
ANDREA BERTONI
DIPENDENTE ALTRO ENTE DI RICERCA Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede exFisica

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2024
 Enhancement and anisotropy of electron Landé factor due to spinorbit 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 holespin qubit in silicon
[Articolo su rivista]
Forghieri, Gaia; Secchi, Andrea; Bertoni, Andrea; Bordone, Paolo; Troiani, Filippo
abstract
Holespin 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 outofequilibrium 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 LuttingerKohn 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
 Twoelectron selective coupling in an edgestate based conditional phase shifter
[Articolo su rivista]
Bellentani, Laura; Forghieri, Gaia; Bordone, Paolo; Bertoni, Andrea
abstract
We investigate the effect of longrange Coulomb interaction on the twoelectron scattering in the integer
quantum Hall regime at bulk filling factor two.We compute the dynamics of the exact twoparticle wave function
by means of a parallel version of the splitstep 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 twoelectron 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 twoqubit 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
MachZehnder 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 statereduction) law that unavoidably yields an uncertainty (variance) associated with the
corresponding mean values. This nonclassical 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) renormalizing 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 MachZehnder interferometer, for classical and quantum
computations, respectively. It is shown that the resulting protocol formally resembles the socalled
collective measurements, although, its practical implementation is substantially different.
2019
 Enhanced Rashba spinorbit coupling in coreshell nanowires by the interfacial effect
[Articolo su rivista]
Wójcik, Paweł; Bertoni, Andrea; Goldoni, Guido
abstract
We report on k → · p → calculations of Rashba spinorbit coupling controlled by external gates in InAs/InAsP coreshell nanowires. We show that charge spilling in the barrier material allows for a stronger symmetry breaking than in homogenous nanomaterials, inducing a specific interfacerelated contribution to spinorbit coupling. Our results suggest additional wavefunction engineering strategies to enhance and control spinorbit coupling.
2019
 Polychromatic emission in a wide energy range from InPInAsInP multishell 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
InPInAsInP multishell 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 nanoheterostructures: the InAs island (axially grown), the thin InAs capping shell (radially grown), the crystalphase 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 InPInAsInP multishell NWs containing various quantum confined domains as polychromatic optically active components in nanodevices for quantum information and communication technologies.
2019
 Quantum computing with quantumHall 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 timedependent numerical modeling of Hall
interferometers operating at the singlecarrier 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 onequbit rotation can be realized. By a
proper design of the twodimensional electron gas potential landscape, an entangling twoqubit
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 timedependent 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 EulerLagrange propagation of structureless classical particles to
unitary Schrödinger propagation of an electronhole wave packet in a mean field and to the full quantum
propagation of the twoparticle complex. A recently proposed beyond meanfield selfenergy approach, adding
internal virtual transitions to the c.m. dynamics, has been generalized to timedependent potentials and turns out
to describe very well full quantum calculations, while being orders of magnitude numerically less demanding.
We show that SAWdriven 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 electronhole dynamic of the SIX. Comparison between
different approximations allow us to show that internal correlation of the electronhole 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 Halledgestate mixing of localized electrons in a twochannel
MachZehnder interferometer
[Articolo su rivista]
Bellentani, Laura; Beggi, Andrea; Bordone, Paolo; Bertoni, Andrea
abstract
We present a numerical study of a multichannel electronicMachZehnder interferometer, based onmagnetically
driven noninteracting edge states. The electron path is defined by a fullscale 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 AharonovBohm oscillations in the transmission probability
of the second channel.We perform timedependent simulations by solving the electron Schrödinger equation
through a parallel implementation of the splitstep Fourier method, and we describe the chargecarrier 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 spinorbit coupling in homogeneous semiconductor nanowires
[Articolo su rivista]
Wojcik, Pawel; Bertoni, Andrea; Goldoni, Guido)
abstract
We use k·p theory to estimate the Rashba spinorbit coupling (SOC) in large semiconductor nanowires. We specifically investigate GaAs and InSbbased devices with different gate configurations to control symmetry and localization of the electron charge density. We explore gatecontrolled 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)PRBMDO1098012110.1103/PhysRevB.91.201413, but not with the much larger values reported in Nat. Commun. 8, 478 (2017)2041172310.1038/s4146701700315y. We discuss possible origins of this discrepancy.
2017
 Dynamics and control of edge states in laserdriven graphene nanoribbons
[Articolo su rivista]
Puviani, Matteo; Manghi, F.; Bertoni, A.
abstract
An intense laser field in the highfrequency 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 flyingqubit architectures.
