Wave-vector filtering effect of the electric-magnetic barrier in HgTe quantum wells

Because of the helicity of electrons in HgTe quantum wells(QWs) with inverted band structures,the electrons cannot be confined by electric barriers since electrons can tunnel the barriers perfectly without backscattering in the HgTe QWs.This behavior is similar to Dirac electrons in graphene.In this paper,we propose a scheme to confine carriers in HgTe QWs using an electric-magnetic barrier.We calculate the transmission of carriers in 2-dimensional HgTe QWs and find that the wave-vector filtering effect of local magnetic fields can confine the carriers.The confining effect will have a potential application in nanodevices based on HgTe QWs.     

Wave-vector filtering effect of electric–magnetic barrier in HgTe quantum wells

Because of helicity of electrons in HgTe quantum wells (QWs) with inverted band structure, the electrons cannot be confined by electric barriers since electrons can tunnel the barriers perfectly without backscattering in HgTe QWs. This behavior is similar to Dirac electrons in graphene. In this paper, we propose a scheme to confine carriers in HgTe QWs using an electric-magnetic barrier. We calculate the transmission of carriers in 2-dimensional HgTe QWs and find that the wave-vector filtering effect of local magnetic fields can confine the carriers. The confining effect will have potential application in nanodevices based on HgTe QWs.     

超构材料中的光学量子自旋霍尔效应

电子的量子自旋霍尔效应的发现推进了当今凝聚态物理学的发展,它是一种电子自旋依赖的具有量子行为的输运效应.近年来,大量的理论和实验研究表明,描述电磁波场运动规律的麦克斯韦方程组内禀了光的量子自旋霍尔效应,存在于界面的倏逝波表现出强烈的自旋与动量关联性.得益于新兴的光学材料:超构材料(metamaterials)的发展,不仅能够任意设定光学参数,同时也能引入很多复杂的自旋-轨道耦合机理,让我们能够更加清晰地了解和验证其中的物理机理.本文对超构材料中量子自旋霍尔效应做了简要的介绍,内容主要包括真空中光的量子自旋霍尔效应的物理本质、电单负和磁单负超构材料能带反转导致的不同拓扑相的界面态、拓扑电路系统中光量子自旋霍尔效应等.     

a-Si ∶H/SiO2多量子阱材料制备及其光学性能和微结构研究

利用等离子体增强化学气相沉积技术制备了a-Si ∶H/SiO₂多量子阱结构材料.对a-Si ∶H/SiO₂多量子阱样品分别进行了3种不同的热处理,其中样品经1100 ℃高温退火可获得尺寸可控的nc-Si:H/SiO₂量子点超晶格结构,其尺寸与非晶硅子层厚度相当.比较了a-Si ∶H/SiO₂多量子阱材料与相同制备工艺条件下a-Si ∶H材料的吸收系数,在紫外/可见短波段前者的吸收系数明显增大,光学吸收边蓝移,说明该材料 关键词： 多量子阱 量子限制效应 光学吸收 能带结构     

Optimization of a GaN-based irregular multiple quantum well structure for a dichromatic white LED

GaN-based irregular multiple quantum well(IMQW) structures assembled two different types of QWs emitting complementary wavelengths for dichromatic white light-emitting diodes(LEDs) are optimized in order to obtain near white light emissions.The hole distributions and spontaneous emission spectra of the IMQW structures are analysed in detail by fully considering the effects of strain,well-coupling,valence band-mixing and polarization effect through employing a newly developed theoretical model from the k · p theory.Several structure parameters such as well material component,well width,layout of the wells and the thickness of barrier between different types of QWs are employed to analyse how these parameters together with the polarization effect influence the electronic and the optical properties of IMQW structure.Numerical results show that uniform hole distributions in different types of QWs are obtained when the number of the QWs emitting blue light is two,the number of the QWs emitting yellow light is one and the barrier between different types of QWs is 8nm in thickness.The near white light emission is realized using GaN-based IMQW structure with appropriate design parameters and injection level.     

