磁性系统中的拓扑结构|本周物理学讲座
报告人：邱子强, 加州大学伯克利分校物理系
时间：8月21日（周二）10:00
单位：北京师范大学
地点：科技楼A区311
巨磁阻现象发现后（2007年物理诺贝尔奖），磁性纳米材料的研究取得了突飞猛进的发展。在众多的研究方向中，非线性磁结构和自旋动力学逐渐成为两个重要的前沿领域。这个报告将重点介绍一下非线性磁结构的研究。我将从历史上数学与物理学中一些有趣的拓扑现象开始，逐渐讨论到二维磁性系统中的拓扑结构，然后介绍近期磁学中涡旋态（vortex）和斯格明子（skyrmion）的概念以及我们的一些研究结果。
报告人：Georg Raithel, University of Michigan
时间：8月21日（周二）09:00
单位：清华大学
地点：理科楼C302
The Rydberg excitation blockade plays a central role in recent experiments that utilize cold atoms excited into Rydberg levels. These include quantum information processing, entangled many-body states atom ensembles that carry multiple Rydberg excitations, and efforts to generate non-classical forms of light. Here, we employ a direct spatial imaging technique to measure pair correlation functions in systems of interacting Rydberg atoms. In this talk, several experiments on the topic are discussed. (1) We have investigated rotary-echo effects on the pair correlation function. The rotary excitation echo depends on both the atom-field detuning and the interaction-induced level shifts in the system, leading to either enhancement or suppression of the pair correlation at distances near the blockade radius. (2) We have studied the trajectories of Rydberg-atom pairs by allowing the atoms to move freely for a variable time, before performing the pair-correlation measurement. We have extracted the C_6 and C_3 coefficients of van-der-Waals and electric-dipole interactions, and the anisotropy of the electric-dipole interaction has been imaged. (3) The expansion and correlation dynamics of a continually excited Rydberg-atom system have been measured.
3Quantum Spin Hall Effect and Spin Bott Index in Quasicrystal Lattice
报告人：黄华卿，University of Utah
时间：8月21日（周二）10:00
单位：清华大学
地点：理科楼C109
Despite the rapid progresses in the field of quantum spin Hall (QSH) effect, most of the QSH systems studied up to now are based on crystalline materials. Here we propose that the QSH effect can be realized in quasicrystal lattices (QLs). We show that the electronic topology of aperiodic and amorphous insulators can be characterized by a spin Bott index Bs. The nontrivial QSH state in a QL is identified by a nonzero spin Bott index Bs = 1, associated with robust edge states and quantized conductance. We also map out a topological phase diagram in which the QSH state lies in between a normal insulator and a weak metal phase due to the unique wavefunctions of QLs. Our findings not only provide a better understanding of electronic properties of quasicrystals but also extend the search of QSH phase to aperiodic and amorphous materials that are experimentally feasible.
4Electromagnetically induced transparency of cesium Rydberg atoms in strong radio-frequency fields
报告人：Jianming Zhao, Shanxi university
时间：8月21日（周二）10:40
单位：清华大学
地点：理科楼C302
We study Rydberg atoms modulated by strong radio-frequency (rf) fields with a RF frequency of 100 MHz. The Rydberg atoms are prepared in a room-temperature cesium cell, and their level structure is probed using electromagnetically induced transparency (EIT). As the rf field increases from the weak- into the strong-field regime, the range of observed rf-induced phenomena progresses from ac level shifts through increasingly pronounced and numerous rf modulation sidebands to complex state mixing and level crossings. These EIT spectroscopy structure can be used to perform a rapid and precise (0.5%) calibration and measurements of electric field and polarization the rf electric field.
报告人：K. L. Ngai，Università di Pisa
时间：8月22日（周三）14:00
单位：中科院物理所
地点：M楼253会议室
Research on the dynamic and thermodynamic properties of glass-forming materials is becoming extremely diversified nowadays. There are many issues and questions one can pick and choose to work on. However without cognizance of the critical and general properties already present and needed to be addressed first, the efforts on the ancillary issues are premature. It is like putting the cart before the horse. Therefore it is important to point this out and to stem the common trend of current research. In this talk I present some of the ten such critical and general properties, and let the audience judge whether these are crucial and should be explained first before considering other issues and questions.
报告人：房栋梁，IMP, CAS
时间：8月24日（周五）10:00
单位：中科院高能所
地点：319 Theoretical Physics Division
Majorana中微子的存在会导致原子核中的无中微子双贝塔衰变，这个过程可以回答关于中微子的质量起源以及绝对质量等重要问题。作为一种罕见的原子核弱衰变，对这个过程的研究依赖于核结构的计算，其中的原子核的跃迁需要通过核矩阵元来表示。我们发展了基于Woods-Saxon势和现实核力的准粒子无规相位近似（QRPA）方法，并将之拓展到变形核的计算。在本次报告中，将给出双贝塔衰变的核矩阵元计算结果，并讨论结果的不确定度，以及各种核多体方法之间的差异。同时还会讨论不同底层衰变机制对结果的影响。
7 Density Wave Probes the Quantum Phase Transition in a Cuprate Superconductor
报告人：Jennifer E Hoffman，Harvard University
时间：8月24日（周五）15:00
单位：清华大学
地点：理科楼郑裕彤讲堂
The strong electronic correlations of the doped antiferromagnetic Mott insulator give rise to unconventional phases beyond high temperature superconductivity in the cuprates. Multiple broken symmetries have been reported, though a complete description of their emergence and interaction is missing. At the heart of this complex phase diagram is evidence of a quantum critical point, where disconnected Fermi arcs jump inexplicably to a large Fermi surface as holes are added. Here, we use sub-unit-cell scanning tunneling microscopy and spectroscopy (STM/S) to track the d-form factor density wave throughout the phase diagram as a local fingerprint of the dominant interactions in (Bi,Pb)2(Sr,La)2CuO6+d. Crucially, we use nanoscale electronic inhomogeneity as a tool to access a range of dopings within a single sample. We discover an abrupt change in the evolution of the modulation wavevector, signaling a commensurate to incommensurate transition that is coincident with the Fermi surface transition. Furthermore, the rapid growth of the wavelength on the overdoped side of the transition is at odds with a simple Fermi surface instability, providing evidence for persistent effects of strong correlations.
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