本周物理学术讲座||声超常材料

报告人：程建春，南京大学

时间： 7月14日（周四）10:30

单位：中国科学院大学

地点：雁栖湖校区教1-115

超常材料是指具有天然材料所不具备的超常物理性质的人工复合结构或复合材料。声波在超常材料中的传播特性主要取决于人工几何结构，而不仅仅是材料组元的物理性质。近年来，在电磁波超构材料的研究基础上，声超常材料及其构成的新声学器件的研究是声学领域的一个热点方向。本报告介绍了我们最近几年在声超常材料及其调控声波方面的若干研究工作，主要包括：1、声二极管；2、基于零折射率材料的声波方向性研究；3、基于超材料的声场旋转；4、亚波长机构声聚焦；5、基于超材料的重力势模拟；6、声学超表面—无色散反射表面；7、基于声学超常表面的单向通道；8、声超材料与声螺旋波。这些研究工作可望应用于各类声检测和成像技术，特别是生物医学超声诊断和治疗。

报告人： 杨志豪，The Education University of Hong Kong

时间： 7月3日（周三）10:30

单位：

地点： ITP New Building 6420

Recommender systems are present in many web applications to guide purchase choices. They increase sales and benefit sellers, but whether they benefit customers by providing relevant products remains less explored. While in many cases the recommended products are relevant to users, in other cases customers may be tempted to purchase the products only because they are recommended. Here we introduce a model to examine the benefit of recommender systems for users, and find that recommendations from the system can be equivalent to random draws if one always follows the recommendations and seldom purchases according to his or her own preference. Nevertheless, with sufficient information about user preferences, recommendations become accurate and an abrupt transition to this accurate regime is observed for some of the studied algorithms. On the other hand, we find that high estimated accuracy indicated by common accuracy metrics is not necessarily equivalent to high real accuracy in matching users with products. This disagreement between the estimated and the real accuracy serves as an alarm for operators and researchers who evaluate recommender systems merely with accuracy metrics. We tested our model with a real dataset and observed similar behaviors. Finally, a recommendation approach with improved accuracy is suggested. These results imply that recommender systems can benefit users, but the more frequently a user purchases the recommended products, the less relevant the recommended products in matching the user taste.

报告人： 周克瑾，英国Diamond光源

时间： 7 月13日（周三）15:00

单位：北京师范大学物理学系

地点：物理楼106

Resonant inelastic X-ray scattering (RIXS) is a powerful bulk-sensitive photon-in / photon-out spectroscopic and scattering probe with elemental sensitivity for the electronic structure of condensed matter. It is a unique tool for studying low energy excitations in complex correlated systems, being direct-ly sensitive to charge-, orbital-, spin-, and lattice-degrees of freedom. Dedicated instrumentation for RIXS with ultra-high resolution in energy and momentum spaces has become available thereby enabling charac-terization of collective excitations such as orbitons, magnons and phonons. In this presentation I will give a brief introduction to RIXS technique and focus on the application in correlated electron systems and unconventional high-Tc superconductors. The status of the next-generation RIXS facility at Dia-mond Light Source will be also updated during the talk.

报告人：Cedric Weber，King's College London

时间： 7 月13日（周三）16:00

单位：北京师范大学物理学系

地点：物理楼106

A recent optimised implementation of DFT was however shown to scale linearly with the number of atoms (ONETEP), and opened the route to large scale DFT calculations for molecules and nano-structures.Nonetheless, one bottleneck of DFT and ONETEP, is that it fails at describing well some of the compounds where strong correlations are present, in particular because the computational scheme has to capture both the band-like character of the uncorrelated part of the compound and the Mott-like features emerging from the local strongly correlated centres. A recent progress has been made in this direction by the dynamical mean-field theory (DMFT), that allows to describe the two limits (metal and insulator) in a remarkable precise way when combined with DFT.The ONETEP+DMFT implementation and strategies to overcome the main bottlenecks of this type of calculations will be discussed, and its applications illustrated by a few case of studies, such as the role of quantum entanglement in Myoglobin and heme systems.

