吴明卫
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COLLOQUIA & SEMINARS 2008:
(COLLOQUIUM)"Spintronics with bubbles: Spin splitting, polarisation, and geometric phase of quantum-confined holes", Associate Prof. Dr. Ulrich Zuelicke (Massey University, NEW ZEALAND), at 4:00 pm, April 3, 2008.
Abstract:
Experiments show, and quantum physics explains, the emergence of two fundamental charged quasi-particles in crystalline solids, called band electrons and holes. While a band electron is very similar to an electron in vacuum, a hole is more like the bubble in a spirit level, i.e., it is a void in the electron liquid! In addition to carrying charge, band electrons and holes behave like tiny permanent magnets due to their spin. Crystal structure renders the spin of band electrons and holes to be very different. Holes are spin-3/2 particles and subject to a strong coupling between their spin and orbital motion. When holes are confined to move in nanometre-size electric circuits, this spin-orbit coupling causes intriguing effects that are never seen in similar structures made with band electrons. We have studied theoretically the interplay between quantum confinement of holes in wires and rings and their spin properties. In the wires, we find novel spin-polarisation states realised at quasi-1D subband edges, which can be probed in transport experiments. In hole rings, we show that the quantum-interference contribution to the two-terminal conductance exhibits an energy-dependent Aharonov-Anandan phase that is unrelated to Rashba or Dresselhaus spin splittings. Instead, confinement-induced heavy-hole - light-hole mixing is found to be the origin of this phase, which has ramifications for magneto-transport measurements in gated hole rings.
(SEMINAR)"Nanospintronics meets relativistic quantum physics: Ubiquity of Zitterbewegung effects", Associate Prof. Dr. Ulrich Zuelicke (Massey University, NEW ZEALAND), at 4:00 pm, April 9, 2008.
Abstract:
Two-level systems are routinely represented in terms of pseudo-spin-1/2 degrees of freedom, and such pseudo-spins are usually considered to be entirely analogous to the real spin angular momentum. Here we consider time reversal (TR) of pseudo-spins from a completely general perspective and find that there exist two different types. One type behaves like ordinary spin, for which all of its three Cartesian components are odd under TR. In addition, a second type of pseudo-spin exists, behaving counter-intuitively in that only one Cartesian component is odd and the other two are even under TR. The second type is not merely of academic interest, as it is realised, eg, by the pseudo-spin representation of the 2D isotropic harmonic oscillator (Schwinger model of spin-1/2). We show that the sublattice-related pseudospin of quasi-relativistic charge carriers in graphene also belongs to the second type and discuss observable implications of this fact.
(COLLOQUIUM)"Optical manipulation of quantum-degenerate excitonic particles", Prof. Dr. Makoto Kuwata-Gonokami (University of Tokyo, JAPAN), at 4:00 pm, April 21, 2008.
(SEMINAR)"Experimental study of electron spin dynamics in low dimensional semiconductor materials", Researcher Dr. Yang Ji (Institute of Semiconductors, CAS), at 4:00 pm, April 25, 2008.
Abstract:
After giving a brief introduction about the experimental techniques--- time resolved polarized photoluminescence (TRPPL) and time resolved Kerr rotation (TRKR), I will talk about our experimental results about the electron spin dynamics in low dimensional materials: spin dynamics in sub-monolayer InAs embedded in GaAs buffer, coherent spin transfer in a GaAs/InGaAs quantum well and electron spin lifetime in a high mobility, low density 2D electron gas. In the last case, experimental evidence was found to support Wu's prediction of the effect of the electron-electron scattering on electron spin dephasing.
(SEMINAR)"Control of electron spin and orbital resonances in quantum dots through spin-orbit interactions", Dr. Peter Stano (Research Center for Quantum Information, Slovak Academy of Sciences, SLOVAKIA), at 4:00 pm, May 5, 2008.
Abstract:
The spin of an electron confined in a semiconductor quantum dot is very well isolated from the environment. That is why it is a perspective candidate for the realization of a quantum bit. However, the qubit isolation goes hand in hand with the manipulation being not easy and vice versa -- a qubit which is easy to manipulate is usualy hard to isolate from the environmental noise. I will discuss the isolation (quantified by the relaxation time) and the manipulation (by resonant electromagnetic field) of the electron spin qubit and show that both these are extremely sensitive to spin-orbit interaction. I will show how the anisotropy of the spin-orbit interaction can be exploited with the goal to keep the relaxation low and the manipulating field influence high.
