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COLLOQUIA & SEMINARS 2006:
 

1. (SEMINAR)"Antiresonance in electron transport through multiple quantum dot structures", Prof. Dr. Y. S. Zheng (Jilin University), at 4:00 pm, Mar. 20, 2006.

1. Abstract:
   It is well-known that electronic transport through quantum dot presents discrete peak structures in the conductance spectrum due to the resonant tunnelling. In contrast, the quantum interference between different Feynman paths in some multiple QD structures can result in a complete vanishing of the electron tunnelling, which is named as the antiresonance. We have recently studied the antiresonance phenomenon in the electron transport though three multiple QD structures, i.e. a QD chain, a QD ring and a double-QD chain. Some interesting results about the antiresonance are found, for example, the sudden resonance-antiresonance transition, the decoupling effect and a well-defined insulating band. These results are instructive for the potential device application. In this talk, I will show the details of our investigation on the antiresonance.

2. (SEMINAR)"Fano Resonance in the Anderson Impurity Systems", Associate Prof. Dr. H. G. Luo (Institute of Theoretical Physics, CAS), at 4:00 pm, March 30, 2006.

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3. (COLLOQUIUM)"Magnetic-Tunnel-Junction Materials, Physics and Prototype Devices", Prof. Dr. X. F. Han (Institute of Physics, CAS), at 4:00 pm, April 14, 2006.

1. Abstract:
   High-temperature superconductors are layered materials. The interlayer transport provides a unique and susceptive probe to the superconducting mechanism since it is controlled by excitations with momenta for which the superconducting gap is maximal. I will show that the interlayer dynamics is governed entirely by the pseudogap energy and the c-axis resistivity satisfies a universal scaling law in the pseudogap phase. It suggests that the superconducting pairing is intimately connected with the pseudogap and the interlayer hopping is coherent.

4. (SEMINAR)"Universal scaling behavior of the c-axis resistivity of high-temperature superconductors", Prof. Dr. T. Xiang (Institute of Theoretical Physics, CAS), at 4:00 pm, April 17, 2006.
   

5. (SEMINAR)"Properties of Conducting-Polymer Nanotubes", Prof. Z. J. Chen (Institute of Physics, CAS), at 4:00 pm, April 20, 2006.
   

6. (SEMINAR)"Colloidal dispersions: effective interaction, structure and dynamics", Prof. Dr. H. R. Ma (Institute of Theoretical Physics, Shanghai Jiao Tong University), at 4:00 pm, May 12, 2006.

1. Abstract:
   Colloidal dispersions are solid particles dispersed in continuous liquids. The particles may carry electric charge or neutral. The interaction, structure and dynamics of this system are very complicate and contains many interesting physics. In this talk I will give the current understandings of effective interactions between colloid particles, the liquid structure and phase transitions of colloids and, finally the short time and long time dynamics of colloids. Emphases will be given to the Monte Carlo evaluation of effective interactions, the pair distribution function and its calculation and experiment measurement, and the important effects of hydrodynamic interactions on the diffusion of colloid particles.

7. (SEMINAR)"Spin battery and spin current", Prof. Dr. D. Y. Xing (Nanjing University), at 4:00 pm, May 27, 2006.
   

8. (SEMINAR)"Unrestricted and Finite Temperature Renormalized Mean field Theory", Prof. Dr. Q. H. Wang (Nanjing University), at 2:00 pm, May 29, 2006.

1. Abstract:
   We developed a variational mean field theory for strongly correlated electron systems. The theory handles the Mott physics by Gutzwiller projection, and is applicable for unrestricted spin-, charge- and superconductong orders at finite temperatures. This is an important extension to earlier theories that deals with homogeneous and ground state properties. As applications of the theory, we discuss some important issues in high-Tc superconductors. First, we discuss the gap inhomogeneities and the characterist >ics of quasiparticle excitations. Second, we discuss the pseudogap temperature, the superfluid density as a function of doping and temperature, and by combining the KT physics we discuss the superconducting phase diagram.

9. (SEMINAR)"Magnetism and superconductivity in layered triangular lattice NaxCoO2 and LixNbO2", Prof. Dr. J. L. Luo (Institute of Physics, CAS), at 4:00 pm, May 31, 2006.
   

