报告（一）

报告题目：Time-reversal-symmetry broken quantum spin Hall state in even-layer MnBi2Te4

报告人: 周晓东  复旦大学

报告时间：12月13日  下午14:30-15：00

报告地点：腾讯会议129-527-605

报告摘要：Being the first intrinsic antiferromagnetic (AFM) topological insulator (TI), MnBi2Te4 is argued to be a topological axion state in its even-layer form due to the antiparallel magnetization between the top and bottom layers. Here we combine both macroscopic transport and microscopic imaging techniques (sMIM and sSQUID) to investigate such axion state in atomically thin MnBi2Te4 with even-layer thickness at zero magnetic field. While transport measurements show a zero Hall plateau signaturing the axion state, sMIM uncovers an unexpected edge state raising questions regarding the nature of the axion state. Based on our model calculation, we propose that the edge state of even-layer MnBi2Te4 at zero field is derived from a pair of gapped helical edge states of the quantum spin Hall effect with broken time-reversal-symmetry, when a crossover from a three-dimensional TI to a two-dimensional TI occurs. Our transport and sSQUID measurement also reveal that such one-dimensional gapped helical edge states display a Luttinger liquid behavior. These findings signify the richness of topological phases in MnB2Te4 that has yet to be fully explored.

报告人简介：周晓东，复旦大学微纳电子器件与量子计算机研究院青年研究员。2008年于中国科学技术大学物理系获学士学位，2014年于清华大学物理系获博士学位。2014-2017年美国哥伦比亚大学博士后，随后于2017年初加入复旦大学。研究工作主要是利用各种扫描探针手段研究低维量子材料，包括高温超导材料、锰氧化物庞磁阻材料、石墨烯和拓扑材料。近期在本征反铁磁拓扑绝缘体MnBi2Te4上开展了一系列研究，发现在偶数薄片MnBi2Te4中存在一维导电边界态，挑战了轴子绝缘体的理论预言。在Phys. Rev. Lett.，Nat. Commun.，ACS Nano，Nano Lett.，Phys. Rev. B等著名学术期刊发表多篇论文。

 报告（二）

报告题目：Magnetoelectric Coupling in Hexagonal Multiferroic Materials

报告人: 王文彬  复旦大学

报告时间：12月13日  下午15:00-15：30

报告地点：腾讯会议129-527-605

报告摘要：Multiferroicity has been subjected to extensive research and broad interest from fundamental and applied perspectives. We used various means to tune and characterize the magnetoelectric coupling properties of the single-layer and double-layer hexagonal ferrites. In oxygen-enriched double-layer hexagonal Lu2FeO4, our experiments revealed that ordering of the oxygen interstitials causes periodic displacements of the Lu and Fe atoms associated with a new type of charge ordering and spin configuration. Moreover, we discovered a series of charge order phases correlated with ferri- and antiferro-magnetic orders in Lu2FeO4 under pressure. The observed charge and spin orders were further supported by the density functional theory calculations, revealing strong coupling among charge, spin and lattice degrees of freedom in this system. In addition, we show that pressure is effective to enhance the magnetic ordering temperature and ferroelectric polarization in single-layer hexagonal Lu0.5Sc0.5FeO3 . These results provide direct insights into the coupling mechanism between multiple order parameters in this class of materials.

报告人简介： 王文彬，复旦大学微纳电子器件与量子计算机研究院 研究员， 2006年获兰州大学学士学位， 2012年获得美国田纳西大学物理系/美国橡树林国家实验室博士学位。2013-2015年在复旦大学物理系做超级博士后。2015-2021年为复旦大学青年研究员。2021年12月晋升为研究员。主要研究凝聚态物理中的复杂氧化物体系，重点研究多铁氧化物中的磁电耦合效应。利用激光脉冲沉积法生长各种高质量的复杂氧化物单晶薄膜，深入研究其序参量之间的耦合机制，共发表SCI论文50余篇。

 报告（三）

报告题目：Topological Lifshitz transition and one-dimensional Weyl mode in HfTe5

报告人: 袁翔华  东师范大学

报告时间：12月13日  下午15:30-16：00

报告地点：腾讯会议129-527-605

报告摘要： Landau band crossings typically stem from the intra-band evolution of electronic states in magnetic fields and enhance the interaction effect in their vicinity. Here in the extreme quantum limit of topological insulator HfTe5, we report the observation of a topological Lifshitz transition from inter-band Landau level crossings using magneto-infrared spectroscopy. By tracking the Landau level transitions, we demonstrate that band inversion drives the zeroth Landau bands to cross with each other after 4.5 T and forms a one-dimensional Weyl mode with the fundamental gap persistently closed. The unusual reduction of the zeroth Landau level transition activity suggests a topological Lifshitz transition at 21 T, which shifts the Weyl mode close to the Fermi level. As a result, a broad and asymmetric absorption feature emerges due to the Pauli blocking effect in one dimension, along with a distinctive negative magneto-resistivity. Our results provide a strategy for realizing one-dimensional Weyl quasiparticles in bulk crystals.

