteristic features in the hole tunneling, which are not captured by the usual single-band heavy-hole approximation. Therefore, a treatment of hole states as multiband Luttinger spinors is required. Within this description the parity symmetry in the vertical direction is lost, and chirality symmetry must be used instead. Effects of spin-orbit coupling on the hole and exciton states, as well as on the optical transitions are discussed. We show that, with increasing interdot distance, the spin-orbit interaction leads to a bonding-antibonding ground-state transition and to quenching of the excitonic emission. These results are relevant to recent experiments.

2008 - 13th International Conference on Modulated Semiconductor Structures [Curatela]
Goldoni, Guido; Sorba, Lucia
abstract

Proceedings 13th International Conference on Modulated Semiconductor Structures

2008 - 17th International Conference on Electronic Properties of Two-Dimensional Systems [Curatela]
Goldoni, Guido; Sorba, Lucia
abstract

Proceedings 17th International Conference on Electronic Properties of Two-Dimensional Systems

2007 - A high-performance full-configuration-interaction study of Wigner crystallization in quantum dots [Relazione in Atti di Convegno]
Rontani, M.; Goldoni, G.
abstract

We study the transition between liquid and solid electron phases of a two-dimensional quantum dot by performing extensive full-configuration- interaction calculations. From the analysis of pair correlation functions we find evidence of localization for values of the dimensionless density parameter λ ≈ 4. © 2007 American Institute of Physics.

2007 - Biexciton Stability in Carbon Nanotubes [Articolo su rivista]
Kammerlander, David; Prezzi, Deborah; Goldoni, Guido; Molinari, Elisa; AND U., Hohenester
abstract

We have applied the quantum Monte Carlo method and tight-binding modeling to calculate the binding energy of biexcitons in semiconductor carbon nanotubes for a wide range of diameters and chiralities. For typical nanotube diameters we find that biexciton binding energies are much larger than previously predicted from variational methods, which easily brings the biexciton binding energy above the room temperature threshold.

2007 - Directionality of acoustic phonon emission in weakly-confined semiconductor quantum dots [Articolo su rivista]
Climente, J. I.; Bertoni, Andrea; Goldoni, Guido; Molinari, Elisa
abstract

The direction of propagation of acoustic phonons emitted by electron relaxation in weakly confined, parabolic quantum dots charged with one or two electrons is studied theoretically. The emission angle strongly depends on the energy of the phonon, the dominant electron-phonon scattering mechanism (deformation potential or piezoelectric field), and the orbital symmetries of the initial and final electron states. This leads to different behaviors for phonons emitted by electrons relaxing between levels of single and coupled quantum dots. Our results establish the basis to control the direction of propagation of phonon modes triggered by transitions in quantum dot systems.

2007 - Effect of electron-electron interaction on the phonon-mediated spin relaxation in quantum dots [Articolo su rivista]
J. I., Climente; Bertoni, Andrea; Goldoni, Guido; Rontani, Massimo; Molinari, Elisa
abstract

We estimate the spin relaxation rate due to spin-orbit coupling and acoustic phonon scattering in weakly confined quantum dots with up to five interacting electrons. The full configuration interaction approach is used to account for the interelectron repulsion, and Rashba and Dresselhaus spin-orbit couplings are exactly diagonalized. We show that electron-electron interaction strongly affects spin-orbit admixture in the sample. Consequently, relaxation rates strongly depend on the number of carriers confined in the dot. We identify the mechanisms which may lead to improved spin stability in few electron (> 2) quantum dots as compared to the usual one and two electron devices. Finally, we discuss recent experiments on triplet-singlet transitions in GaAs dots subject to external magnetic fields. Our simulations are in good agreement with the experimental findings, and support the interpretation of the observed spin relaxation as being due to spin-orbit coupling assisted by acoustic phonon emission.

2007 - Magnetic field dependence of triplet-singlet relaxation in quantum dots with spin-orbit coupling [Articolo su rivista]
Climente, Ji; Bertoni, A; Goldoni, Guido; Rontani, M; Molinari, Elisa
abstract

We estimate the triplet-singlet relaxation time due to spin-orbit coupling assisted by phonon emission in weakly confined quantum dots. Calculations are performed taking into account Coulomb and spin-orbit interactions exactly within the full configuration interaction method, and Fermi golden rule. Our results for two and four electrons show that different triplet-singlet relaxation trends observed in recent experiments under magnetic fields can be understood within a unified theoretical description, as the result of the competition between spin-orbit coupling and phonon emission efficiency. Moreover, we show that properly designed QD structures may give access to very long-lived triplet states as well as to selective population of the triplet Zeeman sublevels.

2007 - Optical near-field mapping of bright and dark quantum dot states [Relazione in Atti di Convegno]
Hohenester, U.; Goldoni, G.; Molinari, E.
abstract

We theoretically investigate scanning nearfield optical microscopy (SNOM) of semiconductor quantum dots. In the nearfield regime bright and dark excitonic states become mixed, opening new channels for the coupling to the electromagnetic field. As a consequence, ultra-narrow luminescence lines appear in the spectra, corresponding to very long lived excitonic states. © 2007 American Institute of Physics.

2007 - Phase lapses in scattering through multielectron quantum dots: Mean-field and few-particle regimes [Articolo su rivista]
A., Bertoni; Goldoni, Guido
abstract

We show that the observed evolution of the transmission phase through multielectron quantum dots with more than approximately ten electrons, which shows a universal (i.e., independent of N) as yet unexplained behavior, is consistent with an electrostatic model, where electron-electron interaction is described by a mean-field approach. Moreover, we perform exact calculations for an open one-dimensional quantum dot and show that carrier correlations may give rise to a nonuniversal (i.e., N-dependent) behavior of the transmission phase, ensuing from Fano resonances, which is consistent with experiments with a few (N < 10) carriers. Our results suggest that in the universal regime, the coherent transmission takes place through a single level, while in the few-particle regime, the correlated scattering state is determined by the number of bound particles.

2007 - Probing spin states in AlGaAs/GaAs few-electron quantum dots by inelastic light scattering [Relazione in Atti di Convegno]
Kalliakos, S.; Pascual Garcia, C.; Pellegrini, V.; Pinczuk, A.; Dennis, B. S.; Pfeiffer, L. N.; West, K. W.; Rontani, M.; Goldoni, G.; Molinari, E.
abstract

Resonant inelastic light scattering provides a venue for the study of spin transitions and interactions in quantum dots (QDs) with few electrons. Here we present the observation of inter-shell excitations in GaAs/AlGaAs nanofabricated QDs containing four electrons. The ground state with four electrons dictated by Hund's rule is a triplet with total spin S = 1 so that peculiar to this configuration there are two monopole inter-shell spin modes with changes ΔS = ±1 that connect the triplet state to the excited states with S = 0 and S = 2, respectively. We present light scattering spectra that offer evidence of these excitations. The light scattering spectra are interpreted by numerical evaluations within a full configuration interaction approach. © 2007 American Institute of Physics.

