拓扑声子晶体 | 本周物理学术讲座
报告人：刘正猷，武汉大学
时间：10月18日（周四）16:00
单位：清华大学
地点：理科楼郑裕彤讲堂
拓扑物态是当前凝聚态及材料物理领域的重要关注焦点。声子晶体是具有周期性结构的人工材料，其中的声子态或声波态也可具有异于寻常的拓扑性质。本报告主要介绍两类拓扑声子晶体，即能谷声子晶体和外尔声子晶体，内容涉及能谷声子晶体中的能谷态的涡旋特性、拓扑相及拓扑相变、能谷边缘态及拓扑输运，以及外尔声子晶体中的Weyl点、费米弧表面态及拓扑负折射效应等等。
2Turning‘off’and‘on’the topological edge states in ultra-thin Na3Bi
报告人：Mark Edmonds，Monash University
时间：10月16日（周二）10:00
单位：清华大学
地点：物理系理科楼C302
Na3Bi in bulk form represents a zero-bandgap topological Dirac semimetal (TDS), but when confined to few-layers is predicted to be a quantum spin Hall insulator with bulk bandgap of 300 meV. Furthermore, application of an electric field to few-layer Na3Bi has been predicted to induce a topological phase transition from conventional to topological insulator.
I will discuss our efforts to grow epitaxial few-layer Na3Bi via molecular beam epitaxy, and probe its electronic structure and response to an electric field using scanning probe microscopy/spectroscopy and angle-resolved photoelectron spectroscopy. We are able to demonstrate that monolayer and bilayer Na3Bi are quantum spin Hall insulators with bandgaps >300 meV. Furthermore, via application of an electric field the bandgap can be tuned to semi-metallic and then re-opened as a conventional insulator with bandgap ~100 meV. The demonstration of an electric field tuned topological phase transition in ultra-thin Na3Bi provides a viable platform for the creation of a topological transistor.
报告人：王宇杰，上海交通大学
时间：10月16日（周二）10:00
单位：中科院理论物理研究所
地点：理论物理所新楼6420报告厅
颗粒物质是由大量宏观尺寸的粒子汇聚成的离散体系，在日常生活和工业生产中被广泛应用，同时是包括地震、泥石流在内很多地质过程的研究载体，理解颗粒输运、阻塞等性质具有重要实际意义。颗粒体系本质上是多体耗散的非平衡态体系，在不同条件下会表现出气液固态，但是其性质与传统连续介质体系有明显区别。现有的工程理论主要是基于经验的本构理论，在实际应用中遇到了很大的困难。从基础物理的角度即从微观结构和动力学研究开始重建颗粒物质的基础理论具有重要科研和应用意义，也对理解非晶态物质的一般特性具有重要意义X射线成像技术在颗粒体系的研究中具有很大的优势，我们介绍过去几年中我们使用X光CT技术研究颗粒体系静态结构及动力学的一些进展。
报告人：Cristina Romero-Canizales，Núcleo de Astronomía of Universidad Diego Portales
时间：10月16日（周二）12:00
单位：北京大学
地点：KIAA, 1st meeting room
The Universe is in constant change. Events such as supernovae and flares from active galactic nuclei give evidence of the transient nature of the sky. Transients are typically discovered by optical surveys observing large regions of the sky at a high cadence. There is however an interest in detecting and studying the counterparts at different wavelengths in order to construct a more complete picture of the transient itself as well as from the object that gave origin to it. In this talk I will present a selection of transients we have followed-up in radio, including results on an interesting transient discovered by the All-Sky Automated Survey for Supernovae (ASAS-SN). I will also make a quick review on what can be done with current radio facilities and how transient radio studies will revolutionize with upcoming facilities.
报告人：李伯杨，北京并行科技股份有限公司
时间：10月16日（周二）14:00
单位：中国科学院大学
地点：雁栖湖校区教1-317
报告人：陈学刚，University of Nebraska-Lincoln
时间：10月16日（周二）14:30
单位：中科院物理研究所
地点：M楼253会议室
在强关联钙钛矿氧化物中，微观尺度上为晶格自由度与其他序参量（电荷、自旋及轨道等）之间强耦合作用，使得强关联钙钛矿氧化物的电子特性和磁特性极易受到载流子和晶格的改变而发生变化，从而在强关联钙钛矿氧化物薄膜中出现一些新的奇特的特性。这些特性难以在强关联钙钛矿氧化物块才中观察到。我们通过铁电材料场效应作用和异质结界面的电荷转移控制电荷转移型Mott绝缘体(Sm,Nd)NiO3的电输运特性，实现巨大的存储特性的增强。我们微观尺度载流子-晶格调控的研究，在基础研究和新型器件的应用研究提供了一种新的可能性。我们在传统的反尖晶石结构亚铁磁NiCo2O4材料中，发现一系列新颖的物理特性，包括较强的室温反常霍尔效应，易磁化轴面外排列，线性磁电阻等。
7CERN IT-Storage Operations: EOS-CERNBox and future strategy
报告人：Luca Mascetti，CERN
时间：10月16日（周二）15:00
单位：中科院高能物理研究所
地点：计算中心二楼会议室
CERN IT-Storage group is responsible for the operations and the development of the infrastructure matching all the storage requirements from the physics data generated by LHC and non-LHC experiments to the personnel users’ files.
