arXiv subjects

G. Goldoni

Publications and source records attributed to G. Goldoni.

At least 19 recordsLinked to original sources

arXiv2023

Enhancement and anisotropy of electron Lande factor due to spin-orbit interaction in semiconductor nanowires

We investigate the effective Lande factor in semiconductor nanowires with strong Rashba spin-orbit coupling. Using the $\mathbf{k}\cdot\mathbf{p}$ theory and the envelope function approach we derive a conduction band Hamiltonian where the tensor $g^*$ is explicitly related to the spin-orbit coupling constant $\alpha_R$. Our model includes orbital effects from the Rashba spin-orbit term, leading to a significant enhancement of the effective Lande factor which is naturally anisotropic. For nanowires based on the low-gap, high spin-orbit coupled material InSb, we investigate the anisotropy of the effective Lande factor with respect to the magnetic field direction, exposing a twofold symmetry for the bottom gate architecture. The anisotropy results from the competition between the localization of the envelope function and the spin polarization of the electronic state, both determined by the magnetic field direction.

cond-mat.mes-hall↗

arXiv2020

Anisotropy of the spin-orbit coupling driven by a magnetic field in InAs nanowires

We use the $\mathbf{k} \cdot \mathbf{p}$ 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 explicitely into account the prismatic symmetry of typical nano-crystals. 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 6-fold anisotropy results from the interplay between the orbital effect of field and the prismatic symmetry of the nanowire. A back-gate potential, breaking the native symmetry of the nano-crystal, couples to the magnetic field inducing a 2-fold 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 2-fold 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.

cond-mat.mes-hall↗

arXiv2018

Enhanced Rashba spin-orbit coupling in core-shell nanowires by the interfacial effect

We report on $\vec{k}\cdot\vec{p}$ 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 homoegenous nano-materials, inducing a specific interface-related contribution to spin-orbit coupling. Our results qualitatively agree with recent experiments [S. Futhemeier \textit{et al.}, Nat. Commun. \textbf{7}, 12413 (2016)] and suggest additional wavefunction engineering strategies to enhance and control spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2014

Unintentional high density p-type modulation doping of a GaAs/AlAs core-multi-shell nanowire

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 \emph{unintentional} p-type doping. Magneto-optical studies of such a GaAs/AlAs core multi-shell 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. Micro-photoluminescence 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 2D hole density in the structure.

cond-mat.mes-hall↗

arXiv2009

Optimal generation of indistinguishable photons from non-identical artificial molecules

We show theoretically that nearly indistinguishable photons can be generated with non-identical semiconductor-based sources. The use of virtual Raman transitions and the optimization of the external driving fields increases the tolerance to spectral inhomogeneity to the meV energy range. A trade-off emerges between photon indistinguishability and efficiency in the photon-generation process. Linear (quadratic) dependence of the coincidence probability within the Hong-Ou-Mandel setup is found with respect to the dephasing (relaxation) rate in the semiconductor sources.

cond-mat.mes-hall↗

arXiv2008

Photoluminescence spectroscopy of trions in quantum dots: a theoretical description

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 et al. Phys. Rev. B {\bf 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 wavefunction lengthscales are also described.

cond-mat.mes-hall↗

arXiv2006

Phonon-induced electron relaxation in weakly-confined single and coupled quantum dots

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.

cond-mat.mes-hall↗

arXiv2005

Reduced electron relaxation rate in multi-electron quantum dots

We use a configuration-interaction approach and Fermi golden rule to investigate electron-phonon interaction in realistic multi-electron 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.

cond-mat.mes-hall↗

arXiv2000

Local absorption spectra of artificial atoms and molecules

We investigate theoretically the spatial dependence of the linear absorption spectra of single and coupled semiconductor quantum dots, where the strong three-dimensional quantum confinement leads to an overall enhancement 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 highly non-monotonic way when the spatial resolution is comparable with the excitonic Bohr radius. We finally discuss how the optical near-field properties of double quantum dots are affected by their coupling.

cond-mat.mtrl-sci↗

arXiv2000

Local optical spectroscopy of semiconductor nanostructures in the linear regime

We present a theoretical approach to calculate the {\it local} absorption spectrum of excitons confined in a semiconductor nanostructure. Using the density-matrix formalism, we derive a microscopic expression for the non-local susceptibility, both in the linear and non-linear regimes, which includes a three-dimensional description of electronic quantum states and their Coulomb interaction. The knowledge of the non-local susceptibility allows us to calculate a properly defined local absorbed power, that depends on the electro-magnetic 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 wavefunction induce new features in the space-resolved optical spectra, particularly in coupled nanostructures. When the spatial extension of the electromagnetic field 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.

cond-mat.mtrl-sci↗

arXiv1999

Enhancement of Coulomb interactions in semiconductor nanostructures by dielectric confinement

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, where the many-body ground states and the addition spectra are predicted to be drastically altered by the dielectric environment.

cond-mat.mtrl-sci↗

arXiv1999

Double photo-ionization of He near a polarizable surface

We calculate the differential cross-section of the direct double photo-ionization of He physisorbed on a polarizable surface. By including the influence of the surface potential in the correlated two-electron final state wavefunction, we show that the differential cross-section carries detailed information on the electronic correlations at the surface. In particular, photo-emission along opposite directions, which is prohibited in the free space, is allowed if the surface potential is long-ranged.

cond-mat.mtrl-sci↗

arXiv1998

Local Optical Spectroscopy in Quantum Confined Systems: A Theoretical Description

A theoretical description of local absorption is proposed in order to investigate spectral variations on a length scale comparable with the extension of the relevant quantum states. A general formulation is derived within the density-matrix formalism including Coulomb correlation, and applied to the prototypical case of coupled quantum wires. The results show that excitonic effects may have a crucial impact on the local absorption with implications for the spatial resolution and the interpretation of near-field optical spectra.

cond-mat.mtrl-sci↗

arXiv1998

Strong exciton binding in quantum structures through remote dielectric confinement

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. Due to the polarization charges induced by the dielectric mismatch at the semiconductor/insulator interfaces, we show that the exciton binding energy can be more than doubled. For conventional III-V quantum wires such remote dielectric confinement allows exciton binding at room temperature.

cond-mat.mtrl-sci↗

arXiv1997

Excitonic Effects in Quantum Wires

We review the effects of Coulomb correlation on the linear and non-linear 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 bandgap, 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.

cond-mat.mtrl-sci↗

arXiv1997

Quantum interference in nanometric devices: ballistic transport across arrays of T-shaped quantum wires

We propose that the 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 the Landauer-Büttiker 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 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.

cond-mat↗

arXiv1997

Shape-independent scaling of excitonic confinement in realistic quantum wires

The scaling of exciton binding energy in semiconductor quantum wires is investigated theoretically through a non-variational, fully three-dimensional approach for a wide set of realistic state-of-the-art structures. We find that in the strong confinement limit the same potential-to-kinetic energy ratio holds for quite different wire cross-sections and compositions. As a consequence, a universal (shape- and composition-independent) parameter can be identified that governs the scaling of the 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.

cond-mat↗

arXiv1996

Band structure and optical anisotropy in V-shaped and T-shaped semiconductor quantum wires

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, usually very difficult to obtain otherwise.

cond-mat↗