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Nature Physics reports Shuang Jia and collaborators’work on Magnetic-tunneling-induced Weyl node annihilation in TaP
《自然•物理》发表量子材料科学中心贾爽及合作者关于“TaP中磁场诱导的外尔费米子湮灭”的研究

Realization of the topological aspect in materials has greatly deepened our understanding on various, low-energy quasi-particles in condensed matter physics. A recent breakthrough in this field is the discovery of topological Weyl semimetals. General speaking, the Weyl quasi-particles in crystals manifest themselves as Weyl fermion, a long-sought crucial elemental particle in quantum field theory. These topologically protected Weyl quasiparticles are stable except the annihilation caused by a collision of two chirality-opposite nodes in momentum space. Such annihilation shall hardly occur when the band structure remains intact.    

ICQM faculties Shuang Jia, Chi Zhang, X.C.Xie,and Prof. Junfeng Wang in Huazhong University of Science and Technology, Prof. Hai-Zhou Lu in South University of Science and Technology of China, Prof. M.Zahid Hasan in Princeton University, Prof. Neupert in University of Zurich and Prof. Hsin Lin of National University of Singapore recently conducted the high-field transport experiments on TaP, an architype Weyl semimetal. They discovered a highly unexpected sign reversal in Hall signal, which can be well explained as a signature of Weyl nodes annihilation by the catalysis of magnetic field. A very strong magnetic field can tunnel the two chirality-opposite lowest Laudau band in TaP when the reciprocal of magnetic length and momentum separation of Weyl nodes is comparable. This annihilation of the two Weyl quasiparticles leads to a sizable gap. This work can be found online in Nature Physics (2017). doi:10.1038/nphys4183.

The first author for this work is PhD student Cheng-Long Zhang in ICQM; the corresponding authors are Shuang Jia, Hai-Zhou Lu and Junfeng Wang. This work is supported by National Basic Research Program of China (Grant 80 Nos. 2013CB921901 and 2014CB239302), National Science Foundation of China and the 1000 Talents Program for Young Scientists of China.

Left:Laudau bands under different strength of magnetic fields. A sizable gap opens in strong magnetic field.

Right:The Hall and magneto-resistance signals for TaP in magnetic field. A sign reversal of Hall signal occurs in 34.4 T(Inset:Weyl fermi pockets of TaP in 1st BZ)。

  拓扑概念在凝聚态物理中的引入极大地加深了人们对各种低能态准粒子的理解。近年来,以外尔半金属为代表的拓扑半金属的发现是继拓扑绝缘体的发现以来又一项重大进展。一般认为,晶体中实现的外尔费米子是具有拓扑保护的稳定准粒子。一对具有相反手性的外尔点只有被移动到同一动量坐标上才会发生湮灭。这种粒子和反粒子的相互湮灭在不改变能带结构的前提下是很难实现的。 

  北京大学物理学院的贾爽研究员,张弛研究员,谢心澄教授和包括华中科技大学的王俊峰教授,南方科技大学的卢海舟教授,普林斯顿大学的Hasan教授,苏黎世大学的Neupert教授以及新加坡国立大学林新教授在内的研究团队最近合作开展了在外尔半金属TaP中的相关高磁场电输运研究。他们发现在磁场的催化作用下,一对手性相反的外尔点会在高磁场下发生湮灭,从而导致霍尔信号的反转。在磁场作用下,外尔半金属中的电子能带首先形成了一套特殊的分立朗道能带。特别是最后一个朗道能带由于其特殊的拓扑结构而具有手性,其在动量空间中是单向的。当磁场很高时,此时电子的磁长度的倒数可和两个相反手性的外尔点的动量距离相比拟,因此物理上允许发生磁隧穿效应而导致最后两个手性的朗道能带发生杂化,最终实现外尔费米子的湮灭和能隙的打开。该工作已在《自然•物理》上在线发表 Nature Physics (2017). doi:10.1038/nphys4183。

  此项工作的通讯作者为贾爽研究员,南方科大卢海舟教授和华中科技大学王俊峰教授;第一作者为量子材料科学中心13级博士生张成龙。同时,这项研究受到国家重大科学研究计划(2014CB239302,2013CB921901)、国家自然科学基金,和中组部“青年千人”计划的支持。

左图:磁场下TaP的朗道能带示意图,描述了其能隙的打开过程。

右图:相应的霍尔和磁电阻的输运表征,在34.4 T 下霍尔信号发生反转(插图:TaP的外尔费米口袋)。