W. Wu, X. Yuan* et al., Nature Materials (2022).

报告人简介：袁翔，复旦大学物理系博士，加州大学伯克利分校联合培养博士，现任华东师范大学青年研究员、博导。主要从事拓扑量子材料的强磁场特性研究，致力于对拓扑半金属开展强磁场红外光谱、电输运等物理性质的联合探索，同时兼顾拓扑纳米应用器件。在SCI学术期刊发表论文30余篇，其中以第一或通讯发表Nature 1篇，Nature Materials 2篇，Nature Communications 2篇等，部分论文获ESI高引。

 报告（四）

报告题目：Nonlinear transport in the quantum limit of HfTe5

报告人: 张成  复旦大学

报告时间：12月13日 下午16:00-16：30

报告地点：腾讯会议129-527-605

报告摘要：The interplay of electron correlations and topological phases gives rise to various exotic phenomena including fractionalization, excitonic instability, and axionic excitation. Recently-discovered transition-metal pentatellurides can reach the ultra-quantum limit in low magnetic fields and serve as good candidates for achieving such a combination. Here, we report evidences of density wave and metal-insulator transition in HfTe5 induced by intense magnetic fields. Using the nonlinear transport technique, we detect a distinct nonlinear conduction behavior in the longitudinal resistivity within the a-c plane, corresponding to the formation of a density wave induced by magnetic fields. In high fields, the onset of the nonlinear conduction in the Hall resistivity indicates an impurity-pinned magnetic freeze-out as the possible origin of the insulating behavior. These frozen electrons can be gradually re-activated into mobile states above a threshold electric field. These experimental evidences call for further investigations into the underlying mechanism for the bulk quantum Hall effect and field-induced phase transitions in pentatellurides.

报告人简介：张成，2014年和2019年分别获复旦大学物理系学士和博士学位，2017至2018年在华盛顿大学物理系联合培养，2019年博士毕业后入职复旦大学微纳电子器件与量子计算机研究院担任青年研究员、博士生导师，入选国家万人计划青年拔尖人才、上海市教委曙光学者、上海市青年拔尖人才和上海市科委扬帆计划等人才项目，担任科技部重点研发计划（青年）项目负责人。主要从事拓扑电子态的量子输运、量子调控和拓扑电子器件研究，代表性工作包括发现基于外尔轨道的三维量子霍尔效应；发现准线性费米弧表面态的低耗散特性并实现二维体系最高面电导率；利用非局域输运方案实现外尔费米子的手性极化操控；利用红外磁光谱首次发现动态手性反常等。在Nature、Nature Materials、Nature Communications等学术期刊共发表论文30余篇，总引用2400余次。

 报告（五）

报告题目：非线性电输运研究

报告人：何攀 复旦大学

报告时间：12月13日 下午16:30-17：00

报告地点：腾讯会议129-527-605

报告摘要：Nonlinear electric transports, which manifest different information from that of their linear counterparts, have received increasing interest recently. In particular, the second-order nonlinear transports, including the nonlinear magnetoresistance and nonlinear Hall effect, have been widely studied in quantum materials with broken inversion symmetry. In this talk, I will first review our studies on a spin-dependent nonlinear magnetoresistance called bilinear magneto-electric resistance (BMER) in a few spin-polarized nonmagnetic materials. BMER can be used to probe the three-dimensional spin texture in momentum space. Later, I will present our recent work on the nonlinear Hall effects in a topological insulator and a graphene moiré superlattice,under both time-reversal invariant and time-reversal breaking conditions.

报告人简介：何攀，2014年在复旦大学物理学系获博士学位。2014-2015年在法国国家科学研究中心从事博士后研究；2015-2020年在新加坡国立大学任研究员/高级研究员（Senior Research Fellow）；2020年9月加入复旦大学工作至今。入选国家海外高层次人才计划（青年项目）和上海市海外高层次人才计划。主要研究方向为量子材料自旋电子学和低维磁学。我们特别关注拓扑量子材料、二维材料、强关联材料以及它们的薄膜异质结构。这些材料具有有趣的自旋（磁）特性，有望实现低能耗的多功能电子器件。通过运用对称性、尺寸效应、层间堆砌方式和拓扑性质等，我们可以设计和调控这些材料的丰富自旋（磁）特性。我们主要运用电学和光学手段来研究各种自旋电子学现象，包括反常霍尔效应、自旋霍尔效应、自旋轨道力矩、自旋相关非线性效应等。目前在Nature Physics，Nature Nanotechnology，Physical Review Letters，Nature Communications等学术期刊上共发表论文近30篇，总引用1200余次，H因子20。

集体讨论：量子材料中电输运性质研究的机遇与挑战

讨论时间：12月13日   下午17:00-17：45

讨论地点：腾讯会议129-527-605

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