本周物理學術講座

知識 07-13

本周物理學術講座

聲超常材料及聲波的測量

報告人：程建春，南京大學

時間： 7月14日（周四）10:30

單位：中國科學院大學

地點：雁棲湖校區教1-115

超常材料是指具有天然材料所不具備的超常物理性質的人工複合結構或複合材料。聲波在超常材料中的傳播特性主要取決於人工幾何結構，而不僅僅是材料組元的物理性質。近年來，在電磁波超構材料的研究基礎上，聲超常材料及其構成的新聲學器件的研究是聲學領域的一個熱點方向。本報告介紹了我們最近幾年在聲超常材料及其調控聲波方面的若干研究工作，主要包括：1、聲二極體；2、基於零折射率材料的聲波方向性研究；3、基於超材料的聲場旋轉；4、亞波長機構聲聚焦；5、基於超材料的重力勢模擬；6、聲學超表面—無色散反射表面；7、基於聲學超常表面的單向通道；8、聲超材料與聲螺旋波。這些研究工作可望應用於各類聲檢測和成像技術，特別是生物醫學超聲診斷和治療。

Do recommender systems benefit users?

報告人：楊志豪，The Education University of Hong Kong

時間： 7月3日（周三）10:30

單位：中科院理論物理研究所

地點：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.

Resonant Inelastic X-ray Scattering from correlated electron systems and unconventional High-Tc Superconductors

報告人：周克瑾，英國Diamond光源

時間：7月13日（周三）15:00

單位：北京師範大學物理學系

地點：物理樓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.

Many body effects in transition metal molecular systems

報告人：Cedric Weber，King"s College London

時間： 7月13日（周三）16:00

單位：北京師範大學物理學系

地點：物理樓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.

Soft X-ray Spectromicroscopy Facility at the Canadian Light Source and Recent Developments

報告人：王建，加拿大光源軟線實驗站

時間： 7月14日（周四）9:00

單位：中科院高能物理研究所

地點：12號廳會議室

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.

Milky Way Dust in 3D

報告人：Greg Green

時間： 7月14日（周四）14:00

單位：清華大學天體物理中心

地點：蒙民偉科技南樓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.

Terasaki Ramps: A Glimpse into the Geometrical Architecture of the Cell

報告人：Greg Huber，Kavli Institute for Theoretical Physics

時間： 7月15日（周五）15:30

單位：中科院理論物理研究所

地點：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?

Chirality selective zero-line transport in graphene

報告人：Jeil Jung，University of Seoul at Seoul

時間： 7月18日（周一）16:00

單位：北京大學量子材料科學中心

地點：物理樓西563會議室

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.

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