EOS is now the key component of the CERN Storage strategy, it allows to operate at high incoming throughput for experiment data-taking while running concurrent complex production work-loads.
This high-performance distributed storage provides now more than 250PB of raw disks and is the key storage component behind the success of CERNBox, the CERN cloud synchronisation service which allows syncing and sharing files on all major mobile and desktop platforms (Linux, Windows, macOS, Android, iOS) aiming to provide offline availability to any data stored in the EOS infrastructure.
CERNBox recorded an exponential growth in the last couple of year in terms of files and data stored thanks to its increasing popularity inside CERN users community and thanks to its integration with a multitude of other CERN services (Batch, SWAN, Microsoft Office).
In parallel CASTOR is being simplified and transitioning from an HSM into an archival system, focusing mainly in the long-term data recording of the primary data from the detectors, preparing the road to the next generation tape archival system, CTA, based on EOS.
In this presentation the speaker will give an overview of the solutions operated by the group with current and future storage developments. He will discuss about the strategy of the group to face future needs, highlighting the challenges ahead and the future evolution.
报告人：Harald Fritzsch，LMU Physics Department
时间：10月17日（周三）15:00
单位：中科院高能物理所
地点：C305
In 1971 Harald Fritzsch and Gell-Mann introduced the color quantum number of the quarks, which explained the bound state structure of the baryons and the decay rate of the neutral pion. One year later they discussed a gauge theory, based on the gauge group of the color transformations SU(3).This theory is similar to quantum electrodynamics, but the gauge bosons of QCD,the gluons, interact not only with the quarks, but also with themselves.This leads to the phenomenon of asymptotic freedom - the coupling constant decreases at high energies. This has been measured at SLAC, DESY, Fermilab and CERN.
At very high energies there should exist a new state of matter, the quark-gluon-plasma, e.g. inside massive neutron stars or in nucleus collisions at high energy. Today the theory of quantum chromodynamics is considered to be the exact theory of the strong interactions and of the nuclear forces.
报告人：徐春祥，东南大学
时间：10月17日（周三）15:10
单位：北京大学
地点：物理楼中楼212室
利用光在ZnO微纳结构中的内壁全反射构建天然的回音壁模（WGM）共振微腔，探讨一系列基础而关键的问题：（1）如何获得高品质紫外激光？（2）如何提升WGM激光性能？如引入表面等离激元，有效实现光场耦合；（3）如何进行激光模式的调控？（4）如何发展电泵浦微激光器件？（5）如何拓展WGM微腔的功能应用？如光探测与生物传感。
10磁性纳米材料研究
报告人：杜娟，中国科学院宁波材料技术与工程研究所
时间：10月18日（周四）10:00
单位：北京师范大学
地点：物理楼106
磁性材料当其晶粒尺寸降低到纳米尺度后会出现新奇的磁学特性，如超高的矫顽力、斯格明子效应、巨磁电阻效应、剩磁增强效应等。这些新奇磁学特性的出现，依赖于纳米尺度磁性材料的可控制备。因此纳米磁性材料的新奇物性及其相关的可控制备技术、纳米晶生长机制及组装机制是凝聚态物理、材料物理、材料化学等领域的国际前沿方向。本报告将详细介绍报告人课题组在纳米永磁材料方面的研究进展，重点介绍磁性纳米颗粒的可控制备、生长机制；磁性纳米颗粒二维颗粒膜的自组装过程及其驱动力；三维纳米晶块体材料的致密化、织构化及复合化方面的系列研究。
报告人：Joel Bregman，Univ. of Michigan
时间：10月18日（周四）14:00
单位：清华大学
地点：蒙民伟科技南楼S727
Hot gas density distributions can be fitted to the X-ray emission and absorption observations of the Milky Way to determine the mass and metallicity, mainly within 50 kpc. We have now included optical depth effects in the critical O VII and O VIII lines and find the same result from three different data sets. An extended spherical hot halo is the main mass component, containing about 3-4E10 Msun within 250 kpc, significantly lower than the mass of missing baryons, about 1.7E11 Msun. The hot halo is rotating at 180 +/- 40 km/s within 50 kpc, the density declines as r^-3/2, and the metallicity is about half solar. Adding a disk component improves the fit but it is a minor contributor to the total mass. The missing baryons most likely lie beyond the virial radius, probably 2-3 R200. In nearby external galaxies, we detect a SZ signal when stacking the nearest dozen spiral galaxies with L ~ L*. This signal suggests a gas mass of about 5E10 Msun within R200, similar to that found around the Milky Way.