2007 - Signatures of molecular correlations in few-electron dynamics of coupled quantum dots [Articolo su rivista]
A., Bertoni; J. I., Climente Plasencia; Goldoni, Guido; M., Rontani; U., Hohenester
abstract

We study the effect of Coulomb interaction on the few-electron dynamics in coupled semiconductor quantum dots by exact diagonalization of the few-body Hamiltonian. The oscillation of carriers is strongly affected by the number of confined electrons and by the strength of the interdot correlations. Single-frequency oscillations are found for either uncorrelated or highly correlated states, while multifrequency oscillations take place in the intermediate regime. Moreover, Coulomb interaction renders few-particle oscillations sensitive to perturbations in spatial directions other than that of the tunneling, contrary to the single-particle case. The inclusion of acoustic phonon scattering does not modify the carrier dynamics substantially at short times, but it can damp oscillation modes selectively at long times.

2006 - Controlling spin phases of few electrons in artificial molecules by magnetic fields [Articolo su rivista]
Rontani, M; Goldoni, Guido; Bellucci, D; Molinari, Elisa
abstract

We investigate the phase diagram of realistic vertically coupled quantum dots under a magnetic field of arbitrary strength and direction. The ground state wave function of four electrons is determined by means of the full configuration interaction method, able to accurately treat electron-electron correlation, which can dominate at experimentally attainable fields. (C) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

2006 - Effect of the Coulomb interaction on the electron relaxation of weakly-confined quantum dot systems using the full configuration interaction approach [Articolo su rivista]
J. I., Climente; Bertoni, Andrea; Rontani, Massimo; Goldoni, Guido; Molinari, Elisa
abstract

We study acoustic-phonon-induced relaxation of charge excitations in single and tunnel-coupled quantum dots containing few confined interacting electrons. The full configuration interaction approach is used to account for the electron-electron repulsion. Electron-phonon interaction is accounted for through both deformation potential and piezoelectric field mechanisms. We show that electronic correlations generally reduce intradot and interdot transition rates with respect to corresponding single-electron transitions, but this effect is lessened by external magnetic fields. On the other hand, piezoelectric field scattering is found to become the dominant relaxation mechanism as the number of confined electrons increases. Previous proposals to strongly suppress electron-phonon coupling in properly designed single-electron quantum dots are shown to hold also in multielectron devices. Our results indicate that few-electron orbital degrees of freedom are more stable than single-electron ones

2006 - Few-particle quantum transmitting boundary method: Scattering resonances through a charged 1d quantum dot [Relazione in Atti di Convegno]
A., Bertoni; Goldoni, Guido
abstract

2006 - Full configuration interaction approach to the few-electron problem in artificial atoms [Articolo su rivista]
Rontani, M; Cavazzoni, C; Bellucci, D; Goldoni, Guido
abstract

We present a new high performance configuration interaction code optimally designed for the calculation of the lowest-energy eigenstates of a few electrons in semiconductor quantum dots (also called artificial atoms) in the strong interaction regime. The implementation relies on a single-particle representation, but it is independent of the choice of the single-particle basis and, therefore, of the details of the device and configuration of external fields. Assuming no truncation of the Fock space of Slater determinants generated from the chosen single-particle basis, the code may tackle regimes where Coulomb interaction very effectively mixes many determinants. Typical strongly correlated systems lead to very large diagonalization problems; in our implementation, the secular equation is reduced to its minimal rank by exploiting the symmetry of the effective-mass interacting Hamiltonian, including square total spin. The resulting Hamiltonian is diagonalized via parallel implementation of the Lanczos algorithm. The code gives access to both wave functions and energies of first excited states. Excellent code scalability in a parallel environment is demonstrated; accuracy is tested for the case of up to eight electrons confined in a two-dimensional harmonic trap as the density is progressively diluted up to the Wigner regime, where correlations become dominant. Comparison with previous quantum Monte Carlo simulations in the Wigner regime demonstrates power and flexibility of the method.

2006 - Phonon-induced electron relaxation in correlated quantum dots [Relazione in Atti di Convegno]
J. I., Climente; A., Bertoni; M., Rontani; Goldoni, Guido; Molinari, Elisa
abstract

We study the electron momentum relaxation in single and vertically coupled quantum dots due to coupling with acoustic phonons. We consider both the single-electron and the few-electron regimes, using a Full Configuration Interaction approach to account for the electron-electron repulsion. Our findings suggest that electronic correlations tend to reduce intradot and interdot transition rates. We also show that external magnetic fields acting on properly designed quantum dots may strongly suppress electron-phonon coupling

2006 - Phonon-induced electron relaxation in weakly confined single and coupled quantum dots [Articolo su rivista]
Climente, Ji; Bertoni, A; Goldoni, Guido; Molinari, Elisa
abstract

We investigate charge relaxation rates due to acoustic phonons in weakly confined quantum dot systems, including both deformation potential and piezoelectric field interactions. Single-electron excited states lifetimes are calculated for single and coupled quantum dot structures, both in homonuclear and heteronuclear devices. Piezoelectric field scattering is shown to be the dominant relaxation mechanism in many experimentally relevant situations. On the other hand, we show that appropriate structure design allows to minimize separately deformation potential and piezolectric field interactions, and may bring electron lifetimes in the range of microseconds.

2006 - Quantum phases of correlated electrons in artificial molecules under magnetic fields [Articolo su rivista]
Bellucci, Devis; Rontani, Massimo; Goldoni, Guido; Molinari, Elisa
abstract

We investigate the stability of few-electron quantum phases in vertically coupled quantum dots under a magnetic field of arbitrary strength and direction. The orbital and spin stability diagrams of realistic devices containing up to five electrons, from strong to weak interdot coupling, is determined. Correlation effects and realistic sample geometries are fully taken into account within the full configuration interaction method. In general, the magnetic field drives the system into a strongly correlated regime by modulating the single-particle gaps. In coupled quantum dots different components of the field, either parallel or perpendicular to the tunneling direction, affect single-dot orbitals and tunneling energy, respectively. Therefore the stability of the quantum phases is related to different correlation mechanisms, depending on the field direction. Comparison of exact diagonalization results with simple models allows one to identify the specific role of correlations.

2006 - Response to "Comment on 'Field-controlled suppression of phonon-induced transitions in coupled quantum dots' [Appl. Phys. Lett. 88, 4729 (2006)]" [Articolo su rivista]
Bertoni, A; Rontani, M; Goldoni, Guido; Troiani, F; Molinari, Elisa
abstract

Risposta ad un Comment

2006 - Scattering resonances in 1D coherent transport through a correlated quantum dot: An application of the few-particle quantum transmitting boundary method [Articolo su rivista]
A., Bertoni; Goldoni, Guido
abstract

We present a method for the calculation of thescattering states of a N + 1th-particle coherently interactingwith N correlated particles confined in a nanostructureand placed within an open domain. The method is based on ageneralization of the quantum transmitting boundary method[C. Lent and D. Kirkner, J. App. Phys. 67, 6353 (1990)].The antisymmetry conditions of the N + 1-identical particlescurrent-carrying state results from a proper choice ofthe boundary conditions. As an example which is relevant tocoherent electronics, we apply the method to compute the exacttransmission functions and phases of an electron crossinga 1D quantum dot with zero, one or two bound electrons.

