基于光子人工微结构材料的类量子现象研究||本周物理学术讲座

报告人：陈鸿，同济大学

时间： 6月9日（周五）15:00

单位：中科院理论物理所

地点：新楼6620报告厅

光子人工微结构材料如光子晶体和超构材料(metamaterials，又称为超材料、特异材料、美特材料等)简介；若干典型类量子现象及应用介绍。如光子石墨烯与光子Landau能级、光子拓扑绝缘体与光子边缘态、光子“Spin Hall”效应、基于超对称原理的光子模式及应用等。我们课题组的相关研究工作简介，如类原子系统量子现象的研究、类凝聚态系统量子现象的研究、类量子现象应用研究等。作为一个典型例子，报告将详细介绍Dirac方程拓扑激发结构的超构材料模拟、拓扑激发诱导的透明金属现象等。我们认为光子人工微结构材料不仅可以提供一个“洁净”的、参数易于调控的类量子现象研究平台，而且可以为新奇量子效应在光子调控中的应用提供新原理。

报告人：吴立明，北京师范大学

时间：6月6日（周二）11:00

单位：清华大学物理系

地点：理科楼B315

热电材料具有热发电和电致冷两个重要的功能，因此它在深太空探测、能源及半导体致冷领域都具有重要的应用。热电材料性能的一个重要评估指标是材料的热电优值(ZT)，它与材料的赛贝克系数、热导率、电导率等性能直接相关。如何能通过材料结构设计调控，协调这几个材料性能以实现材料热电优值的提升是一个重要的科学难题。我们工作通过化学结构调控手段，利用固体化学的阳离子结构剪切、多金属协同掺杂等手段获得了若干高性能的新颖结构热电材料，为新型热电材料的发现及热电机理研究提供了重要的参考。

报告人：Daan Frenkel，University of Cambridge

时间：6月6日（周二）16:00

单位：中科院物理研究所

地点：M楼236报告厅

In an attempt to construct a Statical Mechanics of Powders, Sir Sam Edwards introduced the concept of “granular entropy”, defined as the logarithm of the number of distinct packings of N granular particles in a fixed volume V.In 1989, the proposal was rather controversial but much of the debate was sterile because the granular entropy could not even be computed for systems as small as 20 particles-hardly a good approximation of the thermodynamic limit.In my talk I will describe how granular entropies of much larger systems can now be computed, using a novel algorithm. Interestingly,it turns out the definition of granular  entropy will have to be modified to guarantee that granular entropy is extensive.Which brings us back to the Gibbs paradox and its implications for colloid science.

报告人：尉帅

时间：6月7日（周三）10:00

单位：中科院物理研究所

地点：M253

we examine the electronic conductivity and, then, SC-M transitions in related alloys including those so-called Phase-Change Materials such as Ge2Sb2Te5 and Ge1Sb2Te4 which can be reversibly switched between amorphous and crystalline states and serve as the basis of non-volatile memory devices technology. We find a systematic tendency in SC-M transitions with the metallicity of alloy components, and conclude that Ge-Sb-Te Phase-Change Materials possess SC-M (and liquid-liquid) transitions that have become submerged below the liquidus surface, which controls the liquid-state behavior and has significant impact on application-relevant phase-switching properties.

报告人：张勇，北京师范大学

时间：6月7日（周三）15:30

单位：中科院理论物理研究所

地点：Conference Hall 322

We study tree-level amplitudes from higher-dimensional operators, including F^3 operator of gauge theory, and R^2 , R^3 operators of gravity, in the Cachazo-He-Yuan formulation. As a generalization of the reduced Pfaffian in Yang-Mills theory, we find a new, gauge-invariant object that leads to gluon amplitudes with a single insertion of F^3 , and gravity amplitudes by Kawai-Lewellen-Tye relations. When reduced to four dimensions for given helicities, the new object vanishes for any solution of scattering equations on which the reduced Pfaffian is non-vanishing. This intriguing behavior in four dimensions explains the vanishing of graviton helicity amplitudes produced by the Gauss-Bonnet R^2 term, and provides a scattering-equation origin of the decomposition into self-dual and anti-self-dual parts for F^3 and R^3 amplitudes.

报告人：LIANG Tian，Stanford University

时间： 6月8日（周四）10:00

单位：中科院物理研究所

地点：M楼236

The picture of how a gap closes in a semiconductor has been radically transformed by topological concepts. Instead of the gap closing and immediately re-opening, topological arguments predict that, in the absence of inversion symmetry, a metallic phase protected by Weyl nodes persists over a finite interval of the tuning parameter (e.g. pressure P). The gap re-appears when the Weyl nodes mutually annihilate. We report evidence that Pb1-xSnxTe exhibits this topological metallic phase. Using pressure to tune the gap, we have tracked the nucleation of a Fermi surface droplet that rapidly grows in volume with P. In the metallic state we observe a large Berry curvature which dominates the Hall effect. Moreover, a giant negative magnetoresistance is observed in the insulating side of phase boundaries, in accord with ab initio calculations. The results confirm the existence of a topological metallic phase over a finite pressure interval.