Time-dependent electron transport in HgTe/CdTe quantum wells

Based on the Floquet theory and Keldysh's nonequilibrium Green's function methods, we study the electron transport through the HgTe/CdTe quantum wells (QWs) irradiated by a monochromatic laser field. We find that when the laser field is applied, the edge states are split into a series of sidebands. When the Fermi level lies among these sidebands, the quantized plateau of the conductance is destroyed. Instead, the conductance versus the radiation frequency exhibits the successive oscillation peaks corresponding to the resonant tunneling through the sidebands of the edge states. The resonant interaction between the quasiparticles and the radiation field opens the gaps in the crossing region of the sidebands, which can be tuned by the radiation strength and frequency. This leads to the shift of the oscillation peaks in the conductance. We also show that the amplitudes of the oscillation peaks in the conductance are governed by the radiation strength and frequency.     

Effective spin dephasing mechanism in confined two-dimensional topological insulators

A Kramers pair of helical edge states in quantum spin Hall effect (QSHE) is robust against normal dephasing but not robust to spin dephasing. In our work, we provide an effective spin dephasing mechanism in the puddles of two-dimensional (2D) QSHE, which is simulated as quantum dots modeled by 2D massive Dirac Hamiltonian. We demonstrate that the spin dephasing effect can originate from the combination of the Rashba spin-orbit coupling and electron-phonon interaction, which gives rise to inelastic backscattering in edge states within the topological insulator quantum dots, although the time-reversal symmetry is preserved throughout. Finally, we discuss the tunneling between extended helical edge states and local edge states in the QSH quantum dots, which leads to backscattering in the extended edge states. These results can explain the more robust edge transport in InAs/GaSb QSH systems.     

PbTe/PbSrTe半导体非对称量子阱中的Rashba效应

窄带隙半导体异质结构的自旋效应最近受到了国际上的很大关注.Ⅳ-Ⅵ族半导体具有各向异性和多能谷的特征,因此可以预期Rashba自旋效应在不同取向的Ⅳ-Ⅵ族半导体量子阱结构中存在显著差异.计算了多个取向的Pb_1-ySr_yTe/PbTe/Pb_1-xSr_xTe非对称量子阱中的Rashba分裂能,结果表明［100］取向的PbTe量子阱的Rashba分裂能在阱宽为5.0nm时 关键词： Ⅳ-Ⅵ族半导体 非对称量子阱 Rashba效应 自旋-轨道耦合分裂     

含有量子点的双波长LED的光谱调控

本文通过对含有高In组分量子点的双波长LED进行了模拟计算, 并对器件的能带结构、载流子浓度、复合速率和辐射光谱进行了研究. 通过对器件结构的调整与对比, 发现蓝绿双波长LED的绿光量子阱中加入高In组分量子点后可以拓宽辐射光谱, 使LED光谱具有更高的显色指数, 为实现无荧光粉的白光LED提供指导. 量子点对载流子具有很强的束缚能力, 并且载流子在量子点处具有更短的寿命, 载流子优先在量子点处复合, 量子点处所对应的黄光与量子阱润湿层所对应的绿光的比例随量子点浓度的增大而增大, 载流子浓度较低时以量子点处的黄光辐射为主, 载流子浓度变大后, 量子点复合逐渐达到饱和, 绿光辐射开始占据主导. 对间隔层厚度和间隔层掺杂浓度的调节可以很方便地调控载流子的分布, 从而实现对含有量子点的双波长LED两个活性层辐射速率的调控. 结果表明, 通过对量子点浓度、间隔层厚度、间隔层掺杂浓度的控节可以很好地实现对LED辐射光谱的调控作用. 关键词： GaN 量子点 光谱调控 双波长LED     

Symmetry and spin dephasing in (110)-grown quantum wells

Symmetry and spin dephasing in (110)-grown GaAs quantum wells (QWs) are investigated applying magnetic field induced photogalvanic effect and time-resolved Kerr rotation. We show that magnetic field induced photogalvanic effect provides a tool to probe the symmetry of (110)-grown quantum wells. The photocurrent is only observed for asymmetric structures but vanishes for symmetric QWs. Applying Kerr rotation we prove that in the latter case the spin relaxation time is maximal; therefore, these structures set the upper limit of spin dephasing in GaAs QWs. We also demonstrate that structure inversion asymmetry can be controllably tuned to zero by variation of delta-doping layer positions.     