2006 - Spin excitations in few-electrons AlGaAs/GaAs quantum dots probed by inelastic light scattering [Articolo su rivista]
C. P., Garcia; V., Pellegrini; A., Pinczuk; Rontani, Massimo; Goldoni, Guido; Molinari, Elisa; B. S., Dennis; L. N., Pfeiffer; K. W., West
abstract

We present recent studies of electronic excitations in nanofabricated AlGaAs/GaAs semiconductor quantum dots (QDs) by resonant inelastic light scattering. The resonant light scattering spectra are dominated by excitations from parity-allowed inter-shell transitions between Fock-Darwin levels. In QDs With very few electrons the resonant spectra are characterized by distinct charge and spin excitations that reveal the strong impact of both exchange and correlation effects. A sharp inter-shell spin excitation of the triplet spin QD state with four electrons is identified. (c) 2006 Elsevier B.V. All rights reserved.

2006 - Transmission phases and Fano resonsances in carrier transportthrough a few-electron quasi-1D quantum dot [Relazione in Atti di Convegno]
A., Bertoni; Goldoni, Guido
abstract

We use a novel approach, allowing the inclusion of particle-particle correlation in the calculation of current-carrying states in open systems, to compute the exact transmission amplitude of an electron crossinga quasi-1D quantum dot. The electron state antisymmetry is accounted for by means of a proper choice ofthe boundary conditions. Fano resonances, showing the peculiar phase discontinuity, are found.

2005 - Biexcitons in artificial molecules with in-plane magnetic field [Articolo su rivista]
Bellucci, D.; Troiani, Filippo; Goldoni, Guido; Molinari, Elisa
abstract

We theoretically investigate the effect of a magnetic field perpendicular to the tunneling direction on the ground-state properties of biexcitons in coupled quantum dots. The single-particle states are computed by numerically solving the 3D Scrodinger equation. The biexciton states are obtained by means of a configuration-interaction approach, which fully accounts for the intra- and inter-dot Coulomb correlations. We show that the biexciton ground state undergoes nontrivial transitions as a function of the applied magnetic field, which can be traced back to unexpected carrier localizations. (C) 2004 Elsevier B.V. All rights reserved.

2005 - Collective properties of electrons and holes in coupled quantum dots [Relazione in Atti di Convegno]
Goldoni, Guido; Troiani, F.; Rontani, M.; Bellucci, D.; Molinari, Elisa
abstract

We discuss the properties of few electrons and electron-hole pairs confined in coupled semiconductor quantum dots, with emphasis on correlation effects and the role of tunneling. We shall discuss, in particular, exact diagonalization results for biexciton binding energy, electron-hole localization, magnetic-field induced Wigner molecules, and spin ordering.

2005 - Dark-state luminescence of macroatoms at the near field [Articolo su rivista]
U., Hohenester; Goldoni, Guido; Molinari, Elisa
abstract

We theoretically analyze the optical near-field response of a semiconductor macroatom induced by local monolayer fluctuations in the thickness of a semiconductor quantum well, where the large active volume results in a strong enhancement of the light-matter coupling. We find that in the near-field regime bright and dark excitonic states become mixed, opening new channels for the coupling to the electromagnetic field. As a consequence, ultranarrow luminescence lines appear in the simulated two-photon experiments, corresponding to very long lived excitonic states, which undergo Stark shift and Rabi splitting at relatively small field intensities.

2005 - Evidence of correlation in spin excitations of few-electron quantum dots [Articolo su rivista]
Garcia, Cp; Pellegrini, V; Pinczuk, A; Rontani, M; Goldoni, Guido; Molinari, Elisa; Dennis, Bs; Pfeiffer, Ln; West, Kw
abstract

We report inelastic light scattering measurements of spin and charge excitations in nanofabricated AlGaAs/GaAs quantum dots with few electrons. A narrow spin excitation peak is observed and assigned to the intershell triplet-to-singlet monopole mode of dots with four electrons. Configuration-interaction theory provides precise quantitative interpretations that uncover large correlation effects that are comparable to exchange Coulomb interactions.

2005 - Interacting electrons in artificial molecules with magnetic field of arbitrary direction [Articolo su rivista]
Bellucci, D.; Troiani, Filippo; Goldoni, Guido; Molinari, Elisa
abstract

We theoretically investigate the mechanisms for the magnetic field-induced singlet-triplet transition of two electrons in vertically coupled quantum dots, as a function of the field strength and direction. Our numerical approach is based on a real-space description of single-particle states in realistic samples and exact diagonalization of carrier-carrier Coulomb interaction. The three-electron system is also discussed, and we show that the magnetic field drives non-trivial transitions in the ground and excited states. (C) 2004 Elsevier B.V. All rights reserved.

2005 - Magnetic-field-dependent optical properties and interdot correlations in coupled quantum dots [Articolo su rivista]
Bellucci, D; Troiani, F; Goldoni, Guido; Molinari, Elisa
abstract

We theoretically investigate the properties of neutral and charged excitons in vertically coupled quantum dots, as a function of the in-plane magnetic field. The single-particle states are computed by numerically solving the 3D effective-mass equation, while the neutral- and charged-exciton states are obtained by means of a configuration interaction approach. We show that the field determines an enhancement of the interdot correlations, resulting in unexpected carrier localization. The field effect on the excitonic binding energies is also discussed, and is shown to strongly depend on the charging. (c) 2004 Elsevier B.V. All rights reserved.

2005 - Reduced electron relaxation rate in multi-electron quantum dots [Articolo su rivista]
A., Bertoni; M., Rontani; Goldoni, Guido; Molinari, Elisa
abstract

We use a configuration-interaction approach and the Fermi golden rule to investigate electron-phonon interaction in multielectron quantum dots. Lifetimes are computed in the low-density, highly correlated regime. We report numerical evidence that electron-electron interaction generally leads to reduced decay rates of excited electronic states in weakly confined quantum dots, where carrier relaxation is dominated by the interaction with longitudinal acoustic phonons.

2005 - Suppression of acoustic-phonon-induced electron transitions in coupled quantum dots [Articolo su rivista]
Bertoni, A; Rontani, M; Goldoni, Guido; Troiani, F; Molinari, Elisa
abstract

We calculate the longitudinal-acoustic phonon scattering rate for a vertical double quantum dot (DQD) system and show that a strong modulation of the single-electron excited states lifetime can be induced by an external magnetic field. The results are obtained for typical realistic devices using a Fermi golden rule approach and a three-dimensional description of the electronic quantum states. The DQDs considered are characterized by a weak lateral confinement and the longitudinal-phonon scattering represents the dominant source of decoherence, its tunable suppression can be a valuable tool for an experimental measure of electron-states lifetimes and serve as a signature for coherent delocalization of electrons in the DQD.