(SEMINAR)"Relaxation, dephasing and decoherence in quantum dot spin qubit", Dr. Peter Stano (Research Center for Quantum Information, Slovak Academy of Sciences, SLOVAKIA), at 4:00 pm, May 8, 2008.
Abstract:
The spin of an electron confined in a semiconductor quantum dot is very well isolated from the environment. That is why it is a perspective candidate for the realization of a quantum bit. The dominant source of environmental noise for such a qubit is its interaction with the nuclear spins of surrounding atoms. It will be shown how the spin-spin interaction results in: 1. the relaxation (of the electron spin), that is the loss of memory about the initial electron energy, and 2. the decoherence, that is the loss of memory about the initial phase. Recent experiments will be discussed, which demonstrate that some part (99.9%) of the "decoherence" is revertible (and this part is called dephasing). The reason for this is a very long phase coherence of the environment itself.
(SEMINAR)"Theoretical investigation of basic magnetism of films and polycrystal materials", Prof. H. Y. Wang (Physics Department, Tsinghua University), at 4:00 pm, June 5, 2008.
Abstract:
A general formula for calculation of three-component magnetization of applicable to any spin quantum number is derived. Then, magnetizing of antiferromagnetic films is studied. Under a transverse field, even above Neel point, the magnetization component along the easy-axis of the sublattice is not zero. The physical reason of this phenomenon is pointed out. The magnetizing curves and the hysteresis loops of a ferromagnetic polycrystal sample are calculated. The anomalous hysteresis loops of antiferromagnetically coupled ferromagnetic bilayers are investigated. We also study the mechanism of the exchange bias (EB) of FM/AFM bilayers in both uncompensated and compensate cases. The EB mechanism of the compensate case is analyzed.
(SEMINAR)"LDA+Gutzwiller Method for Correlated Electron Systems", Researcher Dr. Xi Dai (Inst. of Phys., CAS), at 4:00 pm, June 13, 2008.
Abstract:
Combining the density functional theory (DFT) and the Gutzwiller variational approach, a LDA+Gutzwiller method is developed to treat the correlated electron systems from ab-initio. All variational parameters are self-consistently determined from total energy minimization. The method is computationally cheaper, yet the quasi-particle spectrum is well described through kinetic energy renormalization. It can be applied equally to the systems from weakly correlated metals to strongly correlated insulators. The calculated results for SrVO3, Fe, Ni and NaxCoO2, show dramatic improvement over LDA and LDA+U.
(SEMINAR)"Spin torques in Magnetic Tunnel Junctions", Prof. Dr. S. F. Zhang (University of Arizona, USA), at 4:00 pm, June 23, 2008.
Abstract:
There are quite numbers of spin-dependent phenomena developed in the last several years for transition-metal ferromagnetic metals. Among them, current-driven spin torques, spin pumping, spin electromotive force, and inhomogeneous spin damping have received considerable attention. We show that these seemingly different phenomena can be understood and formulated within a single scheme. New predictions such as inhomogeneous damping tensors are explicitly made.
(SEMINAR)"Nonlocal Magnetization Dynamics in Magnetic Heterostructures", Prof. Dr. J. Q. Xiao (University of Delaware, USA), at 4:00 pm, June 24, 2008.
(SEMINAR)"Spin-orbit coupling field at ferromagnetic metal/semiconductor interfaces", Prof. Dr. J. Fabian (University of Regensburg, GERMANY), at 4:00 pm, September 16, 2008.
Abstract:
Spin-orbit coupling in condensed matter physics comes in many different forms. In solids with a center of inversion, the coupling manifests itself as an additional (to Kramers) degeneracy, with spin-modulated Bloch states. In systems lacking spatial inversion, the coupling appears as an effective, momentum dependent magnetic field. Most prominent examples are the Dresselhaus and Bychkov-Rashna spin-orbit Hamiltonians in zinc-blende heterostructures. What happens at metal/semiconductor interfaces is virtually unknown. We have performed ab initio calculations for the important case of Fe/GaAs interface to show that the usual Dresselhaus and Bychkov-Rashba description are valid also here, close to the k=0 point, while at larger momenta the spin-orbit coupling fields assume more exotic forms. Theoretical background as well as experimental ramifications of the discovered spin-orbit fields will be discussed.
