吴明卫
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COLLOQUIA & SEMINARS 2007:
(SEMINAR)"Eletric current induced by optically injected spin current: theoretical study and experimental consequence", Associate Prof. Dr. Shun-Qing Shen (Hongkong University), at 4:00 pm, Jan. 24, 2007.
Abstract:
GaN-based heterostructures are of special interests for the fabrication of high electron mobility transistors (HEMTs) for high power, high temperature and high frequency applications. Owing to the strong spontaneous and piezoelectric polarizations in III-nitrides, the polarization-induced electric field at an AlxGa1-xN/GaN heterointerface is as high as ~ MV/cm. This leads to a significant increase of the sheet density and the narrower confinement of the two-dimensional electron gas (2DEG) in an AlxGa1-xN/GaN heterostructure in comparison with that in an AlxGa1-xAs/GaAs one. Due to the much deeper triangular quantum well and the much higher 2DEG density at AlxGa1-xN/GaN heterointerfaces, the subband structures, the transport properties of the 2DEG and other properties in AlxGa1-xN/GaN heterostructures are probably different from those in AlxGa1-xAs/GaAs ones. On the other hand, GaN-based semiconductors are the potential candidates for spintronics because of the long spin relaxation time in these materials which can persist to room temperature. Although the spin-orbit coupling parameter is thought to be small in wide band-gap semiconductors, the strong polarization-induced electric field in AlxGa1-xN/GaN heterostructures may enhance the spin-orbit interactions. Therefore, the spin properties, especially the spin-orbit interaction, of the 2DEG in GaN-based heterostructures are necessary to be further understood for their potential application in spintronics. Magneto-transport measurement is powerful to understand the transport properties of the 2DEG in semiconductor heterostructures. In this study, based on the growth of AlxGa1-xN/GaN heterostructures of high quality by means of metal organic chemical vapor deposition (MOCVD), the fine subband structures and the 2DEG occupation, the magneto-intersubband scattering (MIS) effect of the 2DEG, the effective mass of the 2DEG, and the zero-field spin splitting of the 2DEG in AlxGa1-xN/GaN heterostructures have been studied in detailed. Meanwhile, the circular photo-galvanic effect (CPGE) induced by the spin-orbit interactions of the 2DEG in AlxGa1-xN/GaN heterostructures has also been investigated.
(SEMINAR)"Anomalous spin polarization in hole nanowires", Dr. Dan Csontos (Institute of Fundamental Sciences, Massey University, New Zealand), at 4:00 pm, March 16, 2007.
Abstract:
Hole semiconductor spintronics is very interesting due most diluted magnetic semiconductors being p-type. More importantly from a physics point of view, the spin-3/2 character of states in the topmost valence band of semiconductors can lead to intricate and sometimes counterintuitive quantum phenomena in hole nanostructures. Two examples are the strong anisotropies of the effective g-factors of two-dimensional hole gases [1], and hole quantum point contacts [2]. Confinement effects appear to drastically influence the spin polarization of hole systems and provide an avenue for hole spin manipulation.
In this work, we theoretically investigate the effects of confinement on spin splitting in cylindrical hole nanowires. An anomalous spin polarization is found to result from a complex interplay between heavy-hole-light-hole splitting, confinement, and symmetry. In particular, we find that the effective g-factors are fluctuating strongly between different wire levels. These findings are universal, i.e., radius-independent, for a given material. Strikingly, some levels have a vanishing g-factor, indicating total lack of polarization. Using an invariant expansion of the spin-3/2 hole density matrix [3], we have investigated in detail the origin of the fluctuations and the complete g-factor suppressions of some of the levels. We found intriguing radially dependent spin textures which directly correlate with the magnitude of the g-factors. Furthermore, we find very interesting dependence of the g-factors on the spin-orbit coupling strength in the valence band. This may open up avenues for hole g-factor manipulation, e.g., in heterostructured nanowires.
(SEMINAR)"Theories for quantum state engineering based on nano-mechanical and other solid state systems", Prof. Dr. C. P. Sun (ITP, CAS), at 4:00 pm, March 29, 2007.
(SEMINAR)"Variational construction of the fractionalized excitation in quantum antiferromagnet", Prof. Dr. T. Li (Renming University), at 3:30 pm, April 9, 2007.