2005 - The crossover between liquid and solid electron phases in quantumdots: A large-scale configuration-interaction study [Articolo su rivista]
M., Rontani; C., Cavazzoni; Goldoni, Guido
abstract

We study the crossover between liquid and solid electron phases in a two-dimensional harmonic trap as the density is progressivelydiluted. We infer the formation of geometrically ordered phases from charge distributions and pair correlation functionsobtained via a large scale configuration interaction calculation.

2004 - Competing mechanisms for singlet-triplet transition in artificial molecules [Articolo su rivista]
Bellucci, D.; Rontani, Massimo; Troiani, Filippo; Goldoni, Guido; Molinari, Elisa
abstract

We study the magnetic field induced singlet/triplet transition for two electrons in vertically-coupled quantum dots by exact diagonalization of the Coulomb interaction. We identify the different mechanisms occurring in the transition, involving either in-plane correlations or localization in opposite dots, depending on the field direction. Therefore, both spin and orbital degrees of freedom can be manipulated by field strength and direction. The phase diagram of realistic devices is determined.

2004 - Field-controlled suppression of phonon-induced transitions in coupled quantum dots [Articolo su rivista]
Bertoni, Andrea; Rontani, Massimo; Goldoni, Guido; Troiani, Filippo; Molinari, Elisa
abstract

We suggest that order-of-magnitude reduction of the longitudinal-acoustic phonon scattering rate, the dominant decoherence mechanism in quantum dots, can be achieved in coupled structures by the application of an external electric or magnetic field. Modulation of the scattering rate is traced to the relation between the wavelength of the emitted phonon and the length scale of delocalized electron wave functions. Explicit calculations for realistic devices, performed with a Fermi golden rule approach and a fully three-dimensional description of the electronic quantum states, show that the lifetime of specific states can achieve tens of microseconds. Our findings extend the feasibility basis of many proposals for quantum gates based on coupled quantum dots. (C) 2004 American Institute of Physics.

2004 - Neutral and charged electron-hole complexes in artificial molecules: quantum transitions induced by the in-plane magnetic field [Articolo su rivista]
Bellucci, D.; Troiani, Filippo; Goldoni, Guido; Molinari, Elisa
abstract

We theoretically investigate the properties of neutral and charged excitons and of the biexciton in vertically coupled quantum dots, as a function of the in-plane magnetic field B-parallel to. The main effect of the field consists in the suppression of the bonding-antibonding splitting, and in the resulting enhancement of the interdot correlations. As a consequence, the excitons form with the additional carrier a bound or an unbound complex depending on the sign of the charging, whereas the biexciton undergoes a transition between different quantum states with increasing B-parallel to. The discussed behaviors and transitions show up in the field dependence of experimentally accessible quantities, such as the charged-exciton and biexciton binding energies.

2004 - Optical near-field mapping of excitons and biexcitons in naturally occurring semiconductor quantum dots [Articolo su rivista]
U., Hohenester; Goldoni, Guido; Molinari, Elisa
abstract

We calculate the near-field optical spectra of excitons and biexcitons in semiconductor quantum dots naturally occurring at interface fluctuations in GaAs-based quantum wells, using a nonlocal description of the response function to a spatially modulated electromagnetic field. The relative intensity of the lowest, far-field forbidden excitonic states is predicted; the spatial extension of the ground biexciton state is found, in agreement with recently published experiments.

2004 - Spin-spin interaction in artificial molecules with in-plane magnetic field [Articolo su rivista]
Bellucci, D; Rontania, M; Goldoni, Guido; Troiani, F; Molinari, Elisa
abstract

We investigate theoretically the spin-spin interaction of two-electrons in vertically coupled QDs as a function of the angle between magnetic field and growth axis. Our numerical approach is based on a real-space description of single-particle states in realistic samples and exact diagonalization of carrier-carrier Coulomb interaction. In particular, the effect of the in-plane field component on tunneling and, therefore, spin-spin interaction will be discussed; the singlet-triplet phase diagram as a function of the field strength and direction is drawn.

2003 - Few-electron liquid and solid phases in artificial molecules at high magnetic field [Capitolo/Saggio]
M., Rontani; Goldoni, Guido; Molinari, Elisa
abstract

Coupled semiconductor quantum dots form artificial molecules where relevant energy scales controlling the interacting ground state can be easily tuned. By applying an external magnetic field it is possible to drive the system from a weak to a strong correlation regime where eventually electrons localize in space in an ordered manner reminiscent of the two-dimensional Wigner crystal. We explore the phase diagram of such "Wigner molecules" analyzing the angular correlation function obtained by the Configuration Interaction solution of the full interacting Hamiltonian. Focus is on the role of tunneling in stabilizing different ground states.

2002 - Optical properties of organic materials: from single molecules to solid state [Capitolo/Saggio]
Ruini, Alice; Mj, Caldas; G., Bussi; A., Ferretti; Bm, Silva; Goldoni, Guido; Molinari, Elisa
abstract

Light-matter interactions in organic conjugated materials have always been of great interest due to their rich abosorption and emission acitivity, and their versatility in, or near, the visible range. The complexity of the molecular structure itself, particularly when assembled in solid-state films or crystals, has precluded theoretical srudies based on realistic models. We here present a study of the behavior of organic structures composed of thienyl units (SC4H2), in two different assemblages: finite oligothiophenes, including one functionalised unit (O2SC4H2), and in infinite polythiophene chains, assembled three-dimenasionally in the realistic herringbone crystalline packing. To do that, we use two different approaches to obtain the excited states of the system, each appropriate to the system under study. We find that subtle symmetry properties are ,in both cases, responsible for dramatic effects in emissive efficiency.

2002 - Raman Signatures of Classical and Quantum Phases in Coupled Dots: a Theoretical Prediction [Articolo su rivista]
Rontani, Massimo; Goldoni, Guido; Manghi, Franca; Molinari, Elisa
abstract

We study electron molecules in realistic vertically coupled quantum dots in a strong magnetic field. Computing the energy spectrum, pair correlation functions, and dynamical form factor as a function of inter-dot coupling via diagonalization of the many-body Hamiltonian, we identify structural transitions between different phases, some of which do not have a classical counterpart. The calculated Raman cross-section shows how such phases can be experimentally singled out.

2002 - The effect of dielectric polarization-induced surface states on many-body configurations in a quantum dot [Articolo su rivista]
Orlandi, A; Goldoni, Guido; Manghi, Franca; Molinari, Elisa
abstract

We compute the exact lowest energy states and addition spectra for N interacting carriers in semiconductor quantum dots (QDs) embedded in low-dielectric-constant matrices. In particular we focus on the transition between the low dielectric mismatch regime, where single-particle states are confined inside the QD, and the large mismatch regime, where surface localized states may be induced by the dielectric potential outside the QD. In this transition, many-body ground-state reconstructions occur which give rise to strong deviations from the regular shell-filling behaviour dictated by Hand's rules. Our calculated addition spectra indicate that such effects could be monitored in transport experiments.