报告人：王建，加拿大光源软线实验站

时间： 7 月14日（周四）9:00

单位：

地点： 12 号厅会议室

The soft X-ray Spectromicroscopy (SM) beamline at CLS consists of an APPLE II type Elliptically Polarizing Undulator (EPU), a plane grating monochromator (PGM, 130-2700 eV), focusing optics, and three endstations, an ambient scanning transmission X-ray microscope (STXM), a cryo-STXM microscope (under commissioning) and an X-ray photoemission electron microscope (X-PEEM). Both STXM and X-PEEM can be used to measure morphology/structure, quantitative distributions (maps) of chemical components in 2D (3D in STXM), molecular orientation, as well as obtain electronic structure of the materials via the spatially-resolved near edge X-ray absorption fine structure (NEXAFS) spectroscopy, for a wide range of samples from the fields of materials sciences, environmental/earth sciences, and life sciences at high spatial resolution (sub 30 nm), high spectral resolution (< 0.05 eV) and in variable sample environments.

报告人：Greg Green

时间： 7月14日（周四）14:00

单位： 清华大学天体物理中心

地点：蒙民伟科技南楼S727

I present a three-dimensional map of interstellar dust reddening, covering three-quarters of the sky out to a distance of several kiloparsecs, based on Pan-STARRS 1 and 2MASS photometry. The map reveals a wealth of detailed structure, from filaments to large cloud complexes. The map has a hybrid angular resolution, ranging from 3.4' to 13.7', and a maximum distance resolution of 25%. The three-dimensional distribution of dust is determined in a fully probabilistic framework, yielding the uncertainty in the reddening distribution along each line of sight, as well as stellar distances, reddenings and classifications for 800 million stars detected by Pan-STARRS 1. The method presented here compares observed stellar photometry with empirical stellar templates, incorporating prior knowledge about the structure of the Galaxy.

报告人：Greg Huber，Kavli Institute for Theoretical Physics

时间： 7月15日（周五）15:30

单位：

地点： ITP New Building Room6620

Biologists have long considered the endoplasmic reticulum (ER) to be an exceedingly important and complex intracellular organelle in eukaryotes. It is a membrane structure, part folded sheet, part branching network, that both envelopes the nucleus and threads its way outward, all the way to the cell’s periphery. Microscopic images attest to its convoluted geometry, but can the complexity of its architecture be understood in a precise, mathematical way? Recently, refined imaging of the ER has revealed beautiful and subtle geometrical forms – "Terasaki ramps'' -- suggestive of Riemann sheets and helical minimal surfaces. What is the physics of these structures, and how do these architectural motifs connect to biological function?

报告人：Jeil Jung，University of Seoul at Seoul

时间： 7月18日（周一）16:00

单位：北京大学量子材料科学中心

地点： 物理楼西563会议室

One-dimensional localized modes exist in few-layer graphene systems at the interfaces between regions with different valley-Hall conductivities. In this presentation we discuss the electronic and transport properties of these kink states or zero-line modes in few layers graphene as a function of their stacking arrangement, interface structure, and applied perpendicular electric fields. We show that the chirality of the current remains robust regardless of their propagation direction or the proximity in momentum space of the opposite channels. Numerical analysis demonstrates that collisions between electrons traveling in opposite directions, due to either disorder or changes in path direction, are strongly suppressed, resulting in extremely long mean free paths of the order of hundreds of micrometers in clean samples. Such properties allows to tailor intersections between one-dimensional chiral channels as topological current splitters, that show very simple yet counterintuitive partition laws that relate current paths to the geometry of the intersection. Our results have implications for electron beam splitter and interferometer device proposals based on chiral transport, and for understanding transport in systems in which multiple topological domains lead to a statistical network of chiral channels. We finally discuss other related systems that can host zero-line modes consisting of graphene nanoroads inserted in AB/BA grain boundaries within BN sheets.