10. (SEMINAR)"Superfluidity in Fermionic Systems: from High Tc Superconductors to Ultralow Temperature Fermi Gases", Dr. Q. J. Chen (James Franck Institute, University of Chicago, USA), at 4:00 pm, June 12, 2006.

1. Abstract:
   Exciting new developments in the field of ultracold atoms make it possible to tune the two-body attractive interactions between fermionic atoms continuously from very weak to very strong; this not only changes the statistics from fermionic to bosonic but also bears on the nature of the superfluidity. In this talk, I discuss recent experiments which elucidate the nature of the superfluid phase as the interaction is continuously tuned via Feshbach resonances. These experiments are interpreted as displaying a "crossover" between BCS superfluidity and Bose-Einstein condensation. Of particular interest is the intermediate or crossover regime where the s-wave scattering length diverges. This regime provides a prototype for studying both high temperature superconductors and also strongly interacting Fermi gases which are also of interest to nuclear and astro-physicists. I will report recent progress in understanding superfluidity in these two condensed matter systems (cold atomic Fermi gases and high Tc superconductors) with particular emphasis on the former.

11. (COLLOQUIUM & SEMINAR)"Quantum Phase Transitions and Strongly Correlated Electrons", Prof. Dr. Qi-Miao Si (Department of Physics and Astronomy, Rice University, USA), at 2:30-3:30 pm (Colloquium) and 4:00 pm-5:00 pm (Seminar), July 3, 2006.

1. Abstract:
   A quantum critical point occurs in a material as it undergoes a smooth transition from one ground state to another at absolute zero. It has been recognized as a mechanism for both unconventional superconductivity and non-Fermi liquid behavior. In this talk, I will review the basic aspects of quantum criticality and summarize some of the developments over the past decade or so. I will in particular consider the magnetic heavy fermion metals, in which extensive recent progresses have been made. Considerable experimental evidence has emerged for the theory of local quantum criticality, which invokes certain quantum entanglement effects beyond the order-parameter fluctuations. I will also briefly survey the relevant issues in a number of other strongly correlated electron systems -- including high temperature superconductors, quantum frustrated magnets, quantum nanostructures -- and in ultracold atoms in optical lattices.

12. (SEMINAR)"Energy and spin relaxation of optically excited electrons in metals and semiconductors", Prof. Dr. M. Aeschlimann (Fachbereich Physik, Technische Universitaet Kaiserslautern, GERMANY), at 10:00 am, August 24, 2006.

1. Abstract:
   The lifetime of excited electrons in metals and semiconductors has been investigated by means of the time- and spin-resolved two-photon photoemission spectroscopy (TR-2PPE). For ferromagnetic materials, these studies have proven the importance of screening effects and the spin-dependent electron density of state with respect to the different electronic relaxation channels. In addition, an important aspect of the dynamics of hot electron damping is the role of spin-flip processes including Stoner excitations and spin-wave emission. In semiconductors we have exploited the possibility of optical excitation of spin polarized electrons using circularly polarized light of an ultrashort pulsed laser system. We have determined the temporal evolution of the energy, momentum and spin behavior of the excited electrons on a femtosecond timescale on a GaAs/metal interface and compared our results with equivalent Faraday rotation measurements.

13. (SEMINAR)"Spin lifetimes at GaAs (100) and (110) surfaces", Prof. Dr. M. Aeschlimann (Fachbereich Physik, Technische Universitaet Kaiserslautern, GERMANY), at 3:00 pm, August 24, 2006.

1. Abstract:
   The development of novel magnetotransport devices has raised interest in the spindependent dynamics of electron scattering and relaxation mechanisms in semiconductor/metal interfaces. Spin transport through a semiconductor/metal interface is still one of the most important obstacle in semiconductor-based spintronics. At many semiconductor surfaces the Fermi-level pinning causes a band bending which significantly affects the carrier and spin dynamics. In addition to pure surface effects, contribution to a different spin polarization decay time might be caused by the increased kinetic energy of the electrons in the band bent region. Due to the large penetration depth of light the carrier as well the spin dynamics in the band bent region can not be probed by pure optical methods such as time-resolved Faraday or Kerr rotation or photoluminecence. We investigate the time evolution of the spin polarization by means of time- and spin-resolved two photon photoemission (TR-2PPE) for different surfaces of the GaAs crystal. High surface and energy sensitivity of TR-2PPE make it ideal for studying the spin dynamics in the Scottky-barrier. We will present measurements on the GaAs(001) and GaAs(110) surface and discuss our results with equivalent bulk measurements (TR Faraday rotation) and theoretical studies.