2001 - Local absorption spectra of single and coupled semiconductor quantum dots [Articolo su rivista]
Simserides, Cd; Hohenester, U; Goldoni, Guido; Molinari, Elisa
abstract

We study theoretically the local absorption spectra of single and double semiconductor quantum dots (QDs). in the linear regime. The three-dimensional confinement leads to an enhancement of the Coulomb correlations, while the spectra depend crucially on the size of the 'local' probe. We show that because of such Coulomb correlations the intensity of certain optical peaks as a function of the resolution can exhibit an unexpected non-monotonic behavior for spatial resolutions comparable with the excitonic Bohr radius. We finally discuss the optical near-field properties of coupled QDs for different coupling strengths. (C) 2001 Elsevier Science B.V. All rights reserved.

2001 - Local optical absorption by confined excitons in single and coupled quantum dots [Articolo su rivista]
Simserides, Cd; Hohenester, U; Goldoni, Guido; Molinari, Elisa
abstract

We investigate optical near-field spectra of single and coupled semiconductor quantum dots. An enhanced role for the Coulomb correlations is predicted, and it is shown that the spectra depend crucially on the spatial resolution of the local probe. The intensity of certain optical peaks as a function of the resolution exhibits an unexpected non-monotonic behavior, which is identified as a fingerprint of Coulomb interactions in zero-dimensional nanostructures.

2001 - Optimal design and quantum limit for second harmonic generation in semiconductor heterostructures [Articolo su rivista]
Goldoni, Guido
abstract

The optimal design for infrared second harmonic generation (SHG) is determined for a GaAs-based quantum device using a recently developed genetic approach. Both the compositional parameters and electric field are simultaneously optimized, and the quantum limit for SHG, set by the trade-off between large dipole moments (favoring electron delocalization) and large overlaps (favoring electron localization), is determined. Optimal devices are generally obtained that have an asymmetric double quantum well shape with narrow barriers and a graded region sideways to the largest well. An electric field is not found to lead to improved SHG if the compositional parameters are optimized.

2001 - Single-electron charging in quantum dots with large dielectric mismatch [Articolo su rivista]
A., Orlandi; M., Rontani; Goldoni, Guido; Manghi, Franca; Molinari, Elisa
abstract

Semiconductor quantum dots characterized by a strong dielectric mismatch with their environment arestudied theoretically through direct diagonalization of the many-body Hamiltonian. The enhancement of theelectron-electron Coulomb interaction, arising from polarization effects, is found to induce a strong increase inaddition energies with increasing dielectric mismatch. For large dielectric mismatch, the excited many-bodystates can undergo reconstructions as the dot is filled with carriers even in the absence of external magneticfields.

2001 - Tailoring of Light Emission Properties of Functionalised Oligothiophenes [Articolo su rivista]
M. J., Caldas; E., Pettenati; Goldoni, Guido; Molinari, Elisa
abstract

We investigate theoretically the light emission properties of short oligothiophenes containing a thienyl-S,S-dioxide moiety, which have recently been shown to exhibit strong photoluminescence efficiency and tunability. We find that the dioxide substitution tends to increase the torsion angle between rings and strongly affects the states at the optical band edges, in a way depending on the position of the substituted ring. We have identified a low-energy transition which is optically active only when the dioxide unit is inserted between two thiophene rings, as a result of increased inter-ring torsional angle also in the excited state. With the dioxide unit in a terminal position, planarity is favored in the excited state, and the transition is optically inactive. For short oligomers, this can lead to nonradiative decay quenching the luminescence. (C) 2001 American Institute of Physics.

2000 - Enhancement of Coulomb interactions in semiconductor nanostructures by dielectric confinement [Articolo su rivista]
Goldoni, Guido; F., Rossi; A., Orlandi; M., Rontani; Manghi, Franca; Molinari, Elisa
abstract

We present a theoretical analysis of the effect of dielectric confinement on the Coulomb interaction in dielectrically modulated quantum structures. We discuss the implications of the strong enhancement of the electron-hole and electron-electron coupling for two specific examples: (i) GaAs-based quantum wires with remote oxide barriers, where combined quantum and dielectric confinements are predicted to lead to room temperature exciton binding, and (ii) semiconductor quantum dots in colloidal environments, when the addition spectra are predicted to depend on the dielectric mismatch. (C) 2000 Elsevier Science B.V. All rights reserved.

2000 - Local absorption spectra of artificial atoms and molecules [Articolo su rivista]
Simserides, C.; Hohenester, U.; Goldoni, Guido; Molinari, Elisa
abstract

We investigate theoretically the spatial dependence of the linear absorption spectra of single and coupledsemiconductor quantum dots, where the strong three-dimensional quantum confinement leads to an overallenhancement of Coulomb interaction and, in turn, to a pronounced renormalization of the excitonic properties.We show that—because of such Coulomb correlations and the spatial interference of the exciton wavefunctions—unexpected spectral features appear whose intensity depends on spatial resolution in a highlynonmonotonic way when the spatial resolution is comparable with the excitonic Bohr radius. We finallydiscuss how the optical near-field properties of double quantum dots are affected by their coupling.

2000 - Local optical spectroscopy of semiconductor nanostructures in the linear regime [Articolo su rivista]
Mauritz, O; Goldoni, Guido; Molinari, Elisa; Rossi, F.
abstract

We present a theoretical approach to calculate the local absorption spectrum of excitons confined in a semiconductor nanostructure. Using the density-matrix formalism, we derive a microscopic expression for the nonlocal susceptibility, both in the linear and nonlinear regimes, which includes a three-dimensional description of electronic quantum states and their Coulomb interaction. The knowledge of the nonlocal susceptibility allows us to calculate a properly defined local absorbed power, which depends on the electromagnetic field distribution. We report on explicit calculations of the local linear response of excitons confined in single and coupled T-shaped quantum wires with realistic geometry and composition. We show that significant interference effects in the interacting electron-hole wave function induce new features in the space-resolved optical spectra, particularly in coupled nanostructures. When the spatial extension of the electromagnetic held is comparable to the exciton Bohr radius, Coulomb effects on the local spectra must be taken into account for a correct assignment of the observed features.

