7月19日低温输运学术报告三则

7月19日唐仲英楼B501，9：00-12：00，我们请到了来自普渡大学、北京大学和复旦大学的三位教授同场做学术报告，其中陈勇教授是普渡大学的终身教授。他们的研究方向都是拓扑材料的量子输运，近年来在这一方向都在Nature子刊发表了多篇优秀的工作。报告信息如附件，题目如下：

Quantum transport in topological insulators

Yong P Chen，普渡大学

低维狄拉克材料的电子输运性质

廖志敏，北京大学

Field-driven phase transition and magneto-optical properties in topological Dirac semimetals

修发贤，复旦大学

Field-driven phase transition and magneto-optical properties in topological Dirac semimetals
Faxian Xiu
State Key Laboratory of Surface Physics and Department of Physics, Fudan University, Shanghai 200433, China
Three-dimensional topological Dirac semimetals (TDSs) are a new kind of Dirac materials that exhibit linear energy dispersion in the bulk and can be viewed as three-dimensional graphene [1-3]. It has been proposed that TDSs can be driven to other exotic phases like Weyl semimetals, topological insulators and topological superconductors by breaking certain symmetries. In this talk, I will first report the Landau level splitting in TDS Cd₃As₂ single crystals under high magnetic fields (up to 60T), which suggests the removal of spin degeneracy by breaking time reversal symmetry [1]. The detected Berry phase develops an evident angular dependence and possesses a crossover from nontrivial to trivial state under high magnetic fields, a strong hint for a fierce competition between the orbit-coupling and the field-generated mass term (Figure 1). Then, I will briefly review our recent progress in chiral anomaly by showing exclusively new approaches to detect the chiral anomaly and the related valley transport in ultra-high mobility Cd₃As₂ Dirac semimetal [4]. Three independent evidences including the E∙B-generated magneto-optical Kerr effect, the negative MR, and the valley transport, are provided as a direct and convincing experimental identification for the chiral anomaly in crystals. Finally, I would like to talk about the magneto-optical measurements in Cd₃As₂/ZrTe₅ Dirac semimetals and some exciting transport measurements under ultra-high magnetic field [5, 6], in which a striking topological phase transition takes place (around 30 T), i.e., the dynamical mass generation. The Dirac electron spontaneously acquires a Dirac mass due to electron-electron interactions. These transitions also manifest themselves as spin density waves in both first and zeroth Landau levels. Our study presents the very first example of the dynamical mass generation phenomenon occurring in three-dimensional massless Dirac fermions in condensed matter physics.
References
[1] J. Cao, et. al, Z. Xia, L. Li, Faxian Xiu, Landau level splitting in Cd3As2 under high magnetic fields, Nature Communications 6, 7779 (2015).
[2] Yanwen Liu, et. al., Faxian Xiu, Gate-tunable quantum oscillations in ambipolar Cd3As2 thin films, NPG Asia Materials 7, e221 (2015).
[3] Enze Zhang, et al. Faxian Xiu, Magnetotransport properties of Cd3As2 nanostructures, ACS Nano 9 (9), 8843–8850 (2015).
[4] Cheng Zhang, et al., Faxian Xiu, Detection of chiral anomaly and valley transport in Dirac semimetals, arXiv:1504.07698 (2015).
[5] Xiang Yuan, et. al., Faxian Xiu, Observation of quasi-two-dimensional Dirac fermions in ZrTe5, arXiv:1510.00907 (2015).
[6] Xiang Yuan, et al., Faxian Xiu, Direct observation of Landau level resonance and mass generation in Dirac semimetal Cd3As2 thin films, arXiv:1601.02316 (2016).