报告人：Igor Shovkovy，Arizona State University

时间： 6月8日（周四）10:00

单位：中科院高能物理研究所

地点：Theoretical Physics Division，319

报告人：Li Yang，the Washington University in St Louis

时间： 6月8日（周四）10:00

单位：北京师范大学物理系

地点：物理楼110

I will present our studies of the unusual electric, optical, thermal, and piezoelectric properties of a new family of two-dimensional (2D) materials, black phosphorus and its corresponding group IV-VI isoelectronic materials. For black phosphorus, we predict the intrinsic, fundamental properties such as quasiparticle band gaps, excitons, and anisotropic thermal conductance, and further engineered properties such as strain-tunable anisotropic electrical conductance and realizing Dirac cones and graphene electronics.

报告人：Shin-ichi Uchida，University of Tokyo

时间： 6月8日（周四）14:00

单位：中科院物理所

地点：M234报告厅

To unravel the mechanism of high-Tc superconductivity is one of the most challenging problems in physics. Equally challenging is to enhance the superconducting transition temperature Tc. The highest superconducting transition temperature Tc at ambient conditions remains 135 K which was recorded in 1993 for Hg-based trilayer cuprate. In contrast to this long silence in Tc, several material families have successively been found to contain high-Tc members. These include MgB2 and iron arsenides/selenides, indicating that there are several routes toward higher Tc. Recently, three different classes of high-Tc materials have shown a signature of higher Tc under extreme conditions, monolayer FeSe films deposited on a SrTiO3 substrate, H2S under extremely high pressures, and YBCO pumped by c-axis polarized THz light pulses, which may be a signal of breaking the long silence. In the first part of my lecture, the materials features in common with many high-Tc families are discussed. Then, I address why copper oxides and iron compounds are so special, and suggest a possibility of finding the third high-Tc materials containing 3d transition-metal elements. In the second part, a focus will be on the hole-doped high-Tc cuprates. Based on the generic phase diagram including the enigmatic pseudogap phase I speculate how high Tc might go up. in the cuprates.

报告人：Jian-Min Wang，Institute of High Energy Physics

时间： 6月8日（周四）14:00

单位：清华大学物理系

地点：蒙民伟科技南楼S727

Supermassive black holes (SMBH) generally exist in both dormant and active galaxies. How to measure SMBH masses is the first step to understand coevolution of SMBH and host galaxies. Based on broad-line region physics, reverberation mapping of AGNs is regarded as a major technique of measuring SMBH mass. In this colloquium, I will introduce: 1) what are made of broad-line regions? structure and dynamics? 2)  the reverberation mapping of AGNs; 3) related accretion physics as ionizing sources and 4) the latest progress of reverberation mapping campaigns during the last five years in China. Finally, implications of measuring black hole masses are discussed for cosmic evolution of SMBHs,  high-z cosmology and detection of nano-Hz gravitational waves.

报告人：Wang YAO，University of Hong Kong

时间： 6月8日（周四）14:00

单位：清华大学物理系

地点：郑裕彤讲堂

In monolayer transition metal dichalcogenides (TMDs), a newly emerged class of 2D semiconductors, the low energy carriers are described by massive Dirac cones located at K and -K corners (valleys) of the Hexagonal Brillouin zone. These massive Dirac Fermions have interesting phenomena associated with their valley index, including the valley optical transition selection rules and valley Hall effects, which enable the use of both valley and spin (via spin-valley coupling) as information carriers in future electronics. Van der Waals stacking of 2D semiconductors into heterostructures further provides a powerful approach towards designer quantum materials that extend the exotic properties of the building blocks. As a generic aspect of these vdW heterostructures, the inevitable lattice mismatch always leads to the formation of moiré pattern (i.e. periodic variation of local atomic registries). I will show that the vdW moiré can endow heterostructures unprecedented properties, including electrically switchable lateral superstructures of topological Insulators, nano-patterned spin optics, and spin-orbit coupled excitonic superlattices.

报告人：Daan Frenkel，英国剑桥大学

时间： 6月8日（周四）18:30

单位：中国科学院大学

地点：国际会议中心一层报告厅

A holy grail of nano-technology is to create truly complex, multi-component structures by self-assembly. Most self-assembly has focused on the creation of "structural complexity". In my talk, I will discuss "Addressable Complexity": the creation of structures that contain hundreds or thousands of distinct building blocks that all have to find their place in a 3D structure. Recent experiments have demonstrated the feasibility of making such structures. Simulation and theory yield surprising insights that can inform the design of novel structures and materials.

报告人：卢征天，中国科学技术大学

时间： 6月12日（周一）10:00

单位：中科院高能物理研究所

地点：化学楼C305

惰性气体氪-81(81Kr)的核不稳定，半衰期为23万年，它是测量年龄在十万到百万年范围的地下水和冰样的理想示踪同位素。该年代范围是碳-14所不能覆盖的。在过去的50年中，许多物理实验为探测81Kr做出了各种努力，但是都没有成功。卢征天教授研究团队提出并发展了原子阱痕量分析方法（Atom Trap Trace Analysis, ATTA），利用它攻克了这一难题，使氪-81测年在地球科学领域的大规模应用成为可能。原子阱痕量分析是一项基于激光的超灵敏同位素检测技术，具备单原子探测能力，可以用来分析环境样品中同位素丰度仅为一百万亿分之一的81Kr。目前该团队与地下水、冰川、海洋等领域的科学家合作开展早期展示性应用研究。