2000 - Nanoscale compositional fluctuations in multiple InGaAs/GaAs quantum wires [Articolo su rivista]
M., Catalano; A., Taurino; M., Lomascolo; L., Vasanelli; M., De Giorgi; A., Passaseo; R., Rinaldi; R., Cingolani; O., Mauritz; Goldoni, Guido; F., Rossi; Molinari, Elisa; P., Crozier
abstract

An accurate analysis of nanoscale compositional fluctuations in InGaAs/GaAs quantum wires grown by metalorganic chemical vapor deposition on V-grooved substrates was performed by means of high-spatial-resolution transmission electron microscopy techniques. Small In fluctuations (2%-3% excess indium), spatially localized over approximately 5 nm, were detected and related to changes in the photoluminescence and photoluminescence excitation spectra. (C) 2000 American Institute of Physics. [S0021-8979(00)07905-6].

2000 - Optimization of semiconductor quantum devices by evolutionary search [Articolo su rivista]
Goldoni, Guido; F., Rossi
abstract

A novel simulation strategy is proposed for searching for semiconductor quantum devices that are optimized with respect to required performances. Based on evolutionary programming, a technique that implements the paradigm of genetic algorithms in more-complex data structures than strings of bits, the proposed algorithm is able to deal with quantum devices with preset nontrivial constraints (e.g., transition energies, geometric requirements). Therefore our approach allows for automatic design, thus avoiding costly by-hand optimizations. We demonstrate the advantages of the proposed algorithm through a relevant and nontrivial application, the optimization of a second-harmonic-generation device working in resonance conditions. (C) 2000 Optical Society of America OCIS codes: 160.6000, 230.5590, 040.4200, 040.3060, 140.5960.

1999 - Double photoionization of He near a polarizable surface [Articolo su rivista]
Goldoni, G.
abstract

We calculate the differential cross section of the direct double photoionization of He physisorbed on a polarizable surface. By including the influence of the surface potential in the correlated two-electron final-state wave function, we show that the differential cross section carries detailed information on the electronic correlations at the surface. In particular, photoemission along opposite directions, which is prohibited in the free space, is allowed if the surface potential is long ranged. © 1999 The American Physical Society.

1999 - Double photo-ionization of he near a polarizable surfaces [Articolo su rivista]
Goldoni, Guido
abstract

We calculate the differential cross section of the direct double photoionization of He physisorbed on apolarizable surface. By including the influence of the surface potential in the correlated two-electron final-statewave function, we show that the differential cross section carries detailed information on the electroniccorrelations at the surface. In particular, photoemission along opposite directions, which is prohibited in thefree space, is allowed if the surface potential is long ranged.

1999 - Local optical spectroscopy in quantum confined systems: a theoretical description [Articolo su rivista]
Mauritz, O.; Goldoni, Guido; Rossi, F.; Molinari, Elisa
abstract

length scale comparable with the extension of the relevant quantum states. A general formulation,including Coulomb correlation, is derived within the density-matrix formalism and applied to theprototypical case of coupled quantum wires. The results show that excitonic effects may have acrucial impact on the local absorption, with implications for the spatial resolution and the interpretationof near-field optical spectra

1999 - Strong exciton binding in hybrid GaAs-based nanostructures [Articolo su rivista]
Goldoni, Guido; Rossi, F; Molinari, Elisa
abstract

We propose a new type of hybrid systems formed by conventional semi-conductor nanostructures with the addition of remote insulating layers, where electron-hole interaction is enhanced by combining quantum and dielectric confinement over different length scales. Due to the polarization charges induced by dielectric mismatch at the semiconductor/insulator interfaces, the exciton binding energy can be strongly enhanced. By a novel theoretical scheme, we show that, for realistic structures based on conventional III-V quantum wires, such remote dielectric confinement allows exciton binding at room temperature. (C) 1999 Elsevier Science B.V. All rights reserved.

1999 - Theory of excitonic confinement in semiconductor quantum wires [Articolo su rivista]
Rossi, F; Goldoni, Guido; Mauritz, O; Molinari, Elisa
abstract

We review our theoretical approach to the optical response of low-dimensional semiconductor structures. The method is based on the density-matrix formalism and can treat low-density (excitonic) and high-density (gain) regimes on the same footing while retaining the full complexity of realistic nanostructures. We discuss in particular its generalization for studying the combined effects of dielectric and quantum confinement. as well as novel developments aimed at the analysis of local absorption spectra. We examine the main effects of electron-hole Coulomb correlation on the optical spectra of semiconductor quantum wires, where it determines the suppression of band-edge singularities and the peculiar scaling properties of excitonic binding and non-linearities. On the basis of our recent results on different types of nanostructure, we present a critical discussion of possible strategies for tailoring electron-hole Coulomb interaction, and predicting its influence on near-field spectra.

1998 - Strong exciton binding in quantum structures through remote dielectric confinement [Articolo su rivista]
Goldoni, Guido; F., Rossi; Molinari, Elisa
abstract

We propose-a new type of hybrid systems formed by conventional semiconductor nanostructures with the addition of remote insulating layers,where the electron-hole interaction is enhanced by combining quantum and dielectric confinement over different length scales. Because of the polarization charges induced by the dielectric mismatch at the semicondcutor/insulator interfaces, we show that the exciton binding energy can be more than doubled. For conventional m-V quantum wires such remote dielectric confinement allows exciton binding at room temperature.

1997 - Band structure and optical anisotropy in V-shaped and T-shaped semiconductor quantum wires [Articolo su rivista]
Goldoni, Guido; Rossi, F.; Molinari, Elisa; Fasolino, Annalisa
abstract

We present a theoretical investigation of the electronic and optical properties of V- and T-shaped quantum wires. Valence-band mixing as well as realistic sample geometries are fully included through an accurate and efficient approach that is described here in detail. We investigate the resulting valence-band structure, which shows some significant peculiarities, such as an anomalously large spin splitting in the lowest heavy-hole subband of T-shaped wires. For both classes of wires we obtain good agreement between calculated optical absorption and recent experimental spectra, and we demonstrate that the analysis of optical anisotropy can be used as an effective tool to extract information on valence states, which is usually very difficult to obtain otherwise.

1997 - Classical molecules in two dimensions [Articolo su rivista]
Peeters, F. M.; Partoens, B.; Schweigert, V. A.; Goldoni, G.
abstract

The ground-state configurations and spectrum of two parallel two-dimensional classical atoms are obtained as a function of the inter-atomic distance (d). The classical particles are confined by parabolic potentials and repel each other through a Coulomb potential. As a function of d we find several configurational transitions which are of first or second order. For first-(second-) order transitions the first (second) derivative of the energy with respect to d is discontinuous, the radial position of the particles changes discontinuously (continuously) and the frequency of the eigenmodes exhibit a jump (softening of a mode). In the limit of an infinite number of electrons the Wigner bilayer system is recovered which moves through five different stable crystalline phases as a function of d. For unequal strength of parabolic confinement we find that the number of configurational transitions increases. Published by Elsevier Science B.V.

