超导体“分段费米面”、电荷密度波、玻璃化和非平衡结晶、全息纠缠熵 | 本周物理讲座

中国物理学会期刊网

1 A glimpse of RIXS and its applications in quantum materials

报告人：周克瑾，Diamond Light Source, UK

时间：3月7日（周二）10:00

单位：中科院物理所

地点：中科院物理所M236会议室

摘要：

Resonant inelastic X-ray scattering (RIXS) is a photon-in photon-out spectroscopic technique utilizing highly intense and monochromatized X-rays to probe the excitations in materials of interest. By selectively working at the resonant thresholds of an element, collective excitations or ordered states, such as magnons, orbitons, plasmons, phonons, charge-density waves, can be probed in the energy-momentum space. Owing to its high cross-section and focused X-ray beam, RIXS is advantageous in measuring micron-size samples and nanometre-thick films.

Over the last decade, RIXS has flourished because of an order of magnitude increase in its energy resolving power. At Diamond Light Source in the UK, the I21-RIXS beamline delivers an energy resolution of 35 meV at Cu L-edge (930 eV) and 15 meV at Oxygen K-edge (532 eV) with high photon throughput. In this talk, I will introduce the principle of the technique and the recent science cases in quantum materials particularly unconventional high-temperature superconductors.

报告人简介：

Dr. Ke-Jin Zhou is the Principal Beamline Scientist of the I21 Resonant Inelastic soft X-ray Scattering (RIXS) beamline, Diamond Light Source, United Kingdom. He obtained Ph.D degree in the Institute of High Energy Physics (IHEP), Chinese Academy of Sciences in 2007. From October 2007 to April 2012, Dr. Zhou was a research fellow at University Pierre and Marie Curie, France, and Swiss Light Source, Paul Scherrer Institute, Switzerland. In 2012, Dr. Ke-Jin Zhou took upon the post of Principal Beamline Scientist at Diamond Light Source, UK, and built up the Beamline of the ultra-high energy resolution RIXS. Since September 2017, I21 has become one of the most sought-after RIXS facilities in the world and produced many ground-breaking results and publications in high profile journals. Dr. Zhou’s scientific focus lies at the core of quantum complex matter including unconventional high Tc superconductors (cuprates, iron-based, and nickelates), artificial oxide heterostructures, as well as low-dimensional quantum materials.

2 Challenges for a QCD Axion at the 10 MeV Scale

报告人：刘佳，北京大学

时间：3月7日（周二）10:00

单位：中科院理论物理所

地点：北楼 322

摘要：

We report on an interesting realization of the QCD axion, with mass in the range O(10) MeV. It has previously been shown that although this scenario is stringently constrained from multiple sources, the model remains viable for a range of parameters that leads to an explanation of the Atomki experiment anomaly. In this article we study in more detail the additional constraints proceeding from recent low energy experiments and study the compatibility of the allowed parameter space with the one leading to consistency of the most recent measurements of the electron anomalous magnetic moment and the fine structure constant. We further provide an ultraviolet completion of this axion variant and show the conditions under which it may lead to the observed quark masses and CKM mixing angles, and remain consistent with experimental constraints on the extended scalar sector appearing in this Standard Model extension. In particular, the decay of the Standard Model-like Higgs boson into two light axions may be relevant and leads to a novel Higgs boson signature that may be searched for at the LHC in the near future.

报告人简介：

刘佳，北京大学物理学院研究员，博雅青年学者。本科毕业于清华大学，硕士毕业于北京大学，博士毕业于纽约大学，之后在德国美因茨大学和芝加哥大学做博士后研究。从事粒子物理理论研究，研究方向为高能物理唯象学和粒子天体物理等。

3 Gravitational waves produced from topological defects

报告人：周也铃，国科大杭州高等研究院

时间：3月7日（周二）10:00

单位：中科院理论物理所

地点：南楼6420

摘要：

Topological defects are a class of important concepts in particle cosmology. They are physical objects formed when a fundamental symmetry is spontaneously broken during the cooling of the expanding Universe. Typical topological defects include monopoles, domain walls and cosmic strings. While the first two lead to a serous cosmological problem and should be diluted by inflation, the last one is safe and have some interesting features favoured by theorists. The most important one is that it is a main source to generate cosmic background of stochastic gravitational waves (GWs). An observation of this background in the future will be helpful for us to construct a fundamental particle theory at super-high energy scale. On the other hand, biased domain walls could avoid the cosmological problem and provide another source for GWs. In this paper, I will introduce mechanisms of GWs from topological defects and discuss their connections to fundamental particle theories.