Academic Training

Ph.D. in Electrical Engineering, University of California, Riverside. Advisor: Professor Jianlin Liu, 2007
M.S. in Materials Science and Engineering, University of California, Los Angeles. Advisor: Professor Jenn-Ming Yang, 2002
B.E. in Materials Science and Engineering, Harbin Institute of Technology, Harbin, China. Advisor: Professor Gaohui Wu, 2000

Professional Experience

Professor, Physics Department, Fudan University, 2012-present
Assistant Professor, Electrical and Computer Engineering, Iowa State University, 2011-2012
Staff Research Associate, Electrical Engineering, University of California, Los Angeles, Advisor: Raytheon Professor Kang L. Wang, 2008-2011

Awards and Honors

2014年复旦大学卓识计划
2013年国家自然科学基金-优秀青年基金
2013年上海市浦江人才计划
2012 年复旦特聘教授，入选国家海外高层次人才计划（青年组）

Bio: Prof. Xiu received his Master degree in Materials Science and Engineering from UCLA in 2002, and his Ph. D. degree from the Electrical Engineering at UC Riverside in 2007. After his Ph.D., he joined UCLA as a staff research associate prior to his tenure-track assistant professor position at Iowa State University. In 2012, he became a Full professor of physics department at Fudan University. Dr. Xiu has interdisciplinary background involving materials science, electrical engineering and physics. He has over 80 peer-reviewed publications and received numerous media attention on his work of dilute magnetic semiconductors and topological insulators. Currently he mainly focuses on the topological Dirac semimetals and their exotic low-temperature transport and magneto-optical properties under high magnetic field.

低维狄拉克材料的电子输运性质
廖志敏
北京大学物理学院

Abstract
狄拉克电子材料中的载流子有效质量为零，能量和动量之间为线性色散关系，这种独特的狄拉克锥形电子能带结构导致许多新奇的量子输运性质。本报告我将汇报我们关于低维狄拉克电子材料（包括石墨烯、拓扑绝缘体Bi₂Se₃纳米结构、狄拉克半金属Cd₃As₂纳米结构）的制备和量子输运性质方面的工作：（1）石墨烯垂直结构的构筑，以及在电流垂直于石墨烯平面的情况下，温度、磁场、门电压对载流子输运的调制作用；（2）在高迁移率的Bi₂Se₃纳米结构中观测到来源于Bi₂Se₃纳米带侧面表面态的Shubnikov-de Haas (SdH)振荡，通过圆偏振光选择性激发拓扑绝缘体的表面态，增强了体系的光热电效应；（3）在狄拉克半金属Cd₃As₂纳米线中观测到负磁电阻效应和A-B效应，给出了外尔费米子手性反常效应以及表面态输运的实验依据。

报告人简介：廖志敏，2002年本科毕业于中山大学物理系，2007年博士毕业于北京大学物理学院，然后留校任讲师，期间2010年1月至2011年1月在爱尔兰圣三一学院从事博士后研究，2011年晋升为副教授。主要研究方向是低维材料的制备和电子输运性质。

Quantum transport in topological insulators
Yong P. Chen, Purdue University
Abstract: Three-dimensional (3D) topological insulators (TI) are a novel class of electronic materials with topologically-nontrivial band structure such that the bulk is gapped and insulating yet the surface has topologically protected gapless conducting states. Such “topological surface states” (TSS) give helically spin polarized Dirac fermions, and offer a promising platform to realize various other novel physics such as topological magnetoelectric effects and Majorana fermions, and may enable technological applications in areas such as spintronics and thermoelectrics. However, it is often challenging to unambiguously access and study the transport properties of TSS in many practical TI materials due to non-negligible bulk conducting states. I will discuss some of our experiments on high-quality “intrinsic” TIs with insulating bulk and surface-dominated conduction that allow us to reveal a number of characteristic quantum transport properties of spin-helical Dirac fermion topological surface states, such as the “half-integer” quantum Hall effect [1-2] and “half-integer” Aharonov-Bohm effect [3]. I will also discuss some issues and cautions related to interpreting commonly measured magnetotransport signatures such as “2D” quantum oscillations, weak antilocalization and linear magnetoresistance that may or may not arise from topological surface states or be unique to topological insulators [4-6]. If time permits, I may discuss other ways of probing TI transport using magnetic (spin-dependent) or superconducting (phase sensitive) electrodes.
References:
[1] Y. Xu et al., Nature Physics 10, 956 (2014)
[2] Y. Xu et al., Nature Communications 7, 11434 (2016)
[3] L. A. Jauregui et al., Nature Nanotechnology 11, 345 (2016)
[4] H. Cao et al., Physical Review Letters 108, 216803 (2012)
[5] L. A. Jauregui et al., Scientific Reports 5, 8452 (2015)
[6] J. Tian et al., Scientific Reports 4, 4859 (2014)