(SEMINAR)"Spatial spin dynamics and polarization in ballistic quantum devices", Prof. Dr. Sam Yong Cho (Chongqing University), at 4:00 pm, Octobter 8, 2008.
Abstract:
Recent advances in nanotechnology have made it possible to transport electron spin coherently over hundreds of micrometers at low temperature. It may enable the realization of quantum spintronics by manipulating the spin degree of freedom of electrons without destroying their phase coherence. The definitions of spin orientation and polarization vectors are introduced within the particle density matrix of scattering states in leads. It is shown that spin-density vector can be defined by the product of the spin orientation vector, being a unit direction vector, and the charge density, corresponding to the amplitude of the spin-density vector, experimentally observable by a spatial charge modulation measurement. When an electron transports through a ballistic semiconductor nanostructure, due to quantum interference of two spin eigenmodes, the electron spin generally undergoes nutation on its precession around the effective magnetic field resulting from spin-orbit interactions. The nutation of electron spin is found to be crucial for spin polarization in the quantum transport. When one of two spin-dependent channels in leads is evanescent, electron spin is shown to be fully polarized for distance from the interface larger than the spin precession length.
(SEMINAR)"Superconductivity and phase diagram in Fe-based high-Tc superconductors", Prof. Dr. X. H. Chen (physics Department), at 4:00 pm, October 27, 2008.
Abstract:
We will talk about the discovery of superconductivity with Tc higher than 40 K in Fe-based superconductors. In this talk, we show you the transport properties (resistivity, Hall coefficient, TEP ) in normal state. These results suggest a quantum phase transition around x=0.14 in SmFeAsO1-xFx system. A electronic phase diagram is proposed. Anisotropy in resistivity and susceptibility is also discussed in single crystals BaFe2As2 and CaFe2As2. The phase diagrams of Ba1-xKxFe2As2 and BaFe2-xCoxAs2 system and isotope effect are also discussed.
(SEMINAR)"Superconductivity and spin-density-wave instability in FeAs-based systems", Researcher Dr. N. L. Wang (Inst. of Phys., CAS), at 2:00 pm, October 28, 2008.
(SEMINAR)"Frustrated Magnetism and Heavy Fermi Liquid in Rare-Earth Iron Pnictides", Prof. Dr. Jian-Hui Dai (Zhejiang University), at 4:00 pm, October 29, 2008.
Abstract:
The recent discovery of iron-based superconductors has triggered intense interests in the homolo- gous rare-earth iron pnictides. These materials have the same layered structure and exhibit a number of exotic magnetic phases in addition to the unconventional superconductivity. Here we discuss the common origin of the ordered or disordered magnetic phases in the parent compounds based on an extended Anderson lattice model where hybridizations of the pnictogen 4p-orbital with both the iron 3d-orbital and the rare-earth 4f-orbital are taken into account. We show that in the strong correlation limit the model exhibits a striped antiferromagnetic long-range order for the d-electrons of irons due to the geometric frustration bridged by pnictogen atoms. The magnetic moment of the d-electrons deceases with increasing itinerancy (or decreasing correlation) of d-electrons, resulting in a quantum phase transition with a peculiar quantum criticality different to the one induced by electron doping. Associated with this transition is a heavy fermion instability driven by the hy- bridization between pnictogen 4p-orbital and rare-earth 4f-orbital. We discuss the relevance of this hybridization to several homogous rare-earth iron pnictides including CeOFeAs and CeOFeP.
(COLLOQUIUM)"Origin of the checkerboard phases in KxC60 fullerides and high Tc cuprates revealed by STM", Prof. Dr. Ya-Yu Wang (Tsinghua University), at 4:00 pm, November 11, 2008.
Abstract:
ne of the main themes in condensed matter research is to unravel the mechanisms of various phases and phase transitions in novel quantum materials. Scanning Tunneling Microscopy (STM) has been a powerful technique in this aspect owing to its ability to map out the atomic scale real space structure and electronic structure simultaneously. In this talk I will discuss two distinct systems: monolayers of KxC60 fullerides and the cuprate high Tc superconductors, where a checkerboard-like pattern has been observed. In KxC60, we demonstrate that the metal-insulator transition from K3C60 to K4C60 is caused by intra-molecular Jahn-Teller effect, and the accompanying checkerboard structure is due to inter-molecular electron hopping. In the cuprates, our results suggest that the checkerboard most likely originate from the charge-density-wave (CDW) formation in the anti-nodal region of the Fermi surface. The possible implications to the pseudogap phase and superconductivity will be discussed. These results illustrate the intricate interplay between the electronic, orientational, and vibronic (lattice) degrees of freedom in novel low dimensional materials.
