Abstract
We study the evolution of the quantum scattering time by gate voltage training in the topological insulator (TI) based on InAs/GaSb/InAs trilayer quantum wells. Depending on the minimal gate ...voltage applied during a gate voltage sweep cycle, the quantum scattering time can be improved by 50% from 0.08 ps to 0.12 ps albeit the transport scattering time is rather constant around 1.0 ps. The ratio of the quantum scattering time versus transport scattering time scales linearly with the charge carrier density and varies from 10 to 30, indicating Coulombic scattering as the dominant scattering mechanism. Our findings may enable to improve bulk and edge properties of TIs based on InAs/GaSb quantum well heterostructures solely by means of an electric field rather than temperature which opens the paths towards their application for macroscopic devices.
We investigate the energy relaxation segmentation in a resonant tunneling heterostructure by assessing the optical and transport dynamics of nonequilibrium charge carriers. The electrical and optical ...properties are analyzed using electronic transport measurements combined with electro- and photoluminescence spectroscopies in continuous-wave mode. Our results suggest that hot electron and hole populations form independent nonequilibrium systems that do not thermalize among them and with the lattice. Consequently, the carrier effective temperature changes independently at different regions of the heterostructure, with a population distribution for holes colder than for electrons.
Carrier relaxation is a key issue in determining the efficiency of semiconductor optoelectronic device operation. Devices incorporating semiconductor quantum dots have the potential to overcome many ...of the limitations of quantum-well-based devices because of the predicted long quantum-dot excited-state lifetimes. For example, the population inversion required for terahertz laser operation in quantum-well-based devices (quantum-cascade lasers) is fundamentally limited by efficient scattering between the laser levels, which form a continuum in the plane of the quantum well. In this context, semiconductor quantum dots are a highly attractive alternative for terahertz devices, because of their intrinsic discrete energy levels. Here, we present the first measurements, and theoretical description, of the intersublevel carrier relaxation in quantum dots for transition energies in the few terahertz range. Long intradot relaxation times (1.5 ns) are found for level separations of 14 meV (3.4 THz), decreasing very strongly to ∼2 ps at 30 meV (7 THz), in very good agreement with our microscopic theory of the carrier relaxation process. Our studies pave the way for quantum-dot terahertz device development, providing the fundamental knowledge of carrier relaxation times required for optimum device design.
In three-dimensional topological insulators, an effective closure of the bulk energy gap with increasing magnetic field expected at a critical point can yield a band crossing at a gapless Dirac node. ...Using high-field magneto-optical Landau-level spectroscopy on the topological crystalline insulator Pb1−xSnxSe, we demonstrate that such a gap closure does not occur, and an avoided crossing is observed as the magnetic field is swept through the critical field. We attribute this anticrossing to orbital parity and spin mixing of the N=0 levels. Concurrently, we observe no gap closure at the topological phase transition versus temperature suggesting that both are due to a single, likely extrinsic, mechanism.
We consider theoretically the role of crossed transitions on the interband optical properties of quantum dots. These transitions, which involve one bound state and one delocalized state, are inherent ...to the joint nature of the valence-to-conduction density of states in quantum dots. We show that they play a crucial role both on the interband absorption and on the broadening of the quantum dot lines.
Background: Depression is characterized by cognitive impairments, including executive dysfunctions. These executive deficits could reflect impairments of more basic executive processes, such as ...updating, set shifting and inhibition. While shifting and inhibition impairments are often reported, studies on depression have been somewhat obscure about specific deficits of the updating process. The main goal of that study was to assess the updating process in young in-patients with depression.
Methods: We used a verbal
n-back task to assess updating process. Load and mental manipulation within working memory (WM) were incremented by using three different levels of complexity (1,2,3-back). Neuropsychological tests and an attentional task (0-back) were also administered to subjects. Twenty-two individuals meeting DSM-IV criteria for Major Unipolar Depression and 22 healthy control subjects, matched on age, verbal IQ and education, were included in the study.
Results: Subjects with depression showed significant deficits at the
n-back task compared to control subjects. They were normal in tasks assessing the short-term maintenance in WM and attention. This suggests that depressed patients exhibit impairment in the updating process. Depressed patients also showed set shifting and inhibition deficits. Only the
n-back task was correlated with the number of hospitalizations and the longitudinal course of the illness.
Conclusions: Our results suggest that young depressed in-patients have widespread executive dysfunctions, including updating, shifting and inhibition processes. We also found a correlation between a longitudinal measure of depression severity and an updating task performance. We suggest that using multiple executive tasks gives the opportunity to distinguish the specific influence of various executive processes on clinical dimensions in depression.
We present gate voltage and temperature dependent transport measurements of InAs/GaSb/InAs triple quantum wells (TQWs) with a designed hybridization gap energy of 4 meV comparable to its traditional ...double quantum well counterpart. Gate voltage dependent measurements enable us to monitor two electron densities deep in the nonhybridized electron regime and further reveal a clear hybridization gap and a Van Hove singularity in the valence band as a result of the hybridized electron-hole band structure of the TQWs. The evolution of the charge carrier densities and types is studied in detail. Electron and hole densities coexist if the Fermi energy is within the gap and the bottom of the valence band at the Γ point. On the contrary, only single carrier types can be found far in the conduction and valence band. Thus, we are able to identify the topological band structure of these TQWs. Furthermore, the temperature evolution of the hybridized gap of the triple quantum well is studied. We find a rather temperature insensitive hybridization gap energy.
The discretization of the electronic spectrum in semiconductor quantum dots implies a strong coupling behavior between the optical phonons and the electron-hole pairs, despite the fact that a pair is ...electrically neutral. The excitonic polarons strongly modify the optical spectra. In particular, the ground excitonic polaron contains one or two phonon components, which leads to the existence of phonon replicas in the luminescence. The population and coherence decay times of the optical transition associated with the ground excitonic polaron are calculated.
We present a detailed experimental and theoretical study of a GaAs∕AlGaAs mid-infrared quantum cascade laser (QCL) under a strong magnetic field. The emitted power and the magnetoresistance show ...strong oscillations as a function of B , due to a magnetic field modulation of the lifetime of the upper state of the laser transition. The analysis of these oscillations shows a modulation of the electron lifetime due to inelastic (LO-phonon emission) and elastic scattering mechanisms. We show that at low temperature, the interface roughness scattering is the most efficient elastic relaxation mechanism; it becomes dominant whenever LO-phonon emission is inhibited by the magnetic field. By comparing experimental and theoretical results, we are able to estimate the relative weight of elastic and inelastic scattering. The magnetic field is thus a powerful spectroscopic tool to study the scattering mechanisms in QCL active regions.
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