双量子点结构中Majorana费米子的噪声特性

Majorana费米子是其自身的反粒子, 在拓扑量子计算中有着重要的应用. 利用粒子数表象下的量子主方程方法, 研究双量子点与Majorana费米子混合结构的电子输运特性, 特别是散粒噪声. 有无Majorana费米子耦合的电流与散粒噪声存在明显差别: 有Majorana费米子耦合时稳态电流差呈反对称, 噪声谱呈现相干振荡并且低频噪声显著增强. 量子点与Majorana费米子对称弱耦合时, 零频噪声由"峰"变为"谷", 并且"边谷"展宽逐渐减小; 当对称强耦合时, 零频噪声的谷深增加, "边谷"向高频端移动. 改变系统与电极的耦合强度时, 零频噪声由谷变成峰. 因此, 稳态电流结合散粒噪声可以探测双量子点结构中Majorana费米子是否存在.     

自旋轨道耦合量子点系统中的量子相干

研究了自旋轨道耦合量子点中的量子相干效应.运用输运电子的全计数统计方法计算系统的平均电流、散粒噪声和偏斜,发现体系存在自旋轨道耦合作用时,散粒噪声值随自旋轨道耦合常数的增加而减小.更重要的是,电流、噪声和偏斜随磁通周期性波动,并且波动周期不受自旋轨道耦合强度大小、自旋极化率以及动力学耦合不对称的影响.     

色关联的色噪声驱动的分段非线性模型的平均首次穿越时间

研究了色关联的乘性高斯色噪声和加性高斯色噪声驱动的分段非线性系统中, 噪声强度和相关时间对平均首次穿越时间的影响. 利用一致有色噪声近似方法和最速下降方法, 推导出系统平均首次穿越时间的表达式. 研究结果表明: 系统的平均首次穿越时间随着乘性噪声的增加会出现单峰结构, 即“共振”现象, 峰值会随着加性噪声强度和噪声之间关联强度的增加而减小. 而平均首次穿越时间作为加性噪声的函数呈单调曲线, 说明乘性噪声和加性噪声对平均首次穿越时间的影响不同. 此外, 乘性和加性噪声关联时间以及互关联时间在正关联时和负关联时 对系统平均首次穿越时间的影响是不同的. 关键词： 色噪声 分段非线性系统 平均首次穿越时间     

铁磁-超导隧道结中的散粒噪声

考虑到铁磁层中的自旋极化效应与粗糙界面散射效应,利用散射理论,讨论铁磁超导隧道结中的散粒噪声.计算表明:磁交换作用可以抑制隧道结系统的平均电流与散粒噪声功率,而粗糙界面散射可以增强系统的平均电流与散粒噪声功率 关键词： 铁磁 超导 隧道结 散粒噪声     

光电耦合器件闪烁噪声模型

对电应力前后光电耦合器件的闪烁噪声（1/f噪声）进行了实验和理论研究.实验发现,应力前后1/f噪声幅值随偏置电流均有相同的变化规律：在低电流区,1/f噪声幅值与偏置电流成正比,在高电流区,1/f噪声幅值与偏置电流的平方成正比,且应力后1/f噪声幅值增大了约7倍.理论分析表明,小电流时该器件的1/f噪声为扩散1/f噪声,大电流时为复合1/f噪声,应力在器件有源区诱生的陷阱是1/f噪声增大的根本原因.基于载流子数涨落和迁移率涨落机制,建立了一个光电耦合器件1/f噪声的定量分析模型,实验结果和模型符合良好.     

Majorana零模式的电导与低压振荡散粒噪声

Majorana零能量模式是自身的反粒子,在拓扑量子计算中有重要应用.本文研究量子点与拓扑超导纳米线混合结构,通过量子点的输运电荷检测Majorana零模式.利用量子主方程方法,发现有无Majorana零模式的电流与散粒噪声存在明显差别.零模式导致稳态电流差呈反对称,在零偏压处显示反常电导峰.电流差随零模式分裂能的增大而减小,随量子点与零模式耦合的增强而增大.另一方面,零模式导致低压散粒噪声相干振荡,零频噪声显著增强.分裂能导致相干振荡愈加明显且零频噪声减小,而量子点与零模式的耦合使零频噪声增强.当量子点与电极非对称耦合时,零模式使电子由反聚束到聚束输运,亚泊松噪声增强为超泊松噪声.稳态电流差结合低压振荡的散粒噪声能够揭示Majorana零模式是否存在.     

夫兰克-赫兹实验中几个问题的研究

测量了F-H实验中控制电压的敏感区,观测了3种不同电路下汞原子的电离曲线并对其成因进行了解释.观测了电离情况下板极电流与反射电压的关系,阐明了在高反射电压下电子在第二栅极附近聚集会增加离子的复合概率.观测了两栅极之间处于"悬浮"状态时板极电流的不稳定,反映了电子和离子扩散的随机性与噪声电压的影响.     

