Smith-Purcell free electron laser based on the semi-elliptical resonator

A novel Smith-Purcell(S-P) free electron laser composed of an electron gun,a semi-elliptical resonator,a metallic reflecting grating and a collector,is presented for the first time.This paper studies the characteristics of this device by theoretical analysis and particle-in-cell simulation method.Results indicate that tunable coherent S-P radiation with a high output peak power at millimeter wavelengths can be generated by adjusting the length of the grating period,or adjusting the voltage of the electron beam.The present scheme has the following advantages:the semi-elliptical resonator can reflect all radiation with the emission angle θ and random azimuthal angles,back onto the electron beam with same-phase and causes the electrons to be modulated,so the output power and efficiency are improved.     

Exchange-induced splitting of radiative exciton levels in a single quantum wire

We report on polarization-resolved micro-photoluminescence experiments performed on a single GaAs/GaAlAs V-shaped quantum wire. At low temperature the micro-photoluminescence spectra are composed of sharp peaks corresponding to excitons localized in naturally formed quantum boxes. We observed splittings of the radiative doublet of these exciton levels into two linearly polarized states due to the exchange interaction. The exchange splittings are of the order of 100 μeV. A theoretical model of the exchange interaction on localized states in quantum wires is developed. It shows that the exchange splitting is characteristic of the uniaxial anisotropy of the localized states and thus related to their extension along the wire axis. The experimental results are discussed within the frame of this model.     

Effects of intersubband interaction on multisubband electron transport in single and double quantum wells

The multisubband electron transport properties are studied for doped single quantum well and gated double asymmetric quantum well structures. The effects due to intersubband interaction and screening of the ionized impurity scattering are also investigated. We show that intersubband coupling plays an essential role in describing the screening properties as well as the effect of ionized impurity scattering on the mobility in a doped single quantum well. For coupled double quantum well structures, negative transconductance is found theoretically which is due to resonant tunneling between the two quantum wells.     

Conductance of Rashba electron in a quantum waveguide with smooth boundary

By using the transfer matrix method, we theoretically investigated Rashba electron's spin transport in a straight waveguide with a stub which has a smooth boundary. It is proved that the two spin states propagate independently. The conductance of the waveguide has been calculated. Conductance quantization is common in this structure when we change the Fermi energy and the width of the stub. If the shape of the stub converges toward the limit–a quadrate stub, the conductance will reduce distinctly due to reflection.     

Stability of charged impurities in a coupled single electron transistor and antidot system

We have fabricated devices on GaAs/AlGaAs heterostructures, containing two-dimensional electron gases, that consist of three point contacts surrounding an etched antidot with an Al/AlO_x/Al single electron transistor. The single electron transistor measurement shows rearrangement of neighboring charged impurities with a characteristic stability time scale of 20 s in one device and greater than 1 h in a second device. We also measured the resistance of the point contact–antidot constriction versus magnetic field. In a device with a 20 s stability time, we see a high noise level and poor reproducibility. In a device with a long stability time, much greater than 1 h, we are able to see reproducible features including Aharonov–Bohm oscillations.     

Macroscopic quantum coherence in a single molecular magnet and Kondo effect of electron transport

We report a Kondo-effect study of electron transport through a quantum dot with embedded biaxial single-molecule magnet based on slave boson mean-field theory and non-equilibrium Green-function technique. It is found the macroscopic quantum coherence of molecule-magnet results in the Kondo peak split of differential conductance due to interaction between electron and molecular magnet. It is also demonstrated that both the peak height and position can be controlled by the sweeping magnetic field and polarization of ferromagnetic electrodes. The characteristic peak split may be used to identify the macroscopic quantum coherence and develop molecule devices.     

电声子相互作用对量子点分子中单电子隧穿的影响

研究了双量子点系统中的电子隧穿动力学过程，在考虑电子与声子相互作用的情况下用基于 正则变换的微扰方法解析地得到了电子动态隧穿电流的表达式. 并且详细分析电子与声子耦 合引起的退相干问题，在此基础上指出了可能的退耦机理. 关键词： 电声子相互作用 双量子点 隧穿     

量子点浮置栅量子线沟道三栅结构单电子场效应管存储特性的数值模拟

通过建立二维薛定谔方程和泊松方程数值模型,对基于硅量子点浮置栅和硅量子线沟道三栅结构单电子场效应管(FET)存储特性进行了研究.通过在不同尺寸、栅压和不同写入电荷条件下,对硅量子线沟道中电子浓度的二维有限元自洽数值求解,研究了在纳米尺度下硅量子线沟道中量子限制效应和电荷分布对于器件特性的影响.模拟结果发现,沟道的导通阈值电压随着尺寸的缩小而提高,并随浮置栅内存储的电子数目的增加而明显升高.然而,这样的增加趋势在受到纳米尺度沟道中高电荷密度的影响下将出现非线性饱和趋势.进一步研究发现,当沟道尺寸较小时,沟道 关键词： 三栅单电子FET存储器 量子效应 薛定谔方程 泊松方程     

Transport properties of a lateral semiconductor quantum dot defined by a single connected metallic front-gate

We present studies on the electric transport in a lateral GaAs/AlGaAs quantum dot defined by a patterned single connected metallic front-gate. This gate design allows to easily couple a large number of quantum dots and therefore holds high potential in the design of new materials with tailor-made band structures based on quantum dot superlattices of controlled shape. Clear Coulomb diamond structures and well pronounced tunneling peaks observed in experiment indicate that single-electron control has been achieved. However, the dependence on electron density in the heterostructure embedding the dot, which is controlled by an additional back-gate, reveals that transport characteristics are strongly influenced supposedly by potential fluctuations in the dot and lead regions.