不同量子阱宽度的InP基In_(0.53)GaAs/In_(0.52)AlAs高电子迁移率晶体管材料二维电子气的性能研究

用Shubnikov-de Haas(SdH)振荡效应,研究了在1.4 K下不同量子阱宽度(10-35 nm)的InP基高电子迁移率晶体管材料的二维电子气特性.通过对纵向电阻SdH振荡的快速傅里叶变换分析,得到不同阱宽时量子阱中二维电子气各子带电子浓度和量子迁移率.研究发现,在Si掺杂浓度一定时,阱宽的改变对于量子阱中总的载流子浓度改变不大,但是随着阱宽的增加,阱中的电子从占据一个子带到占据两个子带,且第二子带上的载流子迁移率远大于第一子带迁移率.当量子阱宽度为20 nm时,处在第二子能级上的电子数与处在第一子能级上的电子数之比达到了最大值0.24.此时有最多的电子位于迁移率高的第二子能级,材料的迁移率也最大.此结果对于优化器件的设计有重要意义.

InAlAs/InGaAs/InAlAs量子阱高迁移移率二维电子气系统中的反弱局域效应研究

研究了Si重δ掺杂In_(0.52)Al_(0.48)As/In_(0.53)Ga_(0.47)As/In_(0.52)Al_(0.48)As单量子阱内高迁移率二维电子气系统中的反弱局域效应.研究表明,强的Rashba自旋轨道相互作用来源于量子阱高的结构反演不对称.高迁移率系统中,粒子的运动基于弹道输运而非扩散输运.因此,旧的理论模型不能用于拟合实验结果.由于最新的模型在实际拟合中过于复杂,一种简单可行的近似用于处理实验结果,并获得了自旋分裂能△_0和自旋轨道耦合常数a两个重要的物理参数.该结果与对纵向电阻的Shubnikov-de Haas-SdH振荡分析获得的结果一致.高迁移率系统中的反弱局域效应研究表明,发展有效的反弱局域理论模型,对于利用Rashba自旋轨道相互作用来设计自旋器件尤为重要.

单边掺杂InAlAs／InGaAs单量子阱中二维电子气的磁输运特性

研究了双子带占据的In0.52Al0.48As／In0.53Ga0.47As单量子阱中磁电阻的Shubnikov-de Haas（SdH）振荡效应和霍耳效应，获得了不同子带电子的浓度、迁移率、有效质量和能级位置．低磁感应强度（B〈1．5T）下由迁移率谱和多载流子拟合相结合的方法得到的各子带电子浓度与通过SdH振荡得到的结果一致．在d^2ρ／dB^2-1／B的快速傅里叶变换谱中，观察到除了通常强烈依赖温度的对应于各子带的频率f1和f2以及f1的倍频（2f1）外，还观察到对温度不敏感的频率f1-f2．这是由于量子阱中不同子带的电子具有相近的有效质量，两个子带之间发生了强烈的磁致子带间散射．

THEORETICAL CALCULATION FOR SHUBNIKOV-DEHAAS OSCILLATIONS IN DEEP MESA STRUCTURES

We have used the rectangular confinement potential to describe Shubnikov-deHaas oscillations produced by one-dimensional electrons confined in deep mesa structures. The edge distortion of the confinement potential caused by electrostatic image forces is taken into account. The model contains no fitting parameters and relates well with experimental data. The comparison with earlier reported parabolic model is presented,

Magnetointersubband oscillations of the two-dimensional electron gas in AlxGa1-xN/GaN heterostructures

Shubmkov-de Haas (SdH) measurements are performed over a temperature range of 1.5-20K in AL(0.22)Ga(0.78)N/GaN heterostructures with two subbands occupied. In addition to an intermodulation between two sets of SdH oscillations from the first and second subbands, a beating in oscillatory magnetoresistance at 12K is observed, due to the mixing of the first subband SdH oscillations and 'magnetointersubband' (MIS) oscillations. A phase shift of pi between the SdH and MIS oscillations is also clearly identified. Our experimental results, i.e. that the SdH oscillations dominate at low temperature and MIS oscillations dominate at high temperature, fully comply with the expected behaviour of MIS oscillations.

Preliminary report on the energy balance for nonlinear oscillations

In this paper, a reliable technique for calculating angular frequencies of nonlinear oscillators is developed. The new algorithm offers a promising approach by constructing a Hamiltonian for the nonlinear oscillator. Some illustrative examples are given. (C) 2002 Published by Elsevier Science Ltd.

Experimental-study of free-surface oscillations of a liquid bridge by optical diagnostics

A non-contact optical method, consisting of a projecting grating technique for the relative measurement of a surface, and a technique of absolute measurement at a fixed point on the surface, are applied to measure the free surface vibration in a liquid bridge of half floating zone with small typical scale of a few of mm for emphasizing the thermocapillary effect in comparison with the effect of buoyancy. The radii variations in both longitudinal and azimuthal directions are obtained, and, then, the feature of surface wave could be analyzed in detail. The results show that there are values of principal oscillatory frequencies at different positions of free surface. The amplitudes of surface waves in longitudinal and azimuthal directions are several mum and several tenths of mum in order of magnitude. The phase of two-dimensional surface waves is different at different height for fixed cross section or at different azimuthal angle for fixed height. The wave features are discussed for the cases of typical parameter ranges.

