Spintronic Transport and Kondo Effect in Quantum Dots

We investigate the spin-dependent transport properties of quantum-dot based structures where Kondo correlations dominate the electronic dynamics. The coupling to ferromagnetic leads with parallel magnetizations is known to give rise to nontrivial effects in the local density of states of a single quantum dot. We show that this influence strongly depends on whether charge fluctuations are present or absent in the dot. This result is confirmed with numerical renormalization group calculations and perturbation theory in the on-site interaction. In the Fermi-liquid fixed point, we determine the correlations of the electric current at zero temperature (shot noise) and demonstrate that the Fano factor is suppressed below the Poissonian limit for the symmetric point of the Anderson Hamiltonian even for nonzero lead magnetizations. We discuss possible avenues of future research in this field: coupling to the low energy excitations of the ferromagnets (magnons), extension to double quantum dot systems with interdot antiferromagnetic interaction and effect of spin-polarized currents on higher symmetry Kondo states such as SU(4).     

Electron Transport Through Coupled Quantum Dots Below the Kondo Temperature

Conductance G, through coupled quantum dots in series isinvestigated below the Kondo temperature, based on the slaveboson formalism of the Anderson model with theantiferromagnetic spin–spin coupling J. Electron transport ischaracterized by the ratio of the dot–dot tunneling couplingV _C to the level broadening in the dots (dot-lead coupling). When V _C /<1, the Kondoresonance is formed in each dot and lead and G is determinedby hopping between the two Kondo states. They are replaced bya spin-singlet state in the dots for low gate voltages. Thegate voltage dependence of G has a sharp peak of 2e ² /h inheight in the crossover region between the spin-singlet andKondo states. When V _c />1, the Kondo levelsare split below and above the Fermi level in the leads for lowgate voltages. The gate voltage dependence of G has a broadpeak, which is fairly robust against J. This broad peak isdivided into two peaks when the energy levels are differentbetween the dots.     

Nonequilibrium Spin Fluctuations in Nonmagnetic Single-Electron Transistors and Quantum Dots

It is shown that nonequilibrium spin fluctuations significantly influence electronic transport in a single-electron transistor, when the spin relaxation on the island is slow. To describe spin fluctuations, the orthodox tunneling theory is generalized by taking into account the electron spin. It is shown that the transition between consecutive charge states can occur via high-spin states, which significantly modifies the shape of Coulomb steps and gives rise to additional resonances at low temperatures.     

退火条件对Sn量子点生长和红外光学性质的影响

首次采用固相外延生长技术在Si(001)表面直接生长Sn量子点,并应用原子力显微镜(AFM)、X射线衍射(XRD)和同步辐射傅里叶红外光谱(FTIR)研究了退火条件对量子点样品的表面形貌、结晶性和红外光学性质的影响.AFM结果表明,随着退火温度的升高和退火时间的延长,量子点的平均尺寸变大,面密度减小.XRD结果显示,外延得到的Sn量子点为四方结构的β-Sn,与衬底的相对取向为Sn(110)//Si(001).由于β-Sn量子点的尺寸仍较大,同步辐射FTIR谱中没有观察到量子点的特征吸收峰.     

Spin Effects in Lateral Quantum Dots

We present the review of our work on spin effects in single lateral quantum dots with the emphasis on the results of Coulomb blockade spectroscopy studies. Realization of a spin-based quantum bit proposal in a lateral quantum dot is discussed. Described are the ways of isolating a single electron spin in a dot containing only one as well as many electrons. Demonstrated is a current readout of spin transitions in a dot by means of spin blockade spectroscopy due to spin polarized injection/detection mechanism in a lateral dot. Discussed are transitions induced both by changing a magnetic field and a number of electrons in a dot with the emphasis on the effects observed close to filling factor in a dot = 2.     

