点击次数：

COLLOQUIA & SEMINARS 2016:

1. (SEMINAR)"Weyl semi-metal: a new topological state in condensed matter", Researcher Dr. Xi Dai (Institute of Physics, CAS), at 4:00 pm, April 14, 2016.

1. Abstract:
   Weyl semi-metal (WSM) is a new topological state found recently in condensed matter. In this talk I will introduce the nonmagnetic WSM material TaAs family. From the first principle calculation, we find twelve pairs of Weyl points in the whole Brillouin zone (BZ) of these materials. In the absence of spin-orbit coupling (SOC), band inversions in mirror invariant planes lead to gapless nodal rings in the energy-momentum dispersion. The strong SOC in these materials then opens full gaps in the mirror planes, generating nonzero mirror Chern numbers and Weyl points off the mirror planes. I will also introduce the main physical properties associated with such exotic quantum state including the Fermi arcs on the surface, the negative MR induced by chiral anomaly and the chiral magnetic effect.

2. (SEMINAR)"Understanding Charge transport mechanisms in Organic Materials", Prof. Dr. Zhi-Gang Shuai (Tsinghua University), at 10:00 am, April 20, 2016.

1. Abstract:
   We present our recent work on the understanding of the charge transport mechanism for organic optoelectronic materials, which has been under controversy. Semiclassical Marcus theory has been extremely popular and insightful for molecular design since Br¨¦das and coworkers. However, the experimental carrier mobility value of organic semiconductors increase rapidly in recent years to well exceed the theoretical limit based on Marcus theory, calling for improvement in computational method beyond the localized semiclassical limit. On the other hand, bandlike transport behavior has been observed for some ultra-pure and closely-packed organic single crystals. We discuss about the effects of charge localization versus delocalization, quantum nuclear tunneling, dynamic disorder, bandlike versus impurity scattering, as well as superexchange effect in D-A mixed stacked crystal.

3. (SEMINAR)"Fidelity Mechanics: Analogues of four thermodynamic laws and Landauer's principle", Prof. Dr. Huan-Qiang Zhou (Chongqing University), at 4:00 pm, May 6, 2016.

1. Abstract:
   Fidelity mechanics, a scheme to investigate critical phenomena in quantum many-body physics, is formulated as an analogue of four laws in thermodynamics and black hole mechanics, thus unveiling a formal connection between critical points and black holes.
   Rich physics is unveiled even for simple, or say, prototypical models in quantum many-body systems, such as one-dimensional quantum XY model and quantum transverse Ising chain in a longitudinal field. This enables us to clarify a confusing point for the so-called long-range entanglement driven order. The latter is believed to be present in the disordered regime for quantum XY chain.
   Fidelity mechanics might also offer an alternative solution to other mysterious problems in physics, including its cosmological implications for the interpretation of dark matter and dark energy.

4. (SEMINAR)"Z2 spin liquid phase on the kagome lattice: a new saddle point", Prof. Dr. Tao Li (Rebmin University), at 4:00 pm, May 16, 2016.

1. Abstract:
   We have performed large scale variational search for the best RVB ansatz for the spin-12 kagome antiferromagnet with both nearest-neighboring(NN) and next-nearest-neighboring(NNN) exchanges assuming only translational symmetry. We find the best RVB state is always fully symmetric and has an mean field ansatz that is gauge equivalent to a previous proposed Z2 spin liquid ansatz. The Z2 state is found to be slightly more stable than the extensively studied U(1) gapless Dirac spin liquid state in both the J2≠0 and the J2=0 case and to possess a small spinon gap for J2<0.2. The breaking of the U(1) gauge symmetry in the Z2 state is found to increase with J2 and to be quite substantial around J2=0.15. However, we find the Z2 state is always very close to the gapless U(1) Dirac spin liquid state, although they have very different RVB parameters. We argue the kagome antiferromagnet should be better understood as a near critical system, rather than a system deep inside a gapped spin liquid phase with well established Z2 topological order.

5. (Ph. D. Dissertation Defense)"Spin dynamics in two-dimensional systems and topological superconductor in single-layer graphene", Mr. Lin Wang, [Committee Members: Prof. Bang-Fen Zhu (Chair, Tsinghua Univ.); Prof. Dr. Yan Chen (Fudan Univ.), Prof. Dr. Takashi Kimura (Kyushu Univ.); Prof. Dr. Mu Wang (APS/Nanjing Univ.); Prof. Dr. Zheng-Yu Weng (Tsinghua Univ.); Prof. Dr. Fu-Chun Zhang (Zhejiang Univ.)], at 9:00 am, September 10, 2016.

1. Please browse SUPERVISED PH. D DISSERTATIONS

6. (SEMINAR)"Ferroelectric control of organic/ferromagnetic spinterface", Dr. Yuan Lu (Institut Jean Lamour, FRANCE), at 10:00 am, December 23, 2016.

1. Abstract:
   Hybrid ferromagnetic metal/organic interface, as known as “spinterface”, can exhibit highly efficient spin-filtering properties and therefore present a promising class of materials for future spintronic devices. The spin-polarization of spinterface at Fermi level can be different or even opposite in sign to that of adjacent ferromagnetic electrode. To achieve desired injection of spin-polarization, one should carefully engineer the organic/ferromagnetic interface to precisely tailor their electronic properties. Here, by using ferroelectric organic material poly(vinylidene fluoride) (PVDF), we demonstrate that the spin-polarization of the PVDF/Co spinterface can be actively controlled by tuning the ferroelectric polarization of PVDF. In particular, we have fabricated organic multiferroic tunnel junctions (MFTJs) based on La0.6Sr0.4MnO3/PVDF/Co structures. The tunneling magnetoresistance (TMR) sign can be changed by electrically switching the ferroelectric polarization of PVDF barrier. This study opens new functionality in controlling the injection of spin polarization into organic materials via the ferroelectric polarization of the barrier.