报告人简介：

周也铃，2010年毕业于兰州大学，2015年于中国科学院高能物理研究所获得博士学位。先后在英国杜伦大学和南安普敦大学从事学术研究，现任国科大杭州高等研究院特聘研究员。主要研究领域为中微子物理与粒子宇宙学，目前在中微子质量与味混合、重子与轻子生成机制、引力波的粒子物理起源等研究方向上做研究。

4 Near Charge Density Wave Instability in Superconductor LaO₀.₅F₀.₅BiS₂ and electron-correlation-induced charge density wave in magnetic kagome FeGe

报告人：万贤纲，南京大学

时间：3月7日（周二）10:00

单位：合肥微尺度物质科学国家研究中心、国际功能材料量子设计中心、中科院强耦合量子材料物理重点实验室、中国科大物理系、科研部

会议链接：https://www.koushare.com/liv‍es/room/697299

摘要：

在能带论中电荷密度波(CDW)结构转变往往和声子的不稳有关，这时人们也可以通过分析不稳的声子来定出其结构变化的情况。我们将报告2个不同的情况。(1) 超导体LaO0.5F0.5BiS2在k=(π，π0)有很强的费米面嵌套，在此处第一性原理计算也发现声子不稳，相应的总能计算也表明有双势井出现，但实验中并未发现有CDW结构转变。我们发现这是由于该材料有很强的非谐效应，原子的量子效应抑制了CDW结构转变。(2) 作为第一种表现出CDW转变的磁性kagome材料，FeGe引起了人们的关注。此前研究发现与AV3Sb5(A=K,Cs,Rb)相似，FeGe有平面内2×2的CDW转变，它也在费米能附近存在van Hove奇点(vHSs)。然而，与具有M点声子不稳的AV3Sb5不同，所有理论计算的FeGe声子频率都是正的。我们发现FeGe费米面嵌套函数的最大值在K点而不是M点。FeGe的能带具有高度三维特征，vHSs在嵌套函数中不起主导作用。我们发现由波矢M嵌套的费米面上2个波函数在同一个Fe位置有很多的权重，所以local库仑相互作用可以大大增强M点的不稳定，从而导致CDW，而由波矢K嵌套的费米面上2个本征态主要分布在不同的子晶格上，从而显著抑制了K点的不稳定性。这表明是电子关联导致了FeGe在M点的CDW结构转变。目前FeGe的具体的CDW结构还没有被确定。我们使用群论分析了FeGe的磁相互作用，发现只有4种CDW结构才能解释该体系在低温时的double cone磁结构。

报告人简介：

万贤纲，南京大学物理学院教授，1990年至2000年在南京大学学习，获得学士、硕士、博士学位。2001起在南京大学历任讲师，副教授，2010年任教授。主要学术成绩为：提出了新型拓扑量子态—Weyl半金属, 该工作被评为“2011年中国百篇最具影响国际学术论文”，该工作也入选美国物理学会PRB期刊创刊50年“里程碑”(MILESTONE)文章，评语是：提出了一种新的量子物质状态外尔半金属(The Weyl semimetal, a new state of quantum matter, is proposed.)；发展了一套计算磁性相互作用的方法并确定多个复杂体系的基态磁构型；基于原子绝缘体基组发展了高效判断拓扑性能的理论方法，并用其对无机材料数据库里面所有非磁材料的拓扑特性进行了分类，构造了拓扑材料数据库。该工作被评为“2019年中国百篇最具影响国际学术论文”，该工作也入选“2019中国十大科技进展新闻”。获中国物理学会2018-2019年度叶企孙物理奖。2019年获首届腾讯基金会“科学探索奖”。2020年获第二届全国创新争先奖。

5 发现超导体“分段费米面”

报告人：郑浩，上海交通大学

时间：3月7日（周二）10:00

单位：中科院物理所

地点：M楼253

摘要：

1965年，Fulde理论预言，超导体中如果库珀对的动量足够大就可以在超导能隙中产生准粒子，从而导致出一种特殊的费米面。但由于普通超导体库珀对动量足够大时，产生准粒子的同时，库珀对也会破裂而失去超导，因此要观察到超导费米面实验上非常困难。该预言一直没有被证实。我们将拓扑表面态费米速度极高的独特优点应用到超导研究中来，设计并制备出拓扑绝缘体/超导复合体系，提出了外场调控库伯对动量新方法，最终在实验上实现了超导 “分段费米面”。该工作证实了50多年前理论预言，并发现可以用磁场方向和大小来调节“分段费米面”的形状和大小，还能调控拓扑性，构建新的拓扑超导，并开辟了调控物态的新方法。