(COLLOQUIUM)"Kondo resonant phenomenon in low-dimensional correlated electron systems", Prof. Dr. G. M. Zhang (Tsinghua University), at 4:00 pm, November 14, 2008.
Abstract:
In this talk, I will offer a comprehensive discussions on Kondo effect, Kondo problem, and Kondo resonance in the conventional metals (alloys) with dilute magnetic impurities. In particular, the various important ideas in the scaling theory and numerical renormalization-group theory for this problem will be introduced, and then a clear and complete physical picture about the Kondo phenomenon is provided. Moreover, various realizations in low dimensional correlated electron systems are going to be discussed fully.
(SEMINAR)"Epitaxial graphene for nanoelectronics", Dr. Xiaosong Wu (Georgia Institute of Technology, USA), at 4:00 pm, November 20, 2008.
Abstract:
The development of solid-state electronics follows the famous Moore's Law. However, a continuation of Moore's Law demands new materials and even a new paradigm. Graphene, though discovered recently, has been envisioned as a material for next generation electronics owing to its extraordinary properties. Although large portion of studies have been focused on exfoliated graphene, epitaxially grown graphene is seen as "the only viable route towards electronic applications". In this talk, I will discuss the phase coherence phenomena in epitaxial graphene, where we found the evidence for the chiral nature of band electrons. Then, I will show our recent work on making an epitaxial-graphene/graphene-oxide junction and its electrical properties. I will discuss why making such devices is an essential step towards epitaxial graphene electronics.
(SEMINAR)"Quantum Zeno and Anti-Zeno Effect: without Rotating Wave Approximation", Prof. Dr. Hang Zheng (Shanghai Jiao-Tong University), at 4:00 pm, November 24, 2008.
Abstract:
The quantum Zeno effect in an atomic system relates to slowing down of the decay process under frequent observation. All the previous studies on this interesting effect have been carried out in the so-called rotating wave approximation (RWA). According to these studies the time interval to experimentally observe the quantum Zeno effect in the decay from an atom, such as hydrogen, needs to be less than a femotosecond. However, for such short time interval, the RWA is no longer valid.
We address the question: What is the effect of the counter-rotating terms on the dynamical evolution of the atom at short time, particularly on quantum Zeno and anti-Zeno effects? It is well-known that the contribution of the counter-rotating terms is insignificant in the calculation of the decay rate for the excited state of an atom in the long time limit.? We find the surprising result that the counter-rotating terms can have great impact on the short time evolution of the population of the excited level, and thus on the quantum Zeno and anti-Zeno effects. We present the analytical study for the multi-level atom coupled to the electromagnetic field in free space, obtain simultaneously the electron self-energy, Lamb shift and the short-time dynamics. For the hydrogen atom, we calculate the effective decay rate and find that, because of the counter-rotating terms, the Zeno time is longer by two orders of magnitude than that obtained with rotating-wave approximation, and there is no anti-Zeno effect. Consequently the experimental measurement of the quantum Zeno effect may be much easier than what was determined with the rotating-wave approximation results.
(SEMINAR)"Quantum Phase Transitions: Fidelity and Tensor Network Algorithms", Prof. Dr. Huan-Qiang Zhou (Chongqing University), at 4:00 pm, December 1, 2008.
Abstract:
The recent work on fidelity approach to quantum phase transitions in the context of tensor network algorithms is summarised.
First, the fidelity, a basic notion of quantum information science, may be used to characterize quantum phase transitions, regardless of what type of internal order is present in quantum many-body states. If the fidelity of two given states vanishes, then there are two cases: (1) they are in the same phase if the distinguishability results from irrelevant local information; or (2) they are in different phases if the distinguishability results from relevant long-distance information. The irrelevant and relevant information are quantified, which allows us to identify unstable and stable fixed points (in the sense of renormalization group theory).
Second, for any D-dimensional quantum lattice system, the fidelity between two ground state many-body wave functions is mapped onto the partition function of a D-dimensional classical statistical vertex lattice model with the same lattice geometry. The fidelity per lattice site, analogous to the free energy per site, is well defined in the thermodynamic limit and can be used to characterize the phase diagram of the model. Remarkably, the fidelity per lattice site may be computed in the context of tensor network algorithms,.
