（7.25）学术报告：Topological Superconductor and Majorana Fermions；Interaction effect in 2D topological insulators

报告地点：强磁场中心磁体研制楼会议室201会议室                                                             

报告时间：7月25日上午 9: 00 （星期四）

主持人：田明亮 研究员                                                                    

报告一： Topological Superconductor and Majorana Fermions           

报告人：胡晓 教授

International Center for Materials Nanoarchitectonics (WPI-MANA)

National Institute for Materials Science (NIMS), Japan                                   

报告人简介：胡晓教授1979年考入北京大学物理系，1981年进入日本东京大学继续大学教育，并于1985年获得理学学士学位。1987和1990年分别在东京大学获理学硕士和博士学位。他于1991-1995年在东京大学从事博士后研究，日本Tohoku大学助教，1995-96年在美国国家标准和技术研究所做访问学者，1996年任Tohoku大学副教授，2007至今兼任Tsukuba大学纯粹和应用科学学院教授。1996-2002任日本国立金属材料研究所（NRIM）研究员。2002-2007年任国立物质/材料研究机构（NIMS）研究组主任。2007年至今任日本世界顶级计划材料纳米结构学国际研究中心（WPI-MANA）首席研究员。胡教授与国内合作关系密切，2006-2010年是中科院“超导和新颖电子系统”国际创新团队成员，2008年担任复旦大学量子调控中心领导小组成员，2011年入选中组部千人计划特聘教授。胡教授的研究兴趣包括：奇异磁性和纳米磁体、新颖超导电性和涡旋态、拓扑量子态和应用。发表论文190 多篇，邀请报告90 多次。他获得的荣誉与奖项包括：1988年中日科学技术交流协会科学研究最高成就奖；1995年获日本基础和金属有关材料应用研究重要贡献Harada奖；2013年他的“超导规范不变位相的量子现象与新功能理论研究”获国立物质/材料研究机构最高成就奖。                               

报告摘要： A vortex in topological superconductor induces two Majorana fermions (MFs), one in the core and the other at the sample edge. In the present work, we notice that edge MFs are very useful in applications since they can be transported and braided much easier than core MFs. Explicitly we consider a finite sample of s-wave superconductor/spin-orbital coupling semiconductor hetero structure with a superconducting vortex at the sample center, which we call a brick. Bricks are connected by constriction junctions, which can be turned on and off adiabatically by applying point gate voltages. We reveal that based on the topological property, namely an edge MF appears when the edge contains odd-number vorticity and should disappear for even-number vorticity, edge MFs can be transported and braided in our device merely by switching on and off the gate voltages with a designed sequence. The adiabatic dynamics of edge MFs has been monitored by solving the time-dependent Bogoliubov-de Gennes (BdG) equation, and the non-Abelian braiding of MFs is confirmed. Therefore, the present work bridges for the first time the fundamental topological features of edge MFs and their adiabatic dynamics, which is important for performing topological quantum computation.

This work is based on collaboration between Q. –F. Liang and Z. Wang.           

报告二： Interaction effect in 2D topological insulators               

报告人：吴从军教授（University of California, San Diego） 

报告人简介：吴从军教授于2005年毕业于美国Stanford 大学物理系并获得理学博士学位。 于2005-2007年在加州大学圣.芭芭拉分校的Kavli理论物理研究所从事博士后研究。他于2007年任加州大学圣地亚哥分校助理教授， 2011年晋升为副教授。 他的研究兴趣包括：量子磁体、超导、轨道物理和量子拓扑态的理论研究。

报告摘要：Interaction effects are particularly important in the 1D helical edge states of the 2D topological insulators. Although the single particle backscattering is forbidden by time-reversal symmetry determined by the Z2 topology, the two-particle correlated backscattering processes are allowed by the time-reversal symmetry, and can destabilize the edge states by opening the gap with spontaneously developing magnetic orderings. Bosonization analysis is employed to give the concrete criterion for the stability of the helical edge states with interactions. Furthermore, we have proved that as a particular example, the Kane-Mele-Hubbard model with purely imaginary next-nearest-neighbor hoppings has a particle-hole symmetry at half filling. Such a symmetry leads to the absence of the sign problem in the determinant quantum Monte Carlo simulations (QMC). Consequentially, the interplay between band topology and strong correlations can be studied at high numeric precisions. It has been found that magnetic correlations are stronger in the edge states than in the bulk in the QMC simulations. With increasing the interaction strength the edge states are nearly destabilized even when the bulk remains paramagnetic and time-reversal invariant.

1）   Congjun Wu, B. Andrei Bernevig, and Shou-Cheng Zhang, " The Helical Liquid and the Edge of Quantum Spin Hall Systems ",  Phys. Rev. Lett. 96 , 106401(2006).

2）Dong Zheng, Guang-Ming Zhang, Congjun Wu, "Particle-hole symmetry and interaction effects in the Kane-Mele-Hubbard model", Phys. Rev. B 84, 205121 (2011) .