1997 - Excitonic effects in quantum wires [Articolo su rivista]
Goldoni, Guido; Rossi, F; Molinari, Elisa
abstract

We review the effects of Coulomb correlation on the linear and nonlinear optical properties of semiconductor quantum wires, with emphasis on recent results for the bound excitonic states. Our theoretical approach is based on generalized semiconductor Bloch equations, and allows full three-dimensional multisubband description of electron-hole correlation for arbitrary confinement profiles. In particular, we consider V- and T-shaped structures for which significant experimental advances were obtained recently. Above band gap, a very general result obtained by this approach is that electron-hole Coulomb correlation removes the inverse-square-root single-particle singularity in the optical spectra at band edge, in agreement with previous reports from purely one-dimensional models. Strong correlation effects on transitions in the continuum are found to persist also at high densities of photoexcited carriers. Below band gap, we find that the same potential- (Coulomb) to kinetic-energy ratio holds for quite different wire cross-sections and compositions. As a consequence, we identify a shape-and barrier-independent parameter that governs a universal scaling law for exciton binding energy with size. Previous indications that the shape of the wire cross-section may have important effects on exciton binding are discussed in the light of the present results.

1997 - Quantum interference in nanometric devices: Ballistic transport across arrays of T-shaped quantum wires [Articolo su rivista]
Goldoni, Guido; F., Rossi; Molinari, Elisa
abstract

We propose that recently realized T-shaped semiconductor quantum wires (T wires) could be exploited as three-terminal quantum interference devices. T wires are formed by intersecting two quantum wells (QWs). By use of a scattering matrix approach and Landauer-Buttiker theory, we calculate the conductance for ballistic transport in the parent QWs and across the wire region as a function of the injection energy. We show that different conductance profiles can be selected by tailoring the widths of the QWs and/or by combining more wires on the scale of the Fermi wavelength. Finally, we discuss the possibility of obtaining spin-dependent conductance of ballistic holes in the same structures.

1997 - Shape-independent scaling of excitonic confinement in realistic quantum wires [Articolo su rivista]
Rossi, F.; Goldoni, Guido; Molinari, Elisa
abstract

The scaling of exciton binding energy in semiconductor quantum wires is investigated theoreticallythrough a nonvariational, fully three-dimensional approach for a wide set of realistic state-of-the-artstructures. We find that in the strong confinement limit the same potential-to-kinetic energy ratio holdsfor quite different wire cross sections and compositions. As a consequence, a universal (shape- andcomposition-independent) parameter can be identified that governs the scaling of the binding energywith size. Previous indications that the shape of the wire cross section may have important effects onexciton binding are discussed in the light of the present results

1997 - Wigner crystallization in quantum electron bilayers [Articolo su rivista]
Goldoni, Guido; Peeters, Fm
abstract

The phase diagram of quantum electron bilayers in zero magnetic field is obtained using density functional theory. For large electron densities the system is in the liquid phase, while for smaller densities the liquid may freeze (Wigner crystallization) into four different crystalline phases; the lattice symmetry and the critical density depend on the interlayer distance. The phase boundaries between different Wigner crystals consist of both first- and second-order transitions, depending on the phases involved, and join the freezing curve at three different triple points.

1996 - Stability and dynamical properties of a double-layer Wigner crystal in two dimensions [Articolo su rivista]
Goldoni, G.; Schweigert, V.; Peeters, F. M.
abstract

We investigate the zero-temperature phase diagram and melting transition of a two-dimensional Wigner crystal (WC) of classical Coulombic particles in a double-layer structure. In contrast with the single-layer WC, we find that five different crystalline phases are stable in different ranges of inter-layer distance/particle density. Phase boundaries consist of both continuous and discontinuous transitions. From the phonon frequencies of the system we evaluate the melting temperature of the WC. We suggest the possibility of structural phase transitions as a function of temperature.

1996 - Stability, dynamical properties, and melting of a bi-layer Wigner crystal [Articolo su rivista]
Goldoni, Guido; F. M., Peeters
abstract

We investigate the stability, dynamical properties, and melting of a two-dimensional ~2D! Wigner crystal~WC! of classical Coulombic particles in a bilayer structure. Compared to the single-layer WC, this systemshows a rich phase diagram. Five different crystalline phases are stable; the energetically favored structure canbe tuned by changing either the interlayer distance or the particle density. Phase boundaries consist of bothcontinuous and discontinuous transitions. We calculated the phonon excitations of the system within theharmonic approximation and we evaluated the melting temperature of the bilayer WC by use of a modifiedLindemann criterion, appropriate to 2D systems. We minimized the harmonic free energy of the system withrespect to the lattice geometry at different values of temperature/interlayer distance and we found notemperature-induced structural phase transition.

1996 - Valence band spectroscopy in V-grooved quantum wires [Articolo su rivista]
Goldoni, Guido; F., Rossi; Molinari, Elisa; A., Fasolino; R., Rinaldi; R., Cingolani
abstract

We present a combined theoretical and experimental study of the anisotropy in the optical absorption of V-shaped quantum wires. By means of realistic band structure calculations for these structures, we show that detailed information on the heavy- and light-hole states can be singled out from the anisotropy spectra independently of the electron confinement, thus allowing accurate valence band spectroscopy.

1995 - Hole subbands and effective masses in p-doped [113]-grown heterojunctions [Articolo su rivista]
GOLDONI, Guido; F. M., Peeters
abstract

We calculate the hole band structure and effective masses in a two-dimensional hole gas (2DHG) in p-doped [113]-grown heterojunctions. A Hamiltonian for the 2DHG is derived which is formally similar to the Luttinger Hamiltonian which describes the hole gas in conventional [001] heterojunctions. We use a limited analytical basis set of uncoupled heavy-hole and light-hole states which allow us to obtain a qualitative description of the differences between [113]- and [001]-grown heterojunctions. We avoid the self-consistent calculation of the electrostatic confining potential by use of an analytical model which proved to be very accurate. In addition to numerical calculations, we derive analytical expressions of the hole subbands which are valid at small in-plane wave vectors. We compute the classical effective masses of the 2DHG as a function of the charge density and we compare with those obtained for conventional [001]-grown structures. We found that the latter are systematically larger in a broad range of carrier concentrations.

1995 - MAGNETIC LEVELS AND INTERBAND ABSORPTION IN QUANTUM-WELL WIRES [Articolo su rivista]
Goldoni, Guido; Fasolino, A.
abstract

We present a theoretical study of the conduction and valence magnetic band structure of Quantum Well Wires in high magnetic fields. We show how the transition from one-dimensional subbands to edge states and to Landau levels can be followed, due to the valence band mixing, by optical experiments by choosing the appropriate, linear or circular, polarization of the light, yielding information on the one-dimensional confinement.

1995 - Magneto-optical study of quantum well electronic structure using disorder-induced resonant acoustic-phonon Raman scattering [Articolo su rivista]
Goldoni, Guido; T., Ruf; V. F., Sapega; A., Fainstein; M., Cardona
abstract

We study the electronic structure of a (100/103)-Å GaAs/Al0.35Ga0.65As multiple quantum-well sample using the resonant enhancement of disorder-induced continuous Raman emission by acoustic phonons at interband magneto-optical transitions between Landau levels. Calculations are compared to fan plots of excitation energy vs resonance magnetic field and material parameters are determined. We find a significant difference between the electron effective mass for confinement (me⊥=0.068m0) and that which describes Landau quantization (me∥=0.073m0). Both masses are enhanced as compared to the bulk value (me=0.0665m0). Our results confirm theoretical predictions on the influence of conduction band nonparabolicity and anisotropy on effective masses in quantum wells.