AgInS2和Ag—Zn—In—S量子点的制备及表征

采用热注入法成功制备出三元AgInS2和四元Ag—Zn—In—S量子点,物性测试得到AgInS2量子点的发射峰为701nm,Ag—Zn—In—S量子点的发射峰593nm,即Ag-Zn-In—S量子点的发射峰相对于AgInS2量子点发生了蓝移,AgInS2和Ag—Zn—In—S量子点都表现出了较长的荧光寿命,分别为169ns和162ns,结合生物组织光学窗口范围限制,相对Ag—Z—In—S,AgInS2量子点更适用于生物标记。     

半导体量子点玻璃的研究进展

介绍了半导体量子点材料禁阻类型,详细阐述了共熔法、溶胶-凝胶法、离子注入法等半导体量子点玻璃材料的制备方法,探讨了半导体量子点玻璃的尺寸效应、禁阻效应、库仑阻塞效应和非线性光学效应等特性及其未来应用前景.     

Electrical Manipulation of Spin Injection into a Single InAs Quantum Dots

We demonstrate spin injection from a n-Zn_0.96Mn_0.04Se layer into individual InAs quantum dots (SQDs) in a p–i–n diode structure using cw polarization resolved magneto-micro photoluminescence spectroscopy. Interestingly, we find that the spin injection efficiency strongly varies from dot to dot. We obtain a single quantum dot circular polarization degree ranging from 2% to almost 50% (at B=4 T) at zero biasing and within the spectral range studied here, we found 2 maxima of the degree of the circular polarization at SQD energies separated by ∼33 meV. Importantly, we demonstrate that the spin injection efficiency can be manipulated by external forward biasing (U _ext).     

水溶性量子点制备条件的优化及表面修饰

以L-半胱氨酸为稳定剂在水溶液中合成CdSe纳米粒子,研究了水浴时间、水浴温度、不同L-半胱氨酸/Cd/Se比例、pH值等因素对其荧光光谱的影响,确定了最佳的合成方案.用CdS对其表面进行修饰,采用透射电镜、X射线衍射、光谱法等表征了Cdse/CdS核壳结构颗粒的形成,结果表明该纳米粒子发光强度明显高于单一的CdSe量子点,光谱峰位置有所红移;合成条件会显著影响CASe/CAS核壳结构量子点的荧光性能.     

ZnO含量对熔融法制备PbSe量子点玻璃的影响

采用高温熔融－退火法在钠硼铝硅酸盐（SiO2-B2O3-Na2O-Al2O3-ZnO-AIF3-Na2O）玻璃中生长了PbSe量子点,通过X射线衍射（XRD）、透射电镜（TEM）、光致荧光（PL）谱等研究了玻璃配料中不同ZnO含量对PbSe量子点尺寸和浓度的影响,结果表明,ZnO含量占总玻璃配料质量比约9．4％时,生成的量子点尺寸比较均匀,直径约为6．5nm,且浓度较高,PL谱强度最强,辐射峰位于1790nm,FWHM为296nm。玻璃配料中加入适量的ZnO有助于PbSe量子点的形成,减少Se元素的挥发,使玻璃中的量子点尺寸分布趋于均-化。     

量子点量子阱ZnS/HgS/ZnS/CdS的制备与光学特性

运用湿化学与表面化学方法制备了ＺｎＳ／ＨｇＳ／ＺｎＳ／Ｃｄｓ量子点量子阱（ＱＤＱＷ）结构,以吸收光谱、光致发光及激光谱表征该结构,并研究了外层ＣｄＳ对材料发光特性的影响,首次观测到ＣｄＳ对中间ＨｇＳ阱层发光的增强作用,并归因子隧道效应的存在。     

Measure of Entanglement States of Two Interacting Electrons in Vertically Coupled Quantum Dots Induced by a Time-Dependent Electric Field

We study the dynamics of two electrons located in two vertically tunnel-coupled quantum dots in the presence of an oscillatory electric field. By solving the time-dependent Schrödinger equation, we predict the dynamical generation of entangled electron states, such as the EPR (Einstein, Podolsky, and Rosen) pairs or Bell states. The Schmidt rank and the von Neumann entropy are evaluated to characterize the degree of entanglement of the two electron states.     

All Optical Spin-Based Quantum Information Processing

We propose a spin-based ultra-fast laser driven implementation of quantum information processing based on the Pauli blocking effect in semiconductors, which acts as a spin dependent switching mechanism for auxiliary exciton states.     