报告人简介：

郑浩，上海交通大学教授，从事凝聚态物理实验研究。2007年毕业于中科院物理所，获得博士学位。之后在英国伯明翰大学、德国基尔大学、美国普林斯顿大学从事博士后研究。2018年获得“求是杰出青年学者奖”，2019年-2022年，连续四年入选科瑞唯安“全球高被引学者”。共发表论文90余篇，（根据Web of science检索）总引用次数8400余次。代表论文包括Science 3篇， Physics Review Letters 15 篇。近年来，主要关注以拓扑半金属和拓扑超导体为代表的拓扑物质研究，尤其是利用扫描隧道显微镜研究其极低温、强磁场下的谱学性质。

6 玻璃化和非平衡结晶过程的密切联系

报告人：谭鹏，复旦大学物理学系

时间：3月7日（周二）10:00

单位：中科院物理所

地点：M楼255会议室

摘要：

玻璃转变和结晶分别形成非晶固体（玻璃态）和晶体。尽管如此，结晶和玻璃化在动力学方面密切联系。我们将详细介绍我们在这一领域的研究中取得的一些进展，如结晶过程中的非经典形核路径、玻璃态结晶原理和玻璃态中多层级的弛豫的极限形式，以及界面、尺寸、非平衡效应等因素如何影响结晶和玻璃转变。

报告人简介：

谭鹏，复旦大学物理学系教授，博士毕业于复旦大学物理学系，曾在香港中文大学从事博士后研究。近年来主要在胶体系统开展相变动力学方面的研究，如玻璃化和结晶的微观过程。

7 全息纠缠熵及其与引力动力学之间的关系

报告人：吴洁强，中国科学院理论物理研究所

时间：3月7日（周二）12:00

单位：中科院理论物理所

地点：南楼6620

摘要：

全息纠缠熵已经提出了15年，并在学界取得了巨大的影响。在这一次的报告中，我会围绕全息纠缠熵介绍一些我认为比较重要的题目，以及一些未来可以进一步研究的问题，具体包括：entanglement wedge nesting及其所需要的能量条件、纠缠熵第一定律的全息描述、JLMS formula、极值曲面在哈密顿系统中的性质等。报告内容以综述为主。

8 First-principles calculations for Dzyaloshinskii-Moriya interaction and its application for the DMI-torque switching of perpendicular magnetization

报告人：杨洪新，南京大学物理学院

时间：3月7日（周二）12:00

单位：江苏省物理学会

会议链接：https://www.koushare.com/lives/room/569762

摘要：

Understanding the magnetic interactions is of fundamental importance in condensed matter physics as well as in the application of spintronic devices. In the past 14 years, the Dzyaloshinskii-Moriya interaction (DMI) has attracted increasing attention. Here, I will present a systemic survey of the first-principles calculation methods for DMI [Nat. Rev. Phys. 5, 43 (2023); PRL. 115, 267210 (2015)], along with an overview of the first-principles calculations of the DMI properties of typical material systems [Nat. Mater. 17, 605 (2018); PRL 128, 167202 (2022); Nano Lett. 22, 10128 (2022)] and the DMI-induced magnetic phenomena, e.g. skyrmions-based logic gates [Natl. Sci. Rev. 9, nwac021 (2022], DMI-torque switching of perpendicular magnetization [PRL 130, 056701 (2023)].

报告人简介：

杨洪新，南京大学物理学院教授。从事自旋电子学理论计算研究，重点致力于反对称交换耦合Dzyaloshinskii-Moriya Interaction (DMI)和垂直磁各向异性等的理论计算方法发展、物理机制探索、材料体系开发和新型自旋电子学器件设计等研究。发展的第一性原理计算DMI的实空间自旋螺旋态短自旋波超原胞算法已是国际上最被广泛使用的方法。发表学术论文100余篇, 重要期刊（Nat. Rev. Phys./Nat. Mater./Nat. Nanotech./PRL/Nat. Comm./Adv. Mater./Nano Lett.）论文23篇。主持国家重点研发子课题、国家海外高层次青年人才、自然科学基金、中科院原始创新等项目。在APS March Meeting及InterMag等做邀请报告，担任2019年APS March Meeting分会主席，组织2019年MMM分会并任分会主席。IEEE senior member。

9 A Fresh Look at the Calculation of Tunneling Actions

报告人：Jose Ramon Espinosa，西班牙马德里IFT ( Instituto de Fisica Teorica)

时间：3月7日 (周二) 16:00

单位：中国科学院理论物理研究所

会议链接：https://www.koush‍are.com/lives/room/567655

摘要：

The calculation of tunneling actions, that control the exponential suppression of the decay of metastable phases (like the unstable electroweak vacuum), can be reformulated as an elementary variational problem in field space. This alternative approach circumvents the use of bounces in Euclidean space by introducing an auxiliary function, a tunneling potential Vt that connects smoothly the metastable and stable phases of the field potential V. The tunneling action is obtained as the integral in field space of an action density that is a simple function of Vt and V and can be considered as a generalization of the thin-wall action to arbitrary potentials. This formalism provides new useful handles for the theoretical understanding of different features of vacuum decay, can be easily extended to include gravitational effects in an elegant way and has a number of interesting properties and applications that I will discuss.

