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COLLOQUIA & SEMINARS 2015:

1. (SEMINAR)"Topological states in resonant photonic crystals", Prof. Dr. E. L. Ivchenko (Ioffe Physical-Technical Institute, RUSSIA), at 4:00 pm, January 9, 2015.

1. Abstract:
   The deeper understanding of topology in physics has led to a new concept of physical objects - topologically ordered system. A topological insulator is an electronic material that has a band gap in its interior like an ordinary insulator but possesses conducting states on its edge or surface. The talk gives an overview about the emerging field of topological photonics and concerns the optical counterparts of the electronic edge states in topological insulators. I will present theoretical and experimental studies of topological edge states of photons, exciton-polaritons and plasmons in periodic one- and two-dimensional nanostructures. The examples of realizations include arrays of coupled optical-ring resonators and multiple quantum wells. A method is presented for measuring topological invariants of a photonic structure through the phase spectroscopy. Particularly, it is shown how the Chern numbers can be deduced from the winding numbers of the reflection coefficient phase.

2. (SEMINAR)"Graphene spin relaxation: the role of adatoms and magnetic moments", Dr. Denis Kochan (University Regensburg, GERMANY), at 4:00 pm, January 13, 2015.

1. Abstract:
   Intrinsic spin-orbit coupling in graphene is relatively weak, some tens of micro eVs. On one hand, this is good, as the projected intrinsic spin relaxation is also slow, on the order of microseconds. On the other hand, a greater value for the spin-orbit interaction is desired for spin manipulation and spin-orbit induced phenomena, such as the spin Hall effect. In this talk I will review the basics of the spin-orbit physics in graphene and show how to effectively increase the value of the spin-orbit interaction (to meVs) by adding adatoms. Specifically, our first-principles calculations show that hydrogen induces local spin-orbit coupling of about 1 meV, while fluorine up to 30 meV. For each adatom I will present a minimal realistic hopping model explaining the first-principles results. Such models are useful for investigating model spin relaxation, spin and charge transport in functionalized graphene.
   Moreover, I will also present phenomenological theory, based on first-principles calculations, of the exchange splitting and spin relaxation in graphene with hydrogen adatoms or vacancies. I will show that resonant scattering and the exchange interaction with the paramagnetic impurities at the adatom site or vacancy can explain the experimentally observed short spin relaxation times, providing a competitive mechanism to that based on spin-orbit coupling.

3. (SECIENSE AND HUMANITIES FORUM)"Academic Honesty", Prof. Bang-Feng Zhu (Tsinghua University), at 7:00 pm, Jaunary 15, 2015.

1. Abstract:
   科研行为大致可分为负责任的、不当的和不端的三类。科研诚信是负责任的科研行为最基本的和最重要的组成部分。近年来我国出现的许多学术不端问题，除了大环境的影响和研究者个人道德品质缺陷外，对于青年学生和研究人员也还存在不懂和不重视的问题。演讲者作为一些学术期刊的编委、国家基金委监督委员会成员、以及中国科协科技工作者道德与权益委员会委员，调查和处理过多个学术不端事件，其中有的广受公众关注。演讲者将结合典型事件和自己的学习思考，阐述学术不端行为的界定和典型案例，讲述科研人员在研究、报告成果、评审、申请经费等科研活动中必须遵守的学术规范，讨论学术规范的一些open questions。讲演者还将通过剖析一个物理界引起广泛关注的案例，进一步阐明一个科技工作者的做人底线、良知和社会责任。

4. (SEMINAR)"Two Extreme Cases for Spin Orbit Coupling", Prof. Dr. Wei Han (Peking University), at 4:00 pm, March 23, 2015.

1. Abstract:
   The spin-orbit coupling is at the heart of efforts for spintronics, which aims to use spin for information storage and logic applications. There are two extreme cases for it, namely large spin orbit coupling and low spin orbit coupling. For materials with large spin orbit coupling such as platinum (Pt) and iridium ? manganese (Ir-Mn), a pure spin current could be generated via the spin Hall effect and lead to spin orbit torques that can largely manipulate the magnetization of FM films. In this talk, I will mainly discuss the role of the interface transparency for efficient spin orbit torque at Pt-FM interfaces and the observation of the facet dependent large spin Hall effect in Ir-Mn.
   For materials with low spin orbit coupling such as graphene, they could be used as a spin channel with long spin lifetime and spin diffusion length. In this talk, I will describe experimental observation of long spin lifetime and spin diffusion length in graphene, which are already much better than conventional metals and semiconductors at room temperature.

5. (SEMINAR)"Thermal vector potential theory of transport induced by temperature gradient", Prof. Dr. Gen Tatara (RIKEN Center for Emergent Matter Science, JAPAN), at 4:00 pm, March 31, 2015.

1. Abstract:
   A microscopic formalism to calculate thermal transport coefficients is presented based on a thermal vector potential, whose time-derivative is related to a thermal force. The formalism is free from unphysical divergences reported to arise when Luttinger's formalism is applied naively, because the equilibrium 'diamagnetic' currents are treated consistently. The mathematical structure for thermal transport coefficients are shown to be identical with the electric ones if the electric charge is replaced by energy. The results indicates that the thermal vector potential couples to energy current via the minimal coupling.

6. (SEMINAR)"Some novel effects in two-dimensional layered materials", Prof. Dr. Qing-Ming Zhang (Renmin University), at 10:00 am, April 28, 2015.