2017
 Exact longwavelength 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 ringconfined 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 timedependent quantum scattering of a spatially indirect exciton by an external potential, taking fully into account the relative quantum dynamics of the electronhole (eh) pair. Exact calculations for an eh wave packet show that transfer of energy between centreofmass (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 pointlike dipole models or meanfield calculations. We show that a properlydesigned local selfenergy 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 fullpropagation calculations. The explicit form of this selfenergy emphasises the dominant role of internal virtual transitions in determining scattering coefficients of indirect excitons.
2016
 Erratum: Excitation energytransfer 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 〈NP0E()(R)NPv〉 and 〈NP0E(+)(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 〈NP0E()(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 〈NP0E()(R)NPv〉 = iT1〈NP0E(+)(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〈MdM∗〉2〈NP0E(+)(R)NPv〉2 + 〈NP0E(+)(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 rewritten 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 〈NP0Ẽ (R) NPv〉 = ∑i,p Cipv 〈pEi〉, (25) 〈NP0dNPv〉 = ∑i,p Cipv 〈pri〉. (26) • In the first sentence of the last paragraph of Sec. II C should be written 〈ie · pcvp〉 instead of ie · pcvp〉. · 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 twobody quantum dynamics of an electronhole pair in semiconductor coupled quantum wells: A timedependent approach
[Articolo su rivista]
Grasselli, Federico; Bertoni, Andrea; Goldoni, Guido
abstract
We simulate the timedependent coherent dynamics of a spatially indirect exciton  an electronhole pair with the two particles confined in different layers  in a GaAs coupled quantum well system. We use a unitary wavepacket propagation method taking into account in full the four degrees of freedom of the two particles in a twodimensional system, including both the longrange Coulomb attraction and arbitrary twodimensional 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 singleslit geometry. We found clear signatures of the internal twobody dynamics, with transient phenomena in the picosecond time scale which might be revealed by optical spectroscopy. Exact results have been compared with meanfield approaches which, neglecting dynamical correlations by construction, turn out to be inadequate to describe the electronhole pair evolution in realistic experimental conditions.
2016
 Excitation energytransfer 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 pointlike transition dipole moment representing the molecule. Optical excitations of the nanoparticle are described through an envelopefunction configuration interaction method for a single electronhole 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 dyefunctionalized 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, wellknown Förster theory, based on the transition dipoledipole 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 dyeNP 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 dyeNP 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 modulationdoped coremultishell nanowires
[Articolo su rivista]
Buscemi, Fabrizio; Royo, Miquel; Goldoni, Guido; Bertoni, Andrea
abstract
We show how a proper radial modulation of the composition of coremultishell nanowires (NWs) critically enhances the control of the freecarrier density in the highmobility core with respect to coresingleshell 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 selfconsistent SchrödingerPoisson approach: free carriers tend to localize in the outer shell and screen the core from the electric field of the dopants.
2016
 Timedependent scattering of a composite particle: A local selfenergy 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 timeresolved energy and localization spectra. Here, we show that timedependent 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 selfenergy 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 WannierMott excitons in one and twodimensional semiconductor heterostructures. The selfenergy approach shows very good agreement with numerically exact Schrödinger propagation for scattering potentials where a meanfield model cannot be applied, at a dramatically reduced computational cost.
2016
 Twobody 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 SuzukiTrotter factorization of the quantum evolution operator, to propagate a twobody complex in arbitrary external potential landscapes taking into account exactly the internal structure. We have simulated spatially indirect WannierMott excitons  optically excited electronhole 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 meanfield methods in this context.