Pressure effect on the electron mobility in AlAs/GaAs quantum wells

By taking the influence of optical phonon modes into account, this paper adopts the dielectric continuum phonon model and force balance equation to investigate the electronic mobility parallel to the interfaces for AlAs/GaAs semiconductor quantum wells (QWs) under hydrostatic pressure. The scattering from confined phonon modes, interface phonon modes and half-space phonon modes are analysed and the dominant scattering mechanisms in wide and narrow QWs are presented. The temperature dependence of the electronic mobility is also studied in the temperature range of optical phonon scattering being available. It is shown that the electronic mobility reduces obviously as pressure increases from 0 to 4GPa, the confined longitudinal optical (LO) phonon modes play an important role in wide QWs, whereas the interface optical phonon modes are dominant in narrow QWs, the half-space LO phonon modes hardly influence the electronic mobility expect for very narrow QWs.     

Direct observation of the carrier transport process in InGaN quantum wells with a pn-junction

A new mechanism of light-to-electricity conversion that uses InGaN/GaN QWs with a p-n junction is reported.According to the well established light-to-electricity conversion theory,quantum wells(QWs) cannot be used in solar cells and photodetectors because the photogenerated carriers in QWs usually relax to ground energy levels,owing to quantum confinement,and cannot form a photocurrent.We observe directly that more than 95% of the photoexcited carriers escape from InGaN/GaN QWs to generate a photocurrent,indicating that the thermionic emission and tunneling processes proposed previously cannot explain carriers escaping from QWs.We show that photoexcited carriers can escape directly from the QWs when the device is under working conditions.Our finding challenges the current theory and demonstrates a new prospect for developing highly efficient solar cells and photodetectors.     

量子态的非相干光时域测量

提出了利用非相干光实现对量子态的时域高分辨率测量的新方法.以测量Autler-Townes效应为例,通过对信号解析表达式的详细讨论,阐述了利用这种方法测量量子态的物理过程,揭示出被测量的演化规律及测量过程对量子态的影响.研究结果表明利用这种方法既可以有效地、高分辨地测量很小的量子态能级间距和很大的原子能级间距,同时又可以测量相干量子态的横向弛豫速率.通过对时域和频域测量方法的比较表明利用非相干光对量子态的时域测量可以全面揭示量子态演化信息. 关键词： 非相干光 量子态测量 Autler-Townes 效应     

Spin–orbit coupling effects on the in-plane optical anisotropy of semiconductor quantum wells

We theoretically study the influence of the spin–orbit coupling(SOC) on the in-plane optical anisotropy(IPOA) induced by in-plane uniaxial strain and interface asymmetry in(001) GaAs/AlGaAs quantum wells(QWs) with different well width. It is found that the SOC has more significant impact on the IPOA for the transition of the first valence subband of heavy hole to the first conduction band(1H1E) than that of 1L1E. The reason has been discussed. The IPOA of(001) InGaAs/InP QWs has been measured by reflectance difference spectroscopy, whose amplitude is about one order larger than that of GaAs/AlGaAs QWs. The anisotropic interface potential parameters of InGaAs/InP QWs are also determined. The influence of the SOC effect on the IPOA of InGaAs/InP QWs when the QWs are under tensile, compressive or zero biaxial strain are also investigated in theory. Our results demonstrate that the SOC has significant effect on the IPOA especially for semiconductor QWs with small well width, and therefore cannot be ignored.     