14. (SEMINAR)"Spin current generation and spin accumulation in 2DEG semiconducting structures", Prof. Dr. C. S. Chu (Taiwan Jiao-Tong University), at 4:00 pm, September 6, 2006.

1. Abstract:
   Two of the major objectives in spintronics are to generate spin current and to generate spin accumulations. In this talk, we will talk about generating dc spin current in a quantum channel. We will also talk about the non- equilibrium spin accumulation set up by a driving electric field, both in a diffusive 2D stripe and in the ballistic range around a point-like scatterer.

15. (COLLOQUIUM)"Physics of single quantum dot as elemental building block for quantum information processing", Prof. Dr. H. Z. Zheng (Member of Chinese Academy of Sciences, Institite of Semiconductors, CAS), at 4:00 pm, September 12, 2006.
    

16. (COLLOQUIUM)"What do THz signal generation, nano devices and state equations have in common?", Prof. Dr. Viktor Krozer (Technical University of Denmark, DENMARK), at 2:30 pm, September 22, 2006.

1. Abstract:
   The talk will present the opportunities and the deficiencies of THz electronics. Especially, signal generation will be discussed, which is currently on of the limiting factor in widespread implementation of THz technology and electronics. The new star on the firmament is believed to be nanotechnology including nanotubes. But generating a strong signal from a tiny device is very difficult. Coupling of such devices can be represented by state equations. We will discuss some of the implications this creates.

17. (SEMINAR)"Spin-polarized transport properties in multilayered ferromagnetic semiconductor nanostructures", Prof. Dr. I. C. da Cunha Lima (Instituto de Fisica, Universidade do Estado do Rio de Janeiro, Brazil) at 4:00 pm, October 12, 2006.

1. Abstract:
   Parabolic confinement potential can be built in semiconductor alloys structures by properly choosing the composition during the growth process. In this sense, wide parabolic quantum wells of Ga1\u2212xAlxAs have been built [1] keeping the Al composition below 35%. This system has the property that the effective g-factor, since it depends on the Al com- position, changes with position in the growth direction. Recently, an exact solution was obtained to treat this case under an external tilted magnetic field, for n-type Ga1\u2212xAlxAs [2]. In the present calculation we apply a transversal electric field in order to modulate the effect of the g-factor. We show that even in this case we find an exact solution to the problem. We explore the consequences of the electric field in the electronic properties of the carriers.
   [1] G. M. Gusev, A. A. Quivy, T. E. Lamas, J. R. Leite, A. K. Bakarov, A. I. Toporov, O. Estibals, and J. C. Portal, Phys. Rev. B 65, 205316 (2002); G. M. Gusev, A. A. Quivy, T. E. Lamas, J. R. Leite, O. Estibals, and J. C. Portal, ibid 67, 155313 (2003).
   [2] I. C. da Cunha Lima, G. M. Gusev, J. R. Leite, J. Supercond. 18, 169 (2005).

18. (SEMINAR)"DNA-a conductor, semiconductor or insulator?", Prof. Dr. S. J. Xie (School of Physics and Microelectronics, Shandong University), at 4:00 pm, October 20, 2006.
    

19. (COLLOQUIUM)"Toward Quantitative Analysis of Nanoelectronics: Status and Challenges", Prof. Dr. H. Guo (Center for the Physics of Materials and Department of Physics, McGill University, Canada) at 4:00 pm, October 30, 2006.

1. Abstract:
   One of the important branches of nanotechnology research is the nano-scale electronics. Nanoelectronic devices operate by the principle of quantum mechanics, their properties are closely related to their atomic structure. It has been a theoretical challenge to calculate device characteristics including relevant microscopic details, especially when one wishes to predict these characteristics without using any phenomenological parameter.
   In this talk, I will speak about the present status of nanoelectronic device theory, the existing theoretical difficulties, and some important problems. I will then report an useful progress we have achieved toward quantitative predictions of non-equilibrium and non-linear charge/spin quantum transport in nanoelectronic devices from atomic point of view. I will give several examples of calculating spin polarized quantum transport in magnetic nanostructures including a very new topic of graphene spintronics. I will end the talk by outlining some existing challenges for developing tools powerful enough for nanoelectronics design automation.