Nelson Studart

Intersubband‐coupling and screening effects on the electron transport in a quasi‐two‐dimensional δ‐doped semiconductor system

The effects due to intersubband coupling and screening on the ionized impurity scattering are studied for a quasi‐two‐dimensional electron system in δ‐doped semiconductors. We found that intersubband coupling plays an essential role in describing the screening properties and the effect of ionized impurity scattering on the mobility in a multisubband system. At the onset of the occupation of a higher subband, the screening due to the intersubband coupling leads to a reduction of the small angle scattering rate in the lower subband. We showed that such an effect is significant in a δ‐doped quantum well and results in a pronounced increase of the quantum mobility at the onset of the occupation of a higher subband.

The structure and spectrum of the anisotropically confined two-dimensional Yukawa system

We have studied the structural and spectral properties of the classical system consisting of a finite number of charged particles, moving in two dimensions (2D), and interacting through a screened Coulomb potential and held together by an anisotropic harmonic potential. It is known that for the bare Coulomb interaction, the system crystallizes in well defined ordered configurations in which the particles are distributed in shells. However, we have found that the occupation of the shells changes considerably as a function of the screening parameter, and for large screening, the shell structure disappears and the particles form a Wigner lattice. We have shown that the eigenmodes of the system stiffen with increasing screening. By increasing the anisotropy of the confining potential, we were able to drive the system from 2D to 1D; this change occurs through a series of structural transitions. These transitions are reflected in the mode spectrum which collapses into a narrower frequency region with increasing anisotropy.

Interface optical phonons in spheroidal quantum dots

Interface optical phonons are studied in the case of a quantum dot (QD) with prolate and oblate spheroidal geometries within the dielectric continuum approach. We considered CdSe or CdS QDs imbedded in a host material which is modelled as an infinite medium. The surface optical phonon modes, the corresponding frequencies, and the electron-phonon interaction Hamiltonian are reported. Comparison is made with previous works which only considered strictly spherical dots. We conclude that deviations from the perfect spherical shape could be responsible for observable physical effects in Raman spectra.

Selective coherent destruction of tunneling in a quantum-dot array

The coherent manipulation of quantum states is one of the main tasks required in quantum computation. In this paper we demonstrate that it is possible to control coherently the electronic position of a particle in a quantum-dot array. By tuning an external ac electric field we can selectively suppress the tunneling between dots, trapping the particle in a determined region of the array. The problem is treated non-perturbatively by a time-dependent Hamiltonian in the effective mass approximation and using Floquet theory. We find that the quasienergy spectrum exhibits crossings at certain field intensities that result in the selective suppression of tunneling.

Quantum dot structures grown on Al containing quaternary material for infrared photodetection beyond 10μm

Different InAs quantum dot structures grown on InGaAlAs lattice matched to InP were investigated for quantum dot infrared photodetectors. Extremely narrow photocurrent peaks were observed, demonstrating great potential for fine wavelength selection. Structures which can detect radiation beyond 10μm were developed. Polarization dependence measurements showed that the structures have a zero-dimensional character and are suitable for detection of normal incident light. On the other hand, structures containing coupled quantum wells showed a hybrid two-dimensional/zero-dimensional behavior.

Decay of excited surface electron states in liquid helium and related relaxation phenomena induced by short-wavelength ripplons

Decay rates of excited surface electron states on liquid helium are theoretically studied for different electron confinement potentials and in the presence of quantizing magnetic field. Contributions of both one-ripplon and two-ripplon scattering processes are analyzed. Regarding the decay rate of the first excited surface level (l=2), two-ripplon emission of short wavelength capillary waves is shown to dominate the conventional one-ripplon scattering in two distinct cases: the ambient temperature is low enough or the surface state excitation energy Δ2−Δ1 does not match an excitation energy of the in-plane motion quantized under a strong magnetic field or in a quantum dot. In these cases magnetic field and confinement cannot essentially reduce the decay rate which is of order of 106s−1 and does not depend on temperature. The importance of these findings for a microwave resonance experiment is discussed.

Inhomogeneous broadening of tunneling conductance in double quantum wells

The lineshape of the tunneling conductance in double quantum wells with a large-scale roughness of heterointerfaces is investigated. Large-scale variations of coupled energy levels and scattering due to the short-range potential are taken into account. The interplay between the inhomogeneous broadening, induced by the non-screened part of large-scale potential, and the homogeneous broadening due to the scattering by short-range potentials is considered. It is shown that the large inhomogeneous broadening can be strongly modified by nonlocal effects involved in the proposed mechanism of inhomogeneity. Related change of lineshape of the resonant tunneling conductance between Gaussian and Lorentzian peaks is described. The theoretical results agree quite well with experimental data.

Theoretical study of a two-dimensional quantum system: electrons on a helium film

An accurate method is presented to calculate the static structure factor of the two dimensional electron system within the mean-field approximation. It provides a simple and efficient way to incorporate local field corrections in different approximations. In particular, the self-consistent theory of Singwi, Tosi, Land and Sjolander (1968) is used to investigate the static and dynamic properties of degenerate electrons on the surface of a helium film deposited over a solid substrate. Numerical results of the local field correction, pair correlation function, correlation energy and the excitation spectra are shown for several values of the coupling parameter and the film thickness. For comparison the random-phase approximation and the Hubbard approximation are also considered. In the limit of thin films over a metal, where the electron interaction is dipolar, the physical properties are qualitatively different from those of other two-dimensional quantum systems.

Electron transport in a quasi-one-dimensional channel on suspended helium films

Quasi-one-dimensional electron systems have been created using a suspended helium film on a structured substrate. The electron mobility along the channel is calculated by taking into account the essential scattering processes of electrons by helium atoms in the vapor phase, ripplons, and surface defects of the film substrate. It is shown that the last scattering mechanism may dominate the electron mobility in the low-temperature limit changing drastically the temperature dependence of the mobility in comparison with that controlled by the electron-ripplon scattering.

Nonperturbative electron dynamics in crossed fields

Intense AC electric fields on semiconductor structures have been studied in photon-assisted tunneling experiments with magnetic field applied either parallel (B_par) or perpendicular (B_per) to the interfaces. We examine here the electron dynamics in a double quantum well when intense AC electric fields F, and tilted magnetic fields are applied simultaneously. The problem is treated non-perturbatively by a time-dependent Hamiltonian in the effective mass approximation, and using a Floquet-Fourier formalism. For B_par=0, the quasi-energy spectra show two types of crossings: those related to different Landau levels, and those associated to dynamic localization (DL), where the electron is confined to one of the wells, despite the non-negligible tunneling between wells. B_par couples parallel and in-plane motions producing anti-crossings in the spectrum. However, since our approach is non-perturbative, we are able to explore the entire frequency range. For high frequencies, we reproduce the well known results of perfect DL given by zeroes of a Bessel function. We find also that the system exhibits DL at the same values of the field F, even as B_par non-zero, suggesting a hidden dynamical symmetry in the system which we identify with different parity operations. The return times for the electron at various values of field exhibit interesting and complex behavior which is also studied in detail. We find that smaller frequencies shifts the DL points to lower field F, and more importantly, yields poorer localization by the field. We analyze the explicit time evolution of the system, monitoring the elapsed time to return to a given well for each Landau level, and find non-monotonic behavior for decreasing frequencies.