2015
 AharonovBohm oscillations and electron gas transitions in hexagonal coreshell nanowires with an axial magnetic field
[Articolo su rivista]
Royo, Miquel; Segarra, Carlos; Bertoni, Andrea; Goldoni, Guido; Planelles, Josep
abstract
We use spindensityfunctional theory within an envelope function approach to calculate electronic states in a GaAs/InAs coreshell nanowire pierced by an axial magnetic field. Our fully threedimensional 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: gateallaround and backgate. At low magnetic fields, we investigate AharonovBohm 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 coremultishell 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 coremultishell 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 quasionedimensional (1D) and quasi2D states, which coexist in such radial
heterostructures, are predicted in the nonresonant approximation from a fully threedimensional multisubband timedependent 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 timedependent 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 twoparticle onedimensional Schrödinger equation by means
of the SplitStep Fourier method, to study the coherent evolution of a spatially indirect exciton (IX)
in semiconductor heterostructures. The mutual Coulomb interaction of the electronhole pair and
the electrostatic potentials generated by external gates and acting on the two particles separately are
taken into account exactly in the twoparticle dynamics. As relevant examples, step/downhill and
barrier/well potential profiles are considered. The space and timedependent 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 twoparticle 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 coreshell nanowires
[Articolo su rivista]
ROYO VALLS, Miguel; Bertoni, Andrea; Goldoni, Guido
abstract
We study the collective excitations and inelastic lightscattering cross section of an electron gas conﬁned
in a GaAs/AlGaAs coaxial quantum well. These systems can be engineered in a coremultishell nanowire
and inherit the hexagonal symmetry of the underlying nanowire substrate. As a result, the electron gas forms
both quasionedimensional channels and quasitwodimensional channels at the quantumwell bents and facets,
respectively. Calculations are performed within the randomphase approximation and timedependent 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
lightscattering selection rules for different scattering geometries. Computational issues stemming from the need
to use a symmetrycompliant grid are also investigated systematically
2013
 Landau levels, edge states, and magnetoconductance in GaAs/AlGaAs coreshell nanowires
[Articolo su rivista]
ROYO VALLS, Miguel; Bertoni, Andrea; Goldoni, Guido
abstract
Magnetic states of the electron gas confined in modulationdoped coreshell 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 envelopefunction approach, and electronelectron
interaction is included in a meanfield selfconsistent approach. Calculations show that in the low freecarrier
density regime, magnetic states can be described in terms of Landau levels and edge states, similar to planar
twodimensional electron gases in a Hall bar. However, at higher carrier density, the dominating electronelectron
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 openboundary Schroedinger equation, we singleout the effects of the twist and show how the presence of a localized state leads to a BreitWigner 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 travelingstates 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 twodimensional 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. Â© 2009 IOP Publishing Ltd.
2009
 Electronic and magnetic states in core multishell nanowires: Edge localization, Landau levels and AharonovBohm 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 quasi1D 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 AharonovBohm oscillations or Landau level formation. We also show that interchannel coupling and magnetic behavior is influenced by the geometric details of the nanowires. Â© 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 freestanding nanowire, together with the prismatic symmetry of the substrate, may induce quantum confinement of carriers in a set of quasi1D 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 AharonovBohm oscillations of the energy levels in the axial configuration, or a dimensional transition of the quantum states from quasi1D 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 quasiID regime by transverse magnetic field. In the case of a spatially modulated magnetic field, the carriers can localise to quasiOD states. (c) 2007 Elsevier B.V. All rights reserved.
2008
 Cylindrical twodimensional electron gas in a transverse magnetic field
[Articolo su rivista]
G., Ferrari; Bertoni, Andrea; Goldoni, Guido; Molinari, Elisa
abstract
We compute the singleparticle states of a twodimensional (2D) electron gas confined to the surface of a cylinder immersed in a magnetic field. The envelopefunction 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 quasi2D (cylindrical) regime to a quasionedimensional 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 tunnelcoupled 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 Landaulevel formation or AharonovBohm behavior.
2007
 Directionality of acoustic phonon emission in weaklyconfined 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 electronphonon 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 electronelectron interaction on the phononmediated 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 spinorbit 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 spinorbit couplings are exactly diagonalized. We show that electronelectron interaction strongly affects spinorbit 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 tripletsinglet 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 spinorbit coupling assisted by acoustic phonon emission.
2006
 Effect of the Coulomb interaction on the electron relaxation of weaklyconfined 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 acousticphononinduced relaxation of charge excitations in single and tunnelcoupled quantum dots containing few confined interacting electrons. The full configuration interaction approach is used to account for the electronelectron repulsion. Electronphonon 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 singleelectron 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 electronphonon coupling in properly designed singleelectron quantum dots are shown to hold also in multielectron devices. Our results indicate that fewelectron orbital degrees of freedom are more stable than singleelectron ones
2004
 Fieldcontrolled suppression of phononinduced transitions in coupled quantum dots
[Articolo su rivista]
Bertoni, Andrea; Rontani, Massimo; Goldoni, Guido; Troiani, Filippo; Molinari, Elisa
abstract
We suggest that orderofmagnitude reduction of the longitudinalacoustic 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 threedimensional 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.