报告人简介：

Jose Ramon Espinosa is a full professor of IFT of Madrid in Spain and now is a visiting professor at CERN. He obtained his PhD in 1994 working on Particle Physics beyond the Standard Model at IEM (CSIC, Madrid) under the supervision of Mariano Quirós. After postdoctoral stays at DESY (Hamburg, Germany), University of Pennsylvania (Philadelphia, USA) and CERN (Geneva, Switzerland), from 1999 to 2008 he joined IMAFF (CSIC, Madrid) and then IFT-UAM/CSIC (Madrid) with a permanent position. From 2008 to 2018 he joined ICREA as Research Professor to work at IFAE. Since 2019 he become a member of IFT, Madrid. He is a world-leading particle physicist in BSM phenomenology and cosmology. He has published over 100 papers with over 17000 citations. Recently, he has discovered a new formalism for quantum tunneling in field theory.

10 Novel Orbital Physics – Unconventional Bose-Einstein Condensation, and itinerant Ferromagnetism in optical lattices

报告人：Congjun WU, Westlake University

时间：3月9日（周四）10:30

单位：南方科技大学物理系

会议链接：https://www.kous‍hare.com/lives/room/278537

摘要：

Orbital is a degree of freedom independent of charge and spin. It plays an important role in physical properties of transition-metal-oxides. The recent developments of cold atom systems in optical lattices have opened up an opportunity to study novel features of orbital physics that are not easily accessible in solid state systems. We predicted that cold bosons, when pumped into high orbital bands of optical lattices, exhibit a class of novel superfluid states spontaneously breaking time-reversal symmetry. In analogy to unconventional superconductivity, their complex-valued condensate wavefunctions possess unconventional symmetries beyond the scope of “no-node” theorem for most states of bosons. This class of unconventional Bose-Einstein condensations have been experimentally realized by a few prominent experimental groups. On the other hand, itinerant ferromagnetism (FM), i.e., FM based on Fermi surfaces instabilities of mobile electrons (fermions), is a hard-core problem of strong correlation physics. The well-known Stoner criterion overestimates the FM tendency by neglecting correlation effects. Furthermore, the paramagnetic metal phase above the Curie temperature, i.e., the Curie-Weiss metal state, is a long-standing challenge. It exhibits a dichotomic nature: The spin channel is incoherent, i.e., local moment-like, while the charge channel remains coherent. In spite of these difficulties, we proved a series of theorems setting up the ground state FM phase in the p-orbital bands. The Curie-Weiss metal phase and the critical scalings of the FM transitions are studied via the sign-problem free quantum Monte-Carlo simulations at high numerical precisions. These results also shed light on the mechanism of itinerant FM and Curie-Weise metal in solid state orbital systems.

报告人简介：

Congjun Wu received his Ph.D. in physics from Stanford University in 2005, and did his postdoctoral research at the Kavli Institute for Theoretical Physics, University of California, Santa Barbara, from 2005 to 2007. He became an Assistant Professor in the Department of Physics at the University of California, San Diego in 2007, an Associate Professor in 2011, and a Professor in 2017. In 2021, he became a Chair Professor at School of Science, Westlake University. He was selected as a New Cornerstone Investigator in 2023, elected to a Fellow of American Physical Society in 2018, and awarded the Sloan Research fellowship in 2008. His research interests are exploring new states of matter and reveling their organizing principles, including quantum magnetism, superconductivity, topological states, mathematical physics, and the numerical method of quantum Monte Carlo simulations.

11 Search for New Physics with Atomic Magnetometers

报告人：Wei Ji（季伟），Helmholtz Institute Mainz

时间：3月9日（周四）14:00

单位：清华大学物理系

地点：理科楼C109

摘要：

Tabletop precision measurements provide powerful tools to search for new physics beyond the standard model, such as dark matter and exotic spin-dependent interactions (5th force). The exotic spin-dependent interaction may be mediated by new ultralight bosons, such as axion or Z', which are also proposed as dark matter candidates. In this talk, I will introduce ultra-high sensitivity alkali-noble gas atomic magnetometers and their application in searching for exotic spin-dependent force. Our recent experiments set stringent limits on different terms of exotic forces between electron-electron, neutron-neutron, or electron-proton. These experimental results represent orders of magnitude improvement by other lab-based experiments. For the Z' boson-mediated term, our result can surpass the limits set by astrophysical methods.

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