Abstract:
Quantum number fractionalization, or more specifically, spin-charge separation, is believed to be responsible for the dramatic incoherent spectral weight in low dimensional quantum antiferrommagenet and its doped version. However, a physical characterization of such fractionalized degree of freedom is still absent except for the one dimensional case, in which they are understood as domain walls in the ground state manifold. This talk reflects our continual effort on the physical characteriztion of fractional excitation in higher than one dimension system in terms of explicit wave function. We find the slave particle subjected to the Gutzwiller projection provides a reasonable wave function for fractionalized excition in general dimension. I will first show how Gutzwiller projection transform the slave particles into topological objects that are equivalent to the antiphase domain walls in the one dimensioanl t-J model. Then I will discuss some interesting properties of the wave function constructed similarly for fractionalized excitations in two dimensional systems with various spin background.
(COLLOQUIUM)"Design and Fabrication of Novel MRAM Demo Device", Prof. Dr. X. F. Han (Institute of Physics, CAS), at 4:00 pm, May 14, 2007.
(COLLOQUIUM)"Option Pricing and Schrodinger Equation", Prof. Y. S. Li, (Department of Mathematics), at 4:00 pm, June 4, 2007.
(SEMINAR)"Ferrodynamics: a new dynamic state in magnetic nanostructures", Prof. Dr. S. F. Zhang (University of Missouri, USA), at 2:30 pm, June 18, 2007.
Abstract:
I will discuss the current research in current-driven magnetization dynamics in magnetic nanostructures. At a sufficiently large current density, the spin angular momentum transfer leads to magnetization reversal and stable magnetization precessional states. Here we focus on a novel state of affair: the current-driven dynamic phase transition in ferromagnetic materials.
(COLLOQUIUM)"Doped Oxides as Catalysts", Prof. Dr. H. Metiu (Department of Chemistry & Biochemistry, University of California, Santa Barbara, USA), at 10:30 am, June 25, 2007.
Abstract:
Oxides are used as catalysts for a large number of oxidation and oxidative dehydrogenation reactions. We use density functional to explore the catalytic properties of two classes of compounds. In one the cation M in the oxide MxOy is replaced with a metal N to form the doped oxide NzMx-zOy. Our intention is to determine to what extent the doped oxide is a better oxidation catalyst than the host oxide. We find that many dopants make the oxygen atoms at the surface of the oxide more ready to engage in oxidation reactions. The oxidation reaction forms an oxygen vacancy on the surface which has to be healed if the system is to be a catalyst. This happens if the system adsorbs oxygen at the vacancy site and activates it so that it will also take part in an oxidation reaction. One oxygen atom is taken away by the oxidation product and the other fills the oxygen vacancy. A good oxidation catalyst will have to reach a good compromise between these steps. We will discuss the activity of a large number of dopants of titania, ceria and zinc oxide. We also examine a system consisting of VOx clusters supported on various oxide surfaces, which can also be viewed as a doped oxide. If there is time we will discuss several issues related to the catalytic activity of very small gold clusters supported on rutile.
(SEMINAR)"Spin dynamics and spin diffusion process in semiconductor 2D structures", Associate Researcher Dr. Bao-Li Liu (Institute of Physics, CAS), at 4:30 pm, July 3, 2007.
Abstract:
The ability to manipulate the relaxation of spin population is a key step toward building a practical spintronics devices in semiconductor 2D structures. The spin-orbit coupling, which is controllable via the structural engineering or electrical field, is the main source of spin relaxation via DP spin relaxation mechanism. We will present our recent experimental results on the spin dynamics of electrons in GaAs/AlGaAs 2D structures by time-resolved Kerr rotation. The possibility of optical control of SO coupling will be also discussed in (001)-orientation 2DEG. Finally we will show the preliminary results on spin diffusion process by the transient spin grating technique in (110)QWs, in which the longer spin relaxation (~1ns) is observed at room temperature.
(COLLOQUIUM)"Investigation on solid state physics and the buildup of neutron source", Prof. Dr. Ding-Sheng Wang (Member of Chinese Academy of Sciences, Institute of Physics, CAS), at 4:00 pm, August 3, 2007.
(SEMINAR)"Spin dephasing in 2D semiconductor heterostructures", Dr. T. Korn (Institut für Experimentelle und Angewandte Physik, Universitaet Regensburg, GERMANY), at 10:00 am, August 4, 2007.