Noise sensing via stochastic quantum Zeno

The dynamics of any quantum system is unavoidably influenced by the external environment. Thus, the observation of a quantum system (probe) can allow the measure of the environmental features. Here, to spectrally resolve a noise field coupled to the quantum probe, we employ dissipative manipulations of the probe, leading to so-called Stochastic Quantum Zeno (SQZ) phenomena. A quantum system coupled to a stochastic noise field and subject to a sequence of protective Zeno measurements slowly decays from its initial state with a survival probability that depends both on the measurement frequency and the noise. We present a robust sensing method to reconstruct the unknown noise power spectral density by evaluating the survival probability that we obtain when we additionally apply a set of coherent control pulses to the probe. The joint effect of coherent control, protective measurements and noise field on the decay provides us the desired information on the noise field.     

Quantum spin Hall effect in inverted InAs/GaSb quantum wells

We review the recent experimental progress towards observing quantum spin Hall effect in inverted InAs/GaSb quantum wells (QWs). Low temperature transport measurements in the hybridization gap show bulk conductivity of a non-trivial origin, while the length and width dependence of conductance in this regime show strong evidence for the existence of helical edge modes proposed by Liu et al. [Phys. Rev. Lett., 2008, 100: 236601]. Surprisingly, edge modes persist in spite of comparable bulk conduction and show only weak dependence on magnetic field. We elucidate that seeming independence of edge on bulk transport comes due to the disparity in Fermi-wave vectors between the bulk and the edge, leading to a total internal reflection of the edge modes.     

Phase-matching quantum key distribution with light source monitoring

The transmission loss of photons during quantum key distribution (QKD) process leads to the linear key rate bound for practical QKD systems without quantum repeaters. Phase matching quantum key distribution (PM-QKD) protocol, an novel QKD protocol, can overcome the constraint with a measurement-device-independent structure, while it still requires the light source to be ideal. This assumption is not guaranteed in practice, leading to practical secure issues. In this paper, we propose a modified PM-QKD protocol with a light source monitoring, named PM-QKD-LSM protocol, which can guarantee the security of the system under the non-ideal source condition. The results show that our proposed protocol performs almost the same as the ideal PM-QKD protocol even considering the imperfect factors in practical systems. PM-QKD-LSM protocol has a better performance with source fluctuation, and it is robust in symmetric or asymmetric cases.     

HgTe/CdS/ZnS多壳层量子点的制备与表征

采用一种简单的方法合成HgTe/CdS/ZnS多壳层量子点。首先,以1-硫代甘油为稳定剂,在水相溶液中制备出HgTe核量子点;然后,采用外延生长法依次在HgTe核量子点表面包覆CdS和ZnS壳层,合成出最终具有稳定近红外发光的HgTe/CdS/ZnS多壳层量子点。该合成方法仅需3个步骤,具有操作简单、成本低廉的优点。实验结果显示,当反应温度为90 ℃、反应溶液pH为11.0、反应加热回流时间为4 min时,HgTe/CdS/ZnS多壳层量子点具有最高荧光量子产率36%。     

Quantitative characterization of the interface roughness of (GaIn)As quantum wells by high resolution STEM

The physical properties of semiconductor quantum wells (QW), like (GaIn)As/GaAs, are significantly influenced by the interface morphology. In the present work, high angle annular dark field imaging in (scanning) transmission electron microscopy ((S)TEM), in combination with contrast simulation, is used to address this question at atomic resolution. The (GaIn)As QWs were grown with metal organic vapor phase epitaxy on GaAs (001) substrates under different, precisely controlled conditions. In order to be able to compare different samples, a carefully applied method to gain reliable results from high resolution STEM micrographs was used. The thickness gradient of the TEM samples, caused by sample preparation, was compensated by the intensity of group V atomic columns, where no alloying takes place. After that, the In concentration map was plotted for the investigated regions based on the intensity of the group III atomic columns. The composition maps show that the Indium distribution across the quantum well is not homogeneous. The growth temperature of the QW can greatly influence the composition fluctuation and the interface morphology, with higher growth temperature resulting in larger composition fluctuations in the QWs and slightly wider interfaces, i.e. larger In-segregation. Growth interruptions are shown to significantly homogenize the elemental depth profile especially along the (GaIn)As/GaAs interface and hence have a positive effect on interface smoothness.