结构对称性对双量子点干涉仪中量子输运的影响

采用非平衡格林函数方法求解量子输运过程,探讨结构对称性对双量子点干涉仪中量子输运的影响,结果表明,调节点一导线间耦合,导致双量子点干涉仪结构对称性和电子传输路径不同,使得电子隧穿并联双量子点结构呈现出一系列的新奇特性。当点一导线间的耦合强度不同,两量子点中阶梯状的平均电子占据数的分离程度不同,且两台阶的平缓程度也不同,证明了结构决定性能,也为设计可控量子器件提供一个理论依据。     

Strong Tunneling in Small Quantum Dots: Kondo Effect in Two Model Systems

We study electron transport through single and double quantumdots with large level spacing and charging energy. Motivatedby recent experiments we focus on linear and nonlinearresponse of two model systems: a single dot with two levelsand a capacitively coupled double dot. At low temperature andstrong coupling to the leads, quantum fluctuations and Kondo-like many-body effects become important and show up, e.g., asresonances in the current-voltage characteristics. Inparticular, we propose a way to observe the splitting of theKondo peak as a function of the applied bias voltage.     

Effects of Magnetic Quantum Dots on Magnetoconductance in Quantum Wires

We present a theoretical study of electronic transport in quantum wires (narrow two-dimensional electron gas) with array of magnetic quantum dots. Each magnetic quantum dot is defined by a small circular region where the strength of perpendicular magnetic field is modulated. By making use of a newly developed calculation method based on the gauge transformations, we calculated the conductance as a function of the external perpendicular magnetic field. Our numerical calculations show that the magnetoconductance is very sensitive to the number of magnetic quantum dots in the field region where the direction of the net magnetic field in dot regions is antiparallel to the external magnetic field.     

CdSe-ZnS QDs/PLGA纳米复合材料的性能及体外降解行为

采用溶液浇铸法制备了不同CdSe-ZnS量子点(Quantum dots QDs)含量的QDs/乳酸-乙醇酸共聚物(PLGA)复合材料薄膜,通过傅里叶变换红外光谱仪、透射电子显微镜(TEM)、光致发光光谱仪(PL)及紫外-可见吸收光谱仪(UV-Vis)等对薄膜的微观结构与光学性能进行了表征,并对不同降解时间后降解液的pH值、薄膜的发光情况和相对分子质量及其分布进行了测试。PL及UV-Vis结果显示,QDs/PLGA纳米复合材料的发光性能良好,吸光度随QDs含量增加而增大;TEM结果显示,QDs在基体中分散良好;在体外降解过程中,凝胶渗透色谱和缓冲液p H值测试结果说明,量子点催化了PLGA的降解;复合材料的荧光效应随着降解时间的延长逐渐减弱。以上结果说明,采用简单的溶液浇铸法制备的CdSe-ZnS QDs/PLGA复合材料发光性能稳定优异,且可以通过检测荧光效应变化以实现动态监测QDs/PLGA复合材料的降解进程。     

The Kondo Effect of Surface Excitons

A recombination radiation line of real excitons in dense two-dimensional electron gas at the [100] silicon surface is observed in luminescence spectra of metal-oxide-semiconductor (MOS) structures. A new effect of anisotropic paramagnetic reduction of the luminescence line indicates a strong influence of the Kondo correlations on electron paramagnetism of the excitons.     

量子点的制备技术及进展

量子点是一种三维受限的低维材料,由于其潜在的应用价值受到了人们的广泛关注.对近年来量子点的各种制备方法进行了总结和评述.     

量子点/环氧树脂复合材料的制备及性能

通过自组装技术将十二胺包覆到CdSe量子点表面实现量子点的氨基改性,并以三乙烯四胺为固化剂制备得到CdSe量子点/环氧树脂复合材料,研究了量子点表面氨基改性对复合材料性能的影响。通过高分辨透射电镜表征量子点的分散情况及粒径大小,X射线能谱表征量子点改性前后元素的变化,动态光散射表征量子点团簇在环氧基体中的粒度分布,紫外-可见光谱表征复合材料的透明性,荧光光谱表征复合材料的荧光性能,冲击试验表征量子点改性前后对复合材料的增韧作用。结果表明,氨基改性量子点/环氧树脂复合材料的透明度为原始量子点/环氧树脂复合材料的2倍,荧光强度为原始量子点/环氧树脂复合材料的4倍。当量子点含量为0.5%时,氨基改性量子点/环氧树脂复合材料的冲击强度达7.28 k J/m~2。