Abstract:
The study of spin dephasing and relaxation processes is at the heart of semiconductor spintronics research. In the talk, I will give a brief introduction into optical orientation of spins in 2D semiconductor heterostructures and the relevant spin dephasing mechanisms. Then I will report on our time-resovled studies of spin dephasing in different types of systems: (a) [110]-grown quantum wells, where the symmetry of the structure strongly influences the spin dephasing, and (b) high-mobility [001]-grown quantum wells. In the [001] QWs, interesting many-body effects were observed by us that had been theoretically predicted by Wu et al. : the spin dephasing strongly depends on the initial spin polarization, and the temperature-dependence for large initial spin polarization is different from that observed for small initial spin polarization. Additionally, a large in-plane spin dephasing anisotropy is present in the [001]-grown quantum wells, which stems from the presence of both Rashba and Dresselhaus spin-orbit fields. The experimental data are in excellent agreement with calculations by Wu et al., and using these calculations, a long in-plane spin lifetime of several nanoseconds is predicted for the [001] QW.
(SEMINAR)"Quantum-Size-Effects in Thin Film Evolution", Prof. Dr. C. Z. Wang (Ames Laboratory, USDOE and Department of Physics, Iowa State University, Ames, Iowa, USA), at 10:00 am, August 31, 2007.
Abstract:
Growth of Pb films on Si(111) surface at low temperatures has been found to exhibit unconventional behavior: Instead of forming three-dimensional (3D) islands of various sizes as commonly observed for nonreactive interfaces, the metal atoms arrange themselves into plateaus or islands of selective heights, with flat tops and steep edges. The extra stability of metal films with specific thickness has an electronic origin and can be explained by the ¡°quantum size effect¡± (QSE) due to electron confinement. Recent experiments also indicate that not only are these films structurally unconventional, their kinetic behavior is also fundamentally altered by quantum-size effects and does not obey traditional classical kinetic model predictions. These new experimental observations require the development of an entirely new theoretical model to describe the kinetics of the system. In this talk, I will review the experimental and theoretical studies on this interesting system. I will also discuss a theoretical model we recently developed and some large-scale quantum mechanics calculations we recently performed for understanding the kinetics of this unconventional growth behavior.
(SEMINAR)"Quantum Dynamics of a Strongly Coupled Single Quantum Dot-Cavity System", Prof. Dr. K. D. Zhu (Shanghai Jiaotong University), at 2:30 pm, Sept. 17, 2007.
(SEMINAR)"Inelastic effects on the transport properties of molecular junction", Assistant Prof. Dr. Yu-Chang Chen (National Chiao Tung University, Taiwan), at 4:00 pm, Sept. 17, 2007.
Abstract:
Understanding electron transport through a single molecular attached to two electrodes is a fundamental issue in molecular electronics. We investigate the effects of electron-phonon inelastic scattering on the transport properties in a single molecular junction in the regime of quasi-ballistic transport. Particularly, we will discuss the small structures occurred in the I-V characteristics and also the current-induced local heating in electrode-alkanethiol-electrode junction due to electrons scattered by the vibrations of molecule. We find that the inelastic profile displays an odd-even effect with chain length which compares well with experimental results. We also compare local temperatures of alkanethiols from theoretical calculations with the measurement from conducting Atomic Force Microscopy (C-AFM).
(COLLOQUIUM)"The scaling of time and the early universe", Emeritus Prof. Kelin Wang (USTC), at 4:00 pm, Sept. 23, 2007.
(COLLOQUIUM)"10-year swimming in the data pool of gene", Prof. Bailin Hao (Member of Chinese Academy of Sciences and the 3rd World Academy of Sciences, Fudan University), at 4:00 pm, Sept. 24, 2007.
(COLLOQUIUM)"Historical and Cultral Origin of Scientific Misconduct", Prof. Bai-Lin Hao (Member of Chinese Academy of Sciences and the 3rd World Academy of Sciences, Fudan University), at 4:00 pm, Sept. 27, 2007.
(SEMINAR)"Optical and Electrical Spin Injection in Semiconductor Quantum Dots", Prof. Dr. Xavier Marie (Laboratory of Physics and Chemistry of Nano-Objects, INSA-CNRS, FRANCE), at 4:00 pm, Oct. 19, 2007.