1995 - Real-space approach to the multi-component envelope function problem in semiconductor heterostructures [Articolo su rivista]
Goldoni, Guido; A., Fasolino
abstract

We describe a real-space numerical method for the solution of the multicomponent-envelope-function problem in semiconductor heterostructures. The method, based on a shooting technique, provides, with a very modest computational effort, an exact solution for arbitrarily shaped one-dimensional confining potentials, including high in-plane magnetic fields. Boundary conditions at interfaces are automatically taken into account. We apply our method to the 4×4 Luttinger Hamiltonian and indicate how larger k⋅p Hamiltonians can be implemented. To demonstrate the flexibility of the method, we show the calculated hole subbands and envelope functions in a GaAs-AlxGa1-xAs quantum well with a magnetic field parallel to the interfaces and with an applied bias along the growth direction.

1995 - Valence band structure, edge states and interband absorption in quantum well wires in high magnetic fields [Articolo su rivista]
Goldoni, Guido; A., Fasolino
abstract

We present a theoretical study of the magnetic band structure of conduction and valence states in quantum-well wires in high magnetic fields. We show that hole mixing results in a very complex behavior of valence edge states with respect to conduction states, a fact which is likely to be important in magnetotransport in the quantum Hall regime. We show how the transition from one-dimensional subbands to edge states and to Landau levels can be followed by optical experiments by choosing the appropriate, linear, or circular, polarization of the light, yielding information on the one-dimensional confinement.

1994 - CURRENT-INDUCED SPIN SEPARATION IN P-DOPED ASYMMETRIC DOUBLE-QUANTUM WELLS [Articolo su rivista]
Goldoni, Guido; Fasolino, A.
abstract

In asymmetric double quantum wells, the ''spin'' splitting of the hole subbands at finite parallel momentum, due to the asymmetric potential, is related to a ''spin'' dependent delocalization of the wavefunction over the two wells. We show that the application of an electric field parallel to the interfaces in a p-doped asymmetric double quantum well can exploit such spin-dependent tunneling probability and separate carriers of opposite spins into the two wells, yielding a macroscopic magnetic moment of different magnitude and either the same or opposite direction in the two wells.

1994 - THEORETICAL-ANALYSIS OF RESONANT MAGNETOTUNNELING SPECTROSCOPY OF HOLES [Articolo su rivista]
Goldoni, Guido; Fasolino, Annalisa
abstract

We calculate the hole energy band structure in GaAs-AlAs Quantum Wells (QWs) in high in-plane magnetic fields and simulate Resonant Magnetotunneling Spectroscopy (RMS) experiments, recently proposed as a probe of the hole energy dispersion. We find that the high magnetic fields used in RMS result in small modifications of the QW hole subbands in thin QWs. However, for wide QWs or very large magnetic fields, the field strongly affects the hole dispersion by changing the heavy hole-light hole mixing. We give a simple expression to evaluate to which extent the mixing is modified as a function of the QW width and field strength.

1993 - Erratum: "Spin" splitting in asymmetric double quantum wells: A mechanism for "spin"-dependent hole delocalization (Physical Review Letters (1993) 70, 13, (2051)) [Articolo su rivista]
Goldoni, G.; Fasolino, A.
abstract

1993 - HOLE STATES IN QUANTUM-WELLS IN HIGH INPLANE MAGNETIC-FIELDS - IMPLICATIONS FOR RESONANT MAGNETOTUNNELING SPECTROSCOPY [Articolo su rivista]
Goldoni, Guido; Fasolino, Annalisa
abstract

We calculate the hole energy-level structure in quantum wells (QW's) in high in-plane magnetic fields. On the basis of our results, we analyze the conditions of validity of resonant magnetotunneling spectroscopy (RMS) as a probe of the unperturbed hole subband dispersion. The high magnetic field used in RMS does net change appreciably, apart from a modest ''spin'' splitting, the hole subbands for thin wells, but have a non-negligible effect on the dispersion for thicker wells. We use a simplified analytical model to evaluate the field-related heavy-hole-light-hole mixing terms and we give their explicit dependence on the field strength and the width of the QW. This allows us to estimate the range of validity of RMS as a probe of hole subbands.

1992 - Spin splitting in asymmetric double quantum wells: a mechanism for spin-dependent hole delocalization [Articolo su rivista]
Goldoni, Guido; A., Fasolino
abstract

We show how the breakdown of Kramers degeneracy for spin-orbit coupled bands in asymmetric double quantum wells may give rise to a new mechanism for tunneling which does not require resonant conditions. The most interesting aspect of this finding is that, in this case, there is a ‘‘spin’’-dependent delocalization of the wave function, leading to preferential tunneling of one ‘‘spin’’ state. We relate this effect to the symmetry properties of the wave functions of both single and asymmetric double quantum wells, and provide a qualitative explanation in terms of perturbation theory.

1991 - Band-structure effects on coherent focusing in a two dimensional hole gas [Articolo su rivista]
Goldoni, Guido; A., Fasolino
abstract

e investigate the coherent focusing in a weak magnetic field of a two-dimensional hole gas realized in a semiconductor heterostructure. By use of a simple model, we study how the coherence effects displayed by ballistic conduction electrons are modified if warped and nonparabolic bands are taken into account. We

1989 - Surface effects in the electronic properties of YBa2Cu3O7 [Articolo su rivista]
CALANDRA BUONAURA, Carlo; Goldoni, Guido; Manghi, Franca; Magri, Rita
abstract

The electronic structure of basal plane surfaces of the high-T, superconducting material YBa,Cu,O, has been determined using a tint-binding Hamilto~~n derived from a first-p~nciple calculation of the bulk phase. The surface local density of states has been obtained for four different surface configurations, which differ for the yttrium and oxygen content. The results show that the presence and state of the surface have a strong influence on the distribution of single particle states. The nature of the surface induced modifications is detailed and the relevance of the results to the understanding of experimental data is shortly discussed.

1989 - Surface effects on the electronic properties of YBa2Cu3O7 [Articolo su rivista]
C., Calandra; Manghi, Franca; Minerva, Tommaso; Goldoni, Guido
abstract

We present the results of a theoretical study of the modifications induced by the surface on the electronic structure of YBa2Cu3O7. Basal plane surfaces terminated with Ba and with Cu-O planes are considered. The calculated local densities of states show that the main structures of the bulk density of states may be drastically modified, specially near the Fermi energy, and new surface derived features appear in the spectrum. Significant changes occur also in the occupancies of the atoms near the surface.