1. Abstract:
   In this talk, I will introduce two pieces of work in two-dimensional layered materials done in our group. In the first one, we report an unusual valley polarization (VP) up to 70% for B exciton in monolayer MoSe2, while that for A exciton is less than 3%, and a small but finite negative VP for A- trion. These results reveal several new intra- and inter-valley scattering processes which significantly affect valley polarization. The second one is the observation of an ultralow-frequency collective compression mode (CCM) in atomically thin black phosphorus (BP). This novel CCM indicates an unusually strong interlayer coupling in BP, which is quantitatively supported by a phonon frequency analysis and first-principles calculations. The CCM and another branch of low-frequency Raman modes shift sensitively with changing number of layers, allowing an accurate determination of the thickness up to tens of atomic layers. These results offer fundamental insights and practical tools for exploring multilayer BP in new device applications.

7. (SEMINAR)"Pure spin current transports in vicinity of transition metal, superconductor and magnetic insulator", Prof. Dr. Takashi Kimura (Kyushu University, JAPAN), at 4:00 pm, May 15, 2015.

1. Abstract:
   Pure spin current, a flow of spin angular momentum without accompanying electric charge current, is a powerful mean for investigating the spin-dependent transports. In this talk, the basic transport properties for the pure spin current created by the nonlocal spin injection are introduced. Then, I show that the recent experimental results on the transports of the pure spin current in the ferromagnetic/nonmagnetic metal hybrid structures with a transition metal, super conductor or ferromagnetic insulator.

8. (SEMINAR)"Low dimensional metal chalcogenide semiconductors: design, synthesis and applications", Researcher Dr. Jun He (National Center for Nanoscience and Technology), at 4:00 pm, October 16, 2015.

1. Abstract:
   While scaling the dimensions of semiconductors down to nano scale, novel properties, such as ultrahigh specific surfaces, quantum confinement effect and strong light-matter interaction, will show out. Among the low dimensional structures, two-dimensional (2D) semiconductors may lead the next resolution in electronics and optoelectronics due to their compatibility with traditional micro-fabrication techniques as well as flexible substrates. Up to now, both layered and non-layered materials have been demonstrated to present in 2D geometry. For the former, even though big breakthroughs have been made, especially on transition metal dichalcogenides (TMDCs), more systematical and deeper studies are needed. Noticeably, inspired by the success of 2D layered materials and the fact that many materials with significant functions have non-layered crystal structures, 2D non-layered materials have attracted increasing attentions.
   In addition, with potential applications in high-speed topological logic devices, the recently discovered topological crystalline insulators (TCIs) have attracted considerable interests. However, owing to the intrinsic istropic crystalline structure, it is not an easy job to synthesize these TCIs into low-dimensional structures, in which the topological surface transport may be strongly enhanced. As a result, reseach on TCIs is limited. Based on above challenges and motivations, our research focuses on the design, synthesis and applications of low dimensional metal chalcogenides semiconductors. In this talk, I will present our recent progress on the following two aspects:
   (1) 2D layered and non-layered metal chalcogenide semiconductors: controllable synthesis, properties, electronic and optoelectronic applications.
   (2) Topological crystalline insulators: synthesis dynamics and surface transport.

9. (HUMANITIES FORUM)"Influence of Taoism on Japanese Culture", Prof. Masaaki Tsuchiya (Senshu University, JAPAN), at 7:30 pm, October 27, 2015.

10. (HUMANITIES FORUM)"Birth of `China' from the perspective of of archaeology", Researcher Dr. Hong Xu (Institute of Archaeology, CASS), at 7:00 pm, October 28, 2015.

11. (SEMINAR)"Spin Rectification vs Spin Pumping", Prof. Dr. Can-Ming Hu (University of Manitoba, CANADA), at 4:00 pm, November 27, 2015.

1. Abstract:
   The art of experimental Condensed Matter Physics (CMP) research is to perform well conceived experiments on specially designed samples, in order to reveal the physics relations hidden by the competing interactions in the many-body system of materials. It is in this context that the CMP research resembles the art work of cinematography, which intends to use staged screenplays to highlight intricate relations formed in a complex society. From this perspective, I will talk about the Manitoba Production of the recent episodes of spintronics, specially focusing on the once controversial issue of spin rectification vs spin pumping, which plagued the field of dynamic spintronics for quite a few years.

12. (SEMINAR)"Ultrafast charge and spin dynamics in topological insulator Bi2Se3", Prof. Dr. Jing-Bo Qi (The Peac Institute of Multiscale Sciences), at 4:00 pm, December 28, 2015.

1. Abstract:
   Topological insulators (TIs) are characterized by an unusual electronic structure exhibiting both insulating bulk and robust metallic surface states (SSs). This unique electronic structure combining external light excitation leads to TIs a great promise for opto-spintronics and ultrafast spintronics applications. Therefore, understanding the charge and spin dynamics in TIs becomes quite essential. Here, a systematic study of these dynamics in prototypical TI Bi_2Se_3 has been carried out using ultrafast pump-probe optical spectroscopy. We unravel that a net spin polarization can not only be generated using circularly polarized light via interband transitions between topological surface states (SSs), but also via transitions between SSs and bulk states. For the first time, our experiment demonstrates that tuning photon energy or temperature can essentially allow for photoexcitation of spin-polarized electrons to unoccupied topological SSs with two distinct spin relaxation times (~25 fs and ~300 fs). We reveal that the intrinsic mechanism leading to such distinctive spin dynamics is the scattering in SSs and bulk states which is dominated by E_g^2 and A_{1g}^1 phonon modes, respectively. These new findings are suggestive of novel ways to manipulate the photoinduced coherent spins in TIs, which may have profound implications in future TI-based ultrafast spintronic devices.