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ANDREA BERTONI

DIPENDENTE ALTRO ENTE DI RICERCA
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 - Quantum estimation and remote charge sensing with a hole-spin qubit in silicon [Articolo su rivista]
Forghieri, Gaia; Secchi, Andrea; Bertoni, Andrea; Bordone, Paolo; Troiani, Filippo
abstract

Hole-spin qubits in semiconductors represent a mature platform for quantum technological applications. Here we consider their use as quantum sensors, and specifically for inferring the presence and estimating the distance from the qubit of a remote charge. Different approaches are considered, based on the use of single or double quantum dots, ground and out-of-equilibrium states, Rabi and Ramsey measurements, and comparatively analyzed by means of the discrimination probability, and of the classical and quantum Fisher information. Detailed quantitative aspects result from the multiband character of the hole states, which we account for by means of the Luttinger-Kohn Hamiltonian. Furthermore, general conclusions can be drawn on the relative efficiency of the above options, and analytical expressions are derived for the Fisher information of a generic qubit within the Rabi and Ramsey schemes.

2020 - Two-electron selective coupling in an edge-state based conditional phase shifter [Articolo su rivista]
Bellentani, Laura; Forghieri, Gaia; Bordone, Paolo; Bertoni, Andrea
abstract

We investigate the effect of long-range Coulomb interaction on the two-electron scattering in the integer quantum Hall regime at bulk filling factor two.We compute the dynamics of the exact two-particle wave function by means of a parallel version of the split-step Fourier method in a 2D potential background reproducing the effect of depleting gates in a realistic heterostructure, with the charge carrier represented by a localized wave packet of edge states.We compare the spatial shift induced by Coulomb repulsion in the final two-electron wave function for two indistinguishable electrons initialized in different configurations according to their Landau index and analyze their bunching probability and the effect of screening. We finally prove the feasibility of this device as a two-qubit conditional phase shifter able to generate controlled entanglement from product states.

2019 - A Proposal for Evading the Measurement Uncertainty in Classical and Quantum Computing: Application to a Resonant Tunneling Diode and a Mach-Zehnder Interferometer [Articolo su rivista]
Pandey, Devashish; Bellentani, Laura; Villani, Matteo; Albareda, Guillermo; Bordone, Paolo; Bertoni, Andrea; Oriols, Xavier
abstract

Measuring properties of quantum systems is governed by a stochastic (collapse or state-reduction) law that unavoidably yields an uncertainty (variance) associated with the corresponding mean values. This non-classical source of uncertainty is known to be manifested as noise in the electrical current of nanoscale electron devices, and hence it can flaw the good performance of more complex quantum gates. We propose a protocol to alleviate this quantum uncertainty that consists of (i) redesigning the device to accommodate a large number of electrons inside the active region, either by enlarging the lateral or longitudinal areas of the device and (ii) re-normalizing the total current to the number of electrons. How the above two steps can be accommodated using the present semiconductor technology has been discussed and numerically studied for a resonant tunneling diode and a Mach-Zehnder interferometer, for classical and quantum computations, respectively. It is shown that the resulting protocol formally resembles the so-called collective measurements, although, its practical implementation is substantially different.

2019 - Coulomb and exchange interaction effects on the exact two-electron dynamics in the Hong-Ou-Mandel interferometer based on Hall edge states [Articolo su rivista]
Bellentani, Laura; Bordone, Paolo; Oriols, Xavier; Bertoni, Andrea
abstract

The electronic Hong-Ou-Mandel interferometer in the integer quantum Hall regime is an ideal system to probe the building up of quantum correlations between charge carriers and it has been proposed as a viable platform for quantum computing gates. Using a parallel implementation of the split-step Fourier method, we simulated the antibunching of two interacting fermionic wave packets impinging on a quantum point contact. Numerical results of the exact approach are compared with a simplified theoretical model based on one-dimensional scattering formalism. We show that, for strongly localized wave packets in a full-scale geometry, the Coulomb repulsion dominates over the exchange energy, this effect being strongly dependent on the energy broadening of the particles. We define analytically the spatial entanglement between the two regions of the quantum point contact, and obtain quantitatively its entanglement-generation capabilities.

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.

2019 - Quantum computing with quantum-Hall edge state interferometry [Articolo su rivista]
Bordone, Paolo; Bellentani, Laura; Bertoni, Andrea
abstract

Electron interferometers based on Hall edge states (ESs) proved to be robust demonstrators of the coherent quantum dynamics of carriers. Several proposals to expose their capability to build and control quantum entanglement and to exploit them as building block for quantum computing devices has been presented. Here, we review the time-dependent numerical modeling of Hall interferometers operating at the single-carrier level at integer filling factor (FF). By defining the qubit state either as the spatial localization (at FF 1) or the Landau index (at FF 2) of a single carrier propagating in the ES, we show how a generic one-qubit rotation can be realized. By a proper design of the two-dimensional electron gas potential landscape, an entangling two-qubit gate can be implemented by exploiting Coulomb interaction, thus realizing a universal set of quantum gates. We also assess how the shape of the edge confining potential affects the visibility of the quantum transformations.

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 - Dynamics and Hall-edge-state mixing of localized electrons in a two-channel Mach-Zehnder interferometer [Articolo su rivista]
Bellentani, Laura; Beggi, Andrea; Bordone, Paolo; Bertoni, Andrea
abstract

We present a numerical study of a multichannel electronicMach-Zehnder interferometer, based onmagnetically driven noninteracting edge states. The electron path is defined by a full-scale potential landscape on the twodimensional electron gas at filling factor 2, assuming initially only the first Landau level as filled.We tailor the two beamsplitters with 50% interchannelmixing and measure Aharonov-Bohm oscillations in the transmission probability of the second channel.We perform time-dependent simulations by solving the electron Schrödinger equation through a parallel implementation of the split-step Fourier method, and we describe the charge-carrier wave function as a Gaussian wave packet of edge states.We finally develop a simplified theoretical model to explain the features observed in the transmission probability, and we propose possible strategies to optimize gate performances.

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 - Dynamics and control of edge states in laser-driven graphene nanoribbons [Articolo su rivista]
Puviani, Matteo; Manghi, F.; Bertoni, A.
abstract

An intense laser field in the high-frequency regime drives carriers in graphene nanoribbons (GNRs) out of equilibrium and creates topologically protected edge states. Using Floquet theory on driven GNRs, we calculate the time evolution of local excitations of these edge states and show that they exhibit a robust dynamics also in the presence of very localized lattice defects (atomic vacancies), which is characteristic of topologically nontrivial behavior. We show how it is possible to control them by a modulated electrostatic potential: They can be fully transmitted on the same edge, reflected on the opposite one, or can be split between the two edges, in analogy with Hall edge states, making them promising candidates for flying-qubit architectures.

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 - 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 - 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

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.

2011 - Effect of quasibound states on coherent electron transport in twisted nanowires [Articolo su rivista]
G., Cuoghi; Bertoni, Andrea; Sacchetti, Andrea
abstract

Quantum transmission spectra of a twisted electron waveguide expose the coupling between traveling andquasibound states. Through a direct numerical solution of the open-boundary Schroedinger equation, we singleout the effects of the twist and show how the presence of a localized state leads to a Breit-Wigner or a Fanoresonance in the transmission.We also find that the energy of quasibound states is increased by the twist, despitethe constant section area along the waveguide. While the mixing of different transmission channels is expectedto reduce the conductance, the shift of localized levels into the traveling-states energy range can reduce theirdetrimental effects on coherent transport.

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.

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 - 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.

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.

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

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.