应变调控下Tl_(2)Ta_(2)O_(7)中的拓扑相变

Topological phase transitions in Tl_(2)Ta_(2)O_(7) under strain regulation

拓扑电子材料因为具有非平庸的拓扑态,所以会展现出许多奇异的物理性质.本文通过第一性原理计算对应变调控下的烧绿石三元氧化物Tl_(2)Ta_(2)O_(7)中的拓扑相变进行了研究.首先分析了原子轨道投影能带,发现体系费米能级附近O原子的(px+py)与pz轨道发生了能带反转,再构造了紧束缚模型计算得到体系的Z2拓扑不变量确定了其拓扑非平庸性,最后研究了表面态等拓扑性质.研究发现未施加应变的Tl_(2)Ta_(2)O_(7)是一个在费米能级处具有二次能带交叉点的半金属,而平面内应变会破缺晶体对称性进而使体系发生拓扑相变.当对体系施加-1%的压缩应变时,它会转变为狄拉克半金属;当对体系施加1%的拉伸应变时,体系相变为拓扑绝缘体.本研究对于在三维材料中调控拓扑相变有着较重要的指导意义,并且为低能耗电子器件的设计提供了良好的材料平台.

Topological electronic materials exhibit many novel physical properties,such as low dissipation transport and high carrier mobility.These extraordinary properties originate from their non-trivial topological electronic structures in momentum space.In recent years,topological phase transitions based on topological electronic materials have gradually become one of the hot topics in condensed matter physics.Using first-principles calculations,we explore the topological phase transitions driven by in-plane strain in ternary pyrochlore oxide Tl_(2)Ta_(2)O_(7).Firstly,we analyze the atomic-orbital-resolved band structure and find that the O(px+py)and pz orbitals of the system near the Fermi level have band inversion,indicating the emergence of topological phase transitions in the system.Then the tight-binding models are constructed to calculate the Z2 topological invariants,which can determine the topologically non-trivial feature of the system.Finally,topological properties such as surface states and a three-dimensional Dirac cone are studied.It is found that Tl_(2)Ta_(2)O_(7) without strain is a semimetal with a quadratic band touching point at Fermi level,while the in-plane strain can drive the topological phase transition via breaking crystalline symmetries.When the system is under the-1%in-plane compression strain and without considering the spin orbit coupling(SOC),the application of strain results in two triply degenerate nodal points formed in the-Z to G direction and G to Z direction,respectively.When the SOC is included,there are two fourfold degenerate Dirac points on the-Z to G path and G to Z path,respectively.Thus,the-1%in-plane compression strain makes the system transit from the quadratic contact point semimetal to a Dirac semimetal.When 1%in-plane expansion strain is applied and the SOC is neglected,there exists one band intersection along Y→G.When the SOC is taken into consideration,the gap is opened.Therefore,the 1%in-plane expansion strain drives Tl_(2)Ta_(2)O_(7) into a strong topological insulator.In addition,the system is also expected to have strong correlation effect and superconductivity due to the possible flat band.This work can guide the study of topological phase transitions in three-dimensional materials and provide a good material platform for the design of low-dissipation electronic devices.