(SEMINAR)"Spin dependent recombination in dilute nitride semiconductors", Prof. Dr. Xavier Marie (Laboratory of Physics and Chemistry of Nano-Objects, INSA-CNRS, FRANCE), at 4:00 pm, Oct. 20, 2007.
(SEMINAR))"Quantum Manipulation of Semiconductor Quantum Dots and Gold Nanoparticles", Prof. Dr. Q. Q. Wang (Wuhan University), at 10:00 am, Oct. 30, 2007.
(SEMINAR)"Spherical crystallography", Prof. Dr. Ze-Xian Cao (Inst. of Phys., CAS), at 4:00 pm, Oct. 31, 2007.
Abstract:
A crystal in the infinite 3D space never can be perfect, to the very least, due to its limited size. On the right contrary, the spherical surface is a most popular manifold of limited extension nested in 3D space, which can be easily filled to suffocation. Consequently, the spherical crystallography differs in some very interesting ways from its counterpart for the flat spaces, and it can model a variety of natural structures as found in viruses, flowers, colloidal clusters, suspended condensates, fullerenes, etc. In this talk, I just want to present a collection of some raw data and my premature reflections upon the matter, an in-depth investigation of this topic calls for your mindful advices.
(SEMINAR)"Spin transport and spin-related charge transport", Prof. Dr. C. C. Xie (Oklahoma State Univ., USA), at 4:00 pm, Nov. 27, 2007.
Abstract:
We find that in order to completely describe the spin transport, apart from the spin current (or the linear spin current), one has to introduce the angular spin current. The two spin currents respectively describe the translational and rotational motion of a spin. Both spin current densities appear naturally in the spin continuity equation. Moreover we predict that the angular spin current, just like the linear spin current, can also induce an electric field. In the second topic, the spin-orbit coupling systems with a zero magnetic field is studied under the equilibrium situation, i.e., without a voltage bias. A persistent spin current is predicted to exist under most circumstances, although the persistent charge current and the spin accumulation are identically zero. In addition, various semiconductor spin devices using spin-orbit interaction are proposed.
(Ph. D. Dissertation Defense)"Spin dynamics and spin manipulation in confined semiconductor nanostructures", Mr. J. L. Cheng, [Committee Members: Prof. Liang-Mo Mei (Chair, Shandong Univ.); Prof. Dr. Jian-Guo Hou (Academician, USTC); Prof. Zhen-Ya Li (Suzhou Univ.); Prof. Rui-Bao Tao (Academician, Fudan Univ.); Prof. Dr. Zhen-Yu Weng (Tsinghua Univ.); Prof. Ding-Yu Xing (Academician-elect, Nanjing Univ.); Prof. Dr. Qi-Kun Xue (Academician, Tsinghua Univ.); Prof. Bang-Fen Zhu (Academician, Tsinghua Univ.)], at 9:00 am, Dec. 2, 2007.
(COLLOQUIUM))"Semiconductor spintronic devices", Prof. Dr. Michael Flatte (University of Iowa, USA), at 4:00 pm, Dec. 17, 2007.
Abstract:
I will describe some of the unusual physical properties of semiconductor spintronic materials, and the novel device functionalities that may be achieved with them. As one example, new, reprogrammable transistor structures are possible that rely on electric-field control of the magnetic properties of semiconductors. A comparison of the fundamental limits of performance of an individual spin transistor with an individual charge-based transistor finds that the spin transistor can have superior room-temperature performance. This includes a lower threshold voltage and lower capacitance than on the semiconductor roadmap for field effect transistors for logic through 2018. Some of the outstanding materials challenges remaining before these devices can be realized will be described.
(SEMINAR))"Optical and electrical manipulation of single spins in semiconductors", Prof. Dr. Michael Flatte (University of Iowa, USA), at 4:00 pm, Dec. 18, 2007.
Abstract:
High-speed, robust manipulation of individual spins is a key goal for quantum information processing and would provide new insight into quantum spin interactions in solids. Some recent scalable proposals will be described for electric-field or optical control of single spins that are either confined to quantum dots or bound to dopants. These proposals rely on indirect control of the spin orientation through the spin-orbit interaction of the host semiconductor.