西安交通大学应用数学系，本科，1996毕业
MIT，数学系硕士，导师Jacob White， 1999
普林斯顿,电子工程系，导师Daniel C. Tsui ，2005
2005-2007，在Rice大学做博士后，
2007年受聘普渡大学电子与计算机工程助理教授，并于2012年获tenure副教授，2016年任教授
研究领域
Experimental condensed matter physics & nanoscience (graphene/2D materials, topological insulators,2D electrons/quantum Hall physics) and cold atom/quantum physics/quantum photonics (Bose-Einsteincondensation, cold molecules, hybrid quantum systems)
获奖情况
Masao Horiba Award for nanoparticle Measurement (2015);
Purdue University Faculty Scholar (2013-2018);
Purdue University Excellence in Research Award (2012, 2013);
NSF CAREER Award (2009-2014);
IBM Faculty Award (2009);
Defense Threat Reduction Agency (DTRA) Young InvestigatorAward (2009-2011);
Miller Family Professorship, Purdue University (2007-2012);
J. Evans Attwell-Welch Postdoctoral Fellowship in Nanoscience, Rice University (2005-2007); International Union ofPure and Applied Physics (IUPAP)
代表论文与引用
Citations: >6,000 (Google Scholar), >4,000 (ISI); H-index: 37 (Google Scholar),
1) Y.Xu, I.Miotkowski, Y.P. Chen, Quantum transport of two-species Dirac fermions in dual-gated
three-dimensional topological insulators, Nature Communications in press (2016)
DOI:101038/ncomms11434
2) Luis A. Jauregui, Michael T. Pettes, Leonid P. Rokhinson, Li Shi, Yong P. Chen, Magnetic field
induced helical mode and topological transitions in a quasi-ballistic topological insulator nanoribbon
with circumferentially quantized surface state sub-bands, Nature Nanotechnology 11, 345 (2016)
3) Jifa Tian, I. Miotkowski, S. Hong and Yong P. Chen, “Electrical injection and detection of spinpolarized
currents in topological insulator Bi2Te2Se”, Scientific Reports 5, 14293 (2015)
4) Y.Xu, I.Miotkowski, C.Liu, J.Tian, H.Nam, N.Alidoust, J.Hu, C-K.Shih, M. Z. Hasan, Y.P.Chen,
“Observation of topological surface state quantum Hall effect in an intrinsic three-dimensional
topological insulator”, Nature Physics 10, 956 (2014)
Prof. Dr. Yong P. Chen Short CV (3-page) 4/15/2016
5) Oleg Yazyev and Yong P. Chen, Polycrystalline graphene and other two-dimensional materials
(invited review), Nature Nanotechnology 9, 755 (2014)
6) P.Yasaei, …, Y.P. Chen, P.Král, and Amin Salehi-Khojin, “Chemical Sensing with Switchable
Transport Channels in Graphene Grain Boundaries”, Nature Communications 5, 4911 (2014)
7) M. Hajlaoui, …, Y.P. Chen, …, M. Marsi, “Tuning a Schottky barrier in a photoexcited topological
insulator with transient Dirac cone electron-hole asymmetry”, Nature Communications 5, 3003 (2014)
8) J.Tian, C.Chang, H.Cao, K.He, X.Ma, Q-K.Xue, Y.P. Chen, “Quantum and Classical
Magnetoresistance in Ambipolar Topological Insulator Transistors with Gate-tunable Bulk and Surface
Conduction”, Scientific Reports 4, 4859 (2014)
9) A.J. Olson, S-J. Wang, R.J. Niffenegger, C-H. Li, C.H. Greene, Y.P. Chen, “Tunable Landau-Zener