Red shift and broadening of backward harmonic radiation from electron oscillations driven by femtosecond laser pulse

The characteristics of backward harmonic radiation due to electron oscillations driven by a linearly polarized fs laser pulse are analysed considering a single electron model. The spectral distributions of the electron's backward harmonic radiation are investigated in detail for different parameters of the driver laser pulse. Higher order harmonic radiations are possible for a sufficiently intense driving laser pulse. We have shown that for a realistic pulsed photon beam, the spectrum of the radiation is red shifted as well as broadened because of changes in the longitudinal velocity of the electrons during the laser pulse. These effects are more pronounced at higher laser intensities giving rise to higher order harmonics that eventually leads to a continuous spectrum. Numerical simulations have further shown that by increasing the laser pulse width the broadening of the high harmonic radiations can be controlled.

Magnetic quantum oscillations for the surface states of topological insulator Bi2Se3

We study quantum oscillations of the magnetization in Bi2Se3 (111) surface system in the presence of a perpendicular magnetic field. The combined spin-chiral Dirac cone and Landau quantization produce profound effects on the magnetization properties that are fundamentally different from those in the conventional semiconductor two-dimensional electron gas. In particular, we show that the oscillating center in the magnetization chooses to pick up positive or negative values depending on whether the zero-mode Landau level is occupied or empty. An intuitive analysis of these features is given and the subsequent effects on the magnetic susceptibility and Hall conductance are also discussed.

Weak measurement of qubit oscillations with strong response detectors: Violation of the fundamental bound imposed on linear detectors

Qubit measurement by mesoscopic charge detectors has received great interest in the community of mesoscopic transport and solid-state quantum computation, and some controversial issues still remain unresolved. In this work, we revisit the continuous weak measurement of a solid-state qubit by single electron transistors (SETs) in nonlinear-response regime. For two SET models typically used in the literature, we find that the signal-to-noise ratio can violate the universal upper bound "4," which is imposed quantum mechanically on linear-response detectors. This different result can be understood by means of the cross correlation of the detector currents by viewing the two junctions of the single SET as two detectors. Possible limitation of the potential-scattering approach to this result is also discussed.

Magnetization of two-dimensional heavy holes with boundaries in a perpendicular magnetic field

The magnetisation of heavy holes in III-V semiconductor quantum wells with Rashba spin-orbit coupling (SOC) in an external perpendicular magnetic field is studied theoretically. We concentrate on the effects on the magnetisation induced by the system boundary, the Rashba SOC and the temperature. It is found that the sawtooth-like de Haas-van Alphen (dHvA) oscillations of the magnetisation will change dramatically in the presence of such three factors. Especially, the effects of the edge states and Rashba SOC on the magnetisation are more evident when the magnetic field is smaller. The oscillation center will shift when the boundary effect is considered and the Rashba SOC will bring beating patterns to the dHvA oscillations. These effects on the dHvA oscillations are preferably observed at low temperatures. With increasing temperature, the dHvA oscillations turn to be blurred and eventually disappear.

Analysis of the temperature-induced transition to current self-oscillations in doped GaAs/AlAs superlattices

We observed the decrease of the hysteresis effect and the transition from the stable to the dynamic domain regime in doped superlattices with increasing temperature. The current-voltage characteristics and the behaviours of the domain boundary are dominated by the temperature-dependent lineshape of the electric field dependence of the drift velocity (V(F)), As the peak-valley ratio in the V(F) curve decreases with increasing temperature, the hysteresis will diminish and temporal current self-oscillations will occur. The simulated calculation, which takes the difference in V(F) curves into consideration, gives a good agreement with the experimental results.

Theoretical analysis and experimental study of Fourier transformation of Franz-Keldysh oscillations in GaAs

Fourier transformation (FT) has been used in the theoretical line shape analysis of Franz-Keldysh oscillations (FKOs) in detail by numerical simulations. FKOs from the surface-intrinsic-n(+) GaAs structure were obtained in photoreflectance (PR) measurements with various modulation light intensities and with different strengths of bias light illumination, which were used to change the static electric field in the intrinsic layer of the sample. The FT spectra of the PR spectra, including the real part, imaginary part, and the modulus, were very consistent with the theoretical line shapes. The ratio of the square root of the reduced mass (root mu (L)/root mu (H)) and the ratio of transition strength of the electron heavy hole to the electron light hole were obtained from the PT spectra. In addition, the electric field in the intrinsic layer of the sample without and with bias illumination and the modulation field induced by photomodulation were also obtained. (C) 2000 American Institute of Physics. [S0021-8979(00)02123-X].

Self-sustained oscillations caused by magnetic field in a weakly-coupled GaAs/AlAs superlattice

We have investigated the influence of transverse magnetic field B up to 14 T at 1.6 K on the tunneling processes of electric field domains in doped weakly coupled GaAs/AlAs superlattices. Three regimes, i.e, stable field domains, current self-sustained oscillations and averaged field distribution are successively observed with increasing B. The mechanisms of switching-over among these regimes are due to B-induced modification of the dependence of the effective electron drift velocity on electric field. The simulated calculation gives a good agreement with the observed experimental results. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Gate controlled Aharonov-Bohm-type oscillations from single neutral excitons in quantum rings

We report on a magnetophotoluminescence study of single self-assembled semiconductor nanorings which are fabricated by molecular-beam epitaxy combined with AsBr3 in situ etching. Oscillations in the neutral exciton radiative recombination energy and in the emission intensity are observed under an applied magnetic field. Further, we control the period of the oscillations with a gate potential that modifies the exciton confinement. We infer from the experimental results, combined with calculations, that the exciton Aharonov-Bohm effect may account for the observed effects.