Third, the tensor network algorithms are exploited to takle a few condensed matter systems, especially the two-dimensional quantum Ising model with transverse and parallel magnetic fields, the quantum two-dimensional anisotropic spin 1/2 antiferromagnetic XYX model in an external magnetic field and the two-dimensional quantum anisotropic compass model.
(SEMINAR)"Spintronics: from spins to devices", Dr. Jiang Xiao (Kavli Institute of NanoScience, Delft University of Technology, The NETHERLANDS), at 4:00 pm, December 4, 2008.
Abstract:
About two decades ago, spintronics appeared on horizon marked by the discovery of giant-magnetoresistance (GMR), where the current flow is affected by the magnetization configuration. Just over one decade ago, the frontier of spintronics was pushed further by the prediction of spin-transfer effect, where the magnetization configuration is affected by the current flow. The current-induced spin-transfer torque can excite magnetization dynamics and even magnetization reversal in magnetic heterostructures. Inversely, we also found that a magnetization dynamics can induced a current flow due to the spin and charge pumping effect. When at thermal equilibrium without bias voltage, the electric noise in a magnetic heterostructure is found to be fundamentally different from that in a normal conductor. In normal conductors, there is only one noise source: thermal agitation of charge carriers, which causes the white Johnson-Nyquist noise. However, in magnetic heterostructures, there is another independent noise source: thermal agitation of the magnetization, which also contributes to the electric noise by spin and charge pumping effect. The noise power spectrum from the latter consists of two absorption lines at zero frequency and at the ferromagnetic resonance frequency on top of a white noise background. The relative intensities depend on the magnetization configuration.
(SEMINAR)"Spin photocurrent and spin relaxation processes in some III-V qunatum wells", Associate Prof. Dr. C. L. Yang (Zhongshan University), at 4:00 pm, December 9, 2008.
Abstract:
This talk consists of the following two parts of experimental work on spin related phenomena in some III-V quantum wells.
The first part is to demonstrate the spin driven photocurrent in a InGaAs/InAlAs two dimensional electron gas system by using the spin-orbit coupling. Conversely, the current will also induce in-plane spin polarization perpendicular to the current, which can be measured by Kerr rotation.
The second part is about the spin relaxation process in InAs/GaAs quantum wells and GaAsN/GaAs quantum wells. Significant enhancement of the spin lifetime in InAs submonolayer will be shown, which is interpreted as the suppressed DP spin relaxation process due to the lateral constriction. The spin lifetime in GaAsN/GaAs quantum wells will be systematically shown with the well width dependence, the nitrogen composition dependence, the pumping density dependence and the temperature dependence.
(COLLOQUIUM)"Property characterization and investigation of materials at nano-scale", Prof. Dr. Ze Zhang (Member of Chinese Academy of Sciences, Beijing University of Technology), at 4:00 pm, December 26, 2008.
Abstract:
In this talk I shall report on some of our recent progress in physical property measurements and corresponding structural characterization, at atomic resolution, of individual nano-materials under simultaneous nano-manipulation from applied fields.
First I shall introduce some innovative methods in measurements of new physical phenomena result from one dimensional quantum confinement of ultra thin metal on semiconductor films. With their innovative sample preparing method and STM analysis, our project colleagues have made a systematic study on superconductivity, surface activity, selective nucleation, Kondou effects, est., deduced from the variation of density of states near by the Fermi surface.
Then I shall present some new methods on in-situ observation, with atomic resolution, of structural evolution of nano-wires under simultaneous nano-manipulation in high resolution transmission electron microscopes. By employing this self-developed new method, we can conduct a continues tensile or bending of one dimensional nanowires and found, for the first time, the brittle-ductile transition at room temperature and extremely large plasticity behavior of both Si- and SiC-nanowires(NWs). In-situ electron microscopy study shows that the unusual large plasticity of Si- and SiC-NMs results from a strain-induced structural evolution process starting from defect generation, aggregation, and finally amorphouzation at the strain concentration region of the NWs. It is not only the first time to report on experimental observation of unusual large plasticity of Si- and SiC-NWs, but also the first to show a new phenomena for amorphous materials with large tensile plasticity from covalent nano-materials.
A special interest of my talk shall focus on the new characterization and investigation methods for one dimensional nanowires and two dimensional nano materials as ultra thin films. Some new characterization methods related to the electron energy loss spectrum and others shall also be introduced.