非晶铟锌氧化物薄膜晶体管的低频噪声特性与分析

本文针对底栅结构非晶铟锌氧化物薄膜晶体管的低频噪声特性开展实验与理论研究.由实验结果可知:受铟锌氧化物与二氧化硅界面处缺陷态俘获与释放载流子效应的影响,器件沟道电流噪声功率谱密度随频率的变化遵循1/fγ(γ≈0.75)的变化规律;此外,器件沟道电流归一化噪声功率谱密度随沟道长度与沟道宽度的增加而减小,证明器件低频噪声来源于沟道的闪烁噪声,可忽略源漏结接触及寄生电阻对器件低频噪声的影响.最后,基于载流子数涨落及迁移率涨落模型,提取γ因子与平均Hooge因子,为评价材料及器件特性奠定基础.     

蓝紫光发光二极管中的低频产生-复合噪声行为研究

本文对GaN基InGaN/GaN多量子阱结构、蓝紫光发光二极管(light-emitting diode, LED)的电流噪声进行了测试,电流测试范围为0.1—180 mA.根据电流噪声的特点,结合LED中载流子之间的产生-复合机制,探讨了电流注入下LED中载流子的产生与复合机制和低频噪声产生的机理.结论表明,随着电流从0.1 mA逐渐增大到27 mA, LED中的电流噪声具有低频产生-复合(generation-recombination, g-r)噪声的特性;当电流逐渐增大到50 mA及以上时,电流噪声的行为接近1/f噪声.采用电子元器件中公认的电流噪声模型,拟合了低频电流噪声功率谱密度与频率之间的关系,结合LED中载流子的输运机理和复合机制,从理论上分析了LED在电流注入时g-r噪声幅值和转折频率的变化规律.本文的结果提供了一种检测和表征多量子阱结构蓝紫光LED在电流逐渐增大过程中发光机制转变的有效手段,为提高其发光量子效率提供理论依据.     

氮化镓基蓝光发光二极管伽马辐照的1/f噪声表征

通过电离辐照对氮化镓基蓝光发光二极管器件有源区光/暗电流产生机制的研究, 建立了电离辐照减小发光二极管有效输出功率电学模型.通过电离辐照对氮化镓基蓝光发光 二极管器件有源区1/f噪声影响机制的研究, 建立了电离辐照增大发光二极管1/f噪声的相关性模型.在I < 1 μA 的小注入区,空间电荷区的复合电流随辐照剂量的增加而增加. 同时, 随着电离辐照产生缺陷的增加, 1/f噪声幅度增大. 在 I> 1 mA 的大注入条件下, 由于串联电阻的影响占主导地位,表面复合速率和电流随辐照剂量的增加而增加.同时, 随着电离辐照产生缺陷的增加, 1/f噪声幅度增大.根据辐照前后电流电压试验结果噪声测试结论, 证实了实验结论与理论推导结果的一致性. 在1 μA < I < 5×10^-5 A 的中值电流情况下, 由于高能载流子散射相关的迁移率涨落与辐照新增缺陷引起的载流子数涨落竞争机制, 随着辐照剂量增大, 1/f噪声在频域变化没有明显规律. 但是, 通过1/f噪声时域多尺度熵复杂度分析方法, 得出随着辐照剂量增大, 1/f噪声时域多尺度熵复杂度的结果. 最终证实1/f噪声幅度可以敏感地反映小注入和大注入情况下氮化镓基蓝光发光二极管电离辐照的可靠性. 噪声幅值越大, 则说明辐照感应N_it越高, 暗电流相关的复合电流越大, 光电流相关的扩散电流比例减少, 使得器件发光效率、光输出功率等性能参数下降, 继而影响器件可靠性, 造成失效率显著增大. 1/f噪声时域多尺度熵复杂度可以敏感地反映中值电流情况下氮 化镓基蓝光发光二极管的电离辐照可靠性.多尺度熵复杂度越大, 则说明辐照感应越多, 复合电流越大,器件可靠性越差.本文结论提供了一种基于 1/f噪声的氮化镓基蓝光发光二极管电离辐照可靠性表征方法. 关键词： f噪声')" href="#">1/f噪声 电离辐照 氮化镓基蓝光发光二极管     

外场对分子纳米结电流-电压特性的影响

针对由金属电极/分子/金属电极组成的分子纳米结,应用扩展主方程的方法,考虑分子纳米结中影响其传输过程的外场、分子内的弛豫过程等因素研究了在外场作用下分子纳米结内的稳定电流和瞬间电流.由于分子内较强的电子-振动耦合,分子纳米结中的电流-电压曲线呈现台阶式非弹性特征.在不同的高斯型脉冲的激发作用下,分子纳米结中电流需要达到稳定的时间也不相同,脉冲宽度在1ps时瞬间电流现象明显,这时分子处于非平衡分布,分子两端的电流存在较大差异.随着脉冲宽度和外场偏压的增加,分子两端的电流趋于平衡.     