报告人：Mathias Klaui，Institute of Physics, Johannes Gutenberg-University Mainz, Germany
时间：10月18日（周四）15:00
单位：中科院物理研究所
地点：M楼253会议室
We probe spin transport in ferrimagnets and insulating antiferromagnets, such as NiO by thermal spin currents resulting from the spin Seebeck effect and by spin pumping measurements and we find short (few nm) spin diffusion lengths. We use a magnonic spin valve device made of collinear magnetic yttrium iron garnet (YIG)/CoO/Co multilayers [3]. By means of microwaves and external magnetic fields YIG is brought into ferromagnetic resonance, resulting in a pure spin current propagating through the sample stack and finally being detected in the Co layer via the inverse spin Hall effect. For parallel and antiparallel alignment of the ferromagnetic layers, we observe a very different amplitude of the magnonic spin current transmitted across the AFM with a spin valve effect amplitude of 120%. Furthermore, by varying the antiferromagnetic layer thickness, we can determine the spin current attenuation length in NiO and CoO to a few nm also for this case. Finally, we carry out non-local spin transport experiments on collinear antiferromagnets. We find that by tuning the spin structure we can realize spin transport over tens of micrometer distances and switching in antiferromagnets can be obtained by spin-orbit torques.
报告人：Jean-Claud Garreau
时间：10月18日（周四）16:00
单位：中科院理论物理所
地点：理论物理所新楼6620报告厅
报告人：Piero Madau，University of California Santa Cruz
时间：10月18日（周四）16:00
单位：北京大学
地点：KIAA-PKU Auditorium
The reionization of the all-pervading intergalactic medium marks a turning point in the history of structure formation in the Universe.A combination of cosmic microwave background data and quasar absorption line spectra shows that this transition must have begun in earnest around redshift 10, and that the Universe was nearly fully reionized some 0.9 Gyr after the Big Bang. The details of this process reflect the nature of the first astrophysical sources of radiation and heating, the power-spectrum of density fluctuations on small scales, the thermodynamics and chemistry of cosmic baryons, the star formation and black hole accretion history, and a complex network of poorly understood feedback mechanisms. These outstanding issues remain crucial missing links in galaxy formation and cosmology research. In this talk I will survey the key aspects of reionization-era phenomenology, describe the diverse range of techniques and tools currently availablein this field, and review some recent challenges in our understanding of cosmic dawn.
报告人：Mathias Klaui，Johannes Gutenberg-Universitat Mainz
时间：10月19日（周五）14:00
单位：清华大学
地点：物理系理科楼C302
In our information-everywhere society IT is a major player for energy consumption. Novel spintronic devices can play a role in the quest for GreenIT if they are stable and can transport and manipulate spin with low power. Firstly to obtain ultimate stability, topologically stabilized spin structures, such as chiral domain walls and skyrmions can be used. We have investigated in detail their dynamics and find that it is governed by the topology of their spin structures. By designing the materials, we can even obtain a skyrmion lattice phase as the ground state of the thin films. By analyzing the thermal energy landscape, we find that depending on the parameters (field, temperature, etc.), stripe domain or skyrmion phases constitute the lowest energy state. Secondly, for ultimately efficient spin manipulation, we use spin-orbit torques: we combine ultimately stable skyrmions with spin orbit torques into a skyrmion racetrack device, where the real time imaging of the trajectories allows us to quantify the novel skyrmion Hall effect. By studying the temperature dependence of the skyrmion Hall effect, we identify the acting mechanisms that lead to different dependences of the skyrmion Hall angle on the current density for the creep and the viscous flow regime. Finally, we recently observed thermally activated skyrmion dynamics showing that skyrmions exhibit diffusion and this is applied to a skyrmion reshuffler device for stochastic computing.
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