transitions in a spin-orbit coupled Bose-Einstein condensate, Phys. Rev. A 90, 013616 (2014)
10) S.Dutta, J.Lorenz, A.Altaf, D. S. Elliott, Y.P. Chen, Photoassociation of ultracold LiRb* molecules:
observation of high efficiency and unitarity-limited rate saturation, Phys. Rev. A 89, 020702(R) (2014)
11) Chris Mann, Damien West, Ireneusz Miotkowski, Yong P. Chen, Shengbai Zhang, Chih-Kang Shih,
“Mapping the 3D surface potential in Bi2Se3”, Nature Communications 4, 2277 (2013)
12) R. He*, T. F. Chung*, … , Y. P. Chen, “Observation of Low Energy Raman Modes in Twisted Bilayer
Graphene” , Nano Lett. 13, 3594 (2013)
13) N.K. Emani, T-F. Chung, X.Ni, A.V. Kildishev,Y.P. Chen, A. Boltasseva, “Electrically Tunable
Damping of Plasmonic Resonances with Graphene”, Nano Lett. 12, 5202 (2012)
14) H.Cao, …, Yong P. Chen, “Quantized Hall effect and Shubnikov--de Haas oscillations in highly
doped Bi2Se3: Evidence for layered transport of bulk carriers”, Phys. Rev. Lett. 108, 216803 (2012)
15) J.Tian, H.Cao, W.Wu, Q.Yu, Y.P. Chen, “Direct Imaging of Graphene Edges: Atomic Structure and
Electronic Scattering”, Nano Lett. 11, 3663 (2011)
16) Q.Yu*, L.A. Jauregui*, … , Yong P. Chen , “Control and characterization of individual grains and
grain boundaries in graphene grown by chemical vapour deposition”, Nature Materials 10, 415 (2011)
17) H.Cao, Q.Yu, …, Y. P. Chen, “Electronic Transport in Chemical Vapor Deposited Graphene
Synthesized on Cu: Quantum Hall Effect and Weak Localization”, Appl. Phys. Lett. 96, 122106 (2010)
18) Jiuning Hu, Xiulin Ruan, Yong P. Chen, “Thermal conductivity and thermal rectification in graphene
nanoribbons:a molecular dynamics study”, Nano Lett. 9, 2730 (2009)
19) S. E. Pollack, D. Dries, M. Junker, Y.P. Chen, T. Corcovilos and R.G. Hulet, “Extreme tunability of
interactions in a 7Li Bose-Einstein condensate”, Phys. Rev. Lett. 102, 090402 (2009)
20) Qingkai Yu, Jie Lian, Sujitra Siripongert, Hao Li, Yong P. Chen, and Shin-Shem Pei, Graphene
segregated on Ni surfaces and transferred to insulators, Applied Physics Letters 93, 113103 (2008)
21) Y.P. Chen, J.Hitchcock, D.Dries, M.Junker, C.Welford, R.G.Hulet, “Phase coherence and superfluidinsulator
transition in a disordered Bose-Einstein condensate”, Phys. Rev. A 77, 033632 (2008)
22) Y.P. Chen, .., L. W. Engel, D. C. Tsui, P. D. Ye, L. N. Pfeiffer, and K. W. West, “Melting of a 2D
Quantum Electron Solid in High Magnetic Field”, Nature Physics 2, 452 (2006)
23) Y.P. Chen, R. M. Lewis, L. W. Engel, D. C. Tsui et al., “Evidence for Two Different Solid Phases of
Two Dimensional Electrons in High Magnetic Fields”, Phys. Rev. Lett. 93, 206805 (2004)
24) Yong Chen, R. M. Lewis, L. W. Engel, D. C. Tsui et al., “Microwave Resonance of the 2D Wigner
Crystal Around Integer Landau Fillings”, Phys. Rev. Lett. 91, 016801 (2003)

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