时变反应扩散系统中螺旋波和湍流的控制

研究了一类参数时变的反应扩散系统中螺旋波和湍流对外电场的响应问题.在数值模拟中,以一类改进的Fitzhugh-Nagumo模型为研究对象(在恰当参数值下可分别描述激发介质和振荡介质),考虑随机和不确定因素(如内外噪声、气压、温度梯度分布和介质形变等)所引起的系统参数涨落对斑图演化的影响,在模拟中选取的参数涨落范围确保系统可以观测到稳定旋转的螺旋波、漫游的螺旋波和湍流,经历一定的暂态过程后,对介质施加极化电场,研究螺旋波和湍流在外电场中的演化.数值计算结果表明：在系统参数发生涨落和外电场强度比较小情况下,主 关键词： 螺旋波 湍流 时变系统 Fitzhugh-Nagumo模型     

Effect of external noise on the current response in superlattices

Within the single-band model and the nearest-neighbor approximation we investigate the effect of external noise on the current response driven by dc–ac field which has a fluctuating component. We find the external noise can destroy the current resonant peaks. When the strength of the external noise becomes enough large, all the current resonant peaks will completely disappear.     

New insights into electron spin dynamics in the presence of correlated noise

The changes in the spin depolarization length in zinc-blende semiconductors when an external component of correlated noise is added to a static driving electric field are analyzed for different values of field strength, noise amplitude and correlation time. Electron dynamics is simulated by a Monte Carlo procedure which takes into account all the possible scattering phenomena of the hot electrons in the medium and includes the evolution of spin polarization. Spin depolarization is studied by examining the decay of the initial spin polarization of the conduction electrons through the D'yakonov-Perel process, the only relevant relaxation mechanism in III-V crystals. Our results show that, for electric field amplitudes lower than the Gunn field, the dephasing length shortens with increasing noise intensity. Moreover, a nonmonotonic behavior of spin depolarization length with the noise correlation time is found, characterized by a maximum variation for values of noise correlation time comparable with the dephasing time. Instead, in high field conditions, we find that, critically depending on the noise correlation time, external fluctuations can positively affect the relaxation length. The influence of the inclusion of the electron-electron scattering mechanism is also shown and discussed.     

Transient absorption–dispersion properties of four-level atomic system via elliptically polarized probe light and magnetic field

A novel four-level atomic system which interacted by an elliptically polarized probe field and a control laser field in the presence of external magnetic field is proposed. Here, we are interested in the transient properties of a weak probe field due to its potential application on quantum computing and quantum communication. It is shown that the external magnetic field and relative phase between two electric field components of the probe field can influence the probe absorption and dispersion.     

Theoretical Analysis of the Fluctuation Theorem Applied to Electric Circuits

We carry out a theoretical analysis of the fluctuations in the Johnson-Nyquist noise in an RC electric circuit being driven by a non-ideal constant current source. Using the appropriate Langevin equation, we derive the fluctuation theorem for the system. This analysis exhibits some interesting features. Firstly, the average of the dissipation function is not necessarily just an instantaneous entropy production, i.e. the work being done on the circuit by the external power source divided by the ambient temperature. Secondly, by appropriate selection of the values for the circuit components, the transient response of the system can be made identical to the nonequilibrium steady state response. In the limit of an ideal current source, the average of the dissipation function reduces to the entropy production. Our analysis accurately reproduces published experimental results.     

Non-Gaussian noise in quantum wells infrared photodetectors

Non-Gaussian dark current noise has been observed in quantum wells infrared photo detectors. The non-Gaussian component of the noise was ascribed to fluctuations of spatial distribution of electric field in the device. Non-Gaussian noise was found in both n- and p-type QWIPs, however, it was significantly less pronounce. In n-type devices non-Gaussian noise manifests itself only as randomly distributed excess current bursts. In p-type QWIPs the non-Gaussian noise takes form of bias dependent random telegraph-like fluctuations with a finite time of transition between the levels. The lifetime at both levels is Poisson distributed and the average lifetime, together with the level spacing, strongly depend on bias voltage. At low voltages the system stays predominantly in the low current level while at higher voltages the average lifetime of the high current level is longer. The transient time of passing between the states has been related to the charging time constant of the system determined by QWIP capacitance and contacts resistance.     

Delayed electrical response of polyethylene observed in positronium formation

The transient and steady effects of positronium yield in polyethylene, due to the application and removal of an external electric field, have been studied. The time-dependent electric field effective in the Ps formation region was deduced from yield measurements.     

Pattern selection processes and noise induced pattern-forming transitions in periodic systems with transient dynamics

We apply the phase field crystal method for nonequilibrium patterning to stochastic systems with an external source in which transient dynamics is essential. Considering a prototype model for a one-component periodic system subjected to external influence kind of irradiation we study properties of pattern selection processes and external noise induced pattern-forming transitions. These processes are examined by means of the structure function dynamics analysis. Nonequilibrium pattern-forming transitions are analyzed numerically.