Nontrivial Topological Phases in Ternary Borides M_(2)XB_(2)(M=W,Mo;X=Co,Ni)

The nontrivial band topologies protected by certain symmetries have attracted significant interest in condensed matter physics.The discoveries of nontrivial topological phases in real materials provide a series of archetype materials to further explore the topological physics.

Predicting Dirac semimetals based on sodium ternary compounds

Predicting new Dirac semimetals,as well as other topological materials,is challenging since the relationship between crystal structure,atoms and band topology is complex and elusive.Here,we demonstrate an approach to design Dirac semimetals via exploring chemical degree of freedom.Based on understanding of the well-known Dirac semimetal,Na3Bi,three compounds in one family,namely Na_(2)MgSn,Na_(2)MgPb,and Na2CdSn,are located.Furthermore,hybrid-functional calculations with improved accuracy for estimation of band inversion show that Na_(2)MgPb and Na_(2)CdSn have the band topology of Dirac semimetals.The nontrivial surface states with Fermi arcs on the(100)and(010)surfaces are shown to connect the projection of bulk Dirac nodes.Most importantly,the candidate compounds are dynamically stable and have been experimentally synthesized.The ideas in this work could stimulate further predictions of topological materials based on understanding of existing ones.

Evolution of Weyl nodes in Ni-doped thallium niobate pyrochlore Tl_(2-x)Ni_(x)Nb_(2)O_(7)

The magnetic Weyl semimetal(WSM)is important for fundamental physics and potential applications due to its spontaneous magnetism,robust band topology,and enhanced Berry curvature.It possesses many unique quantum effects,including a large intrinsic anomalous Hall effect,Fermi arcs,and chiral anomaly.In this work,using ab initio calculations,we propose that Nidoped pyrochlore Tl2Nb2O7is a magnetic WSM caused by the exchange field splitting on bands around its quadratic band crossing point.The exchange field tuned by Ni 3d on-site Coulomb interaction parameter U drives the evolution of Weyl nodes and the resulting topological phase transition.As Weyl nodes can exist at generic points in the Brillouin zone and are hard to identify exactly,their creation and annihilation,i.e.,the change in their number,chirality,and distribution,have been consistently confirmed with a combined theoretical approach,which employs parity criterion,symmetry indicator analysis,and the Wilson loop of the Wannier center.We find that Weyl nodes remain in a large range of U and are close to the Fermi level,which makes the experimental observation very possible.We think that this method and our proposal of magnetic WSM will be useful in finding more WSMs and add to the understanding of the topological phase transition.

具有高阶费米弧的可调狄拉克半金属:Kagome晶格Pd_(3)Pb_(2)X_(2)

近十年来,狄拉克半金属因其独特的物性而受到了广泛的关注和研究.然而它的特征仍然不清楚.体-边界对应关系在识别拓扑态方面取得了巨大成功,但是人们在理论上发现该对应关系并不适用于狄拉克半金属.本文通过第一性原理计算和低能有效模型分析提出kagome晶格材料Pd_(3)Pb_(2)X_(2)(X=S,Se)是独特的第一类狄拉克半金属.它们的表面没有费米弧,这不同于人们所熟知的狄拉克半金属Na_(3)Bi和Cd_(3)As_(2).该结果充分表明表面费米弧的存在不是狄拉克半金属的必要条件.由于体-铰链对应,非平庸的拓扑不变量保证在它们的特定铰链上存在高阶费米弧和分数电荷.这些铰链上的独特性质是三维狄拉克点的直接和拓扑结果.此外,Pd_(3)Pb_(2)X_(2)也可以通过外部应力驱动为第二类狄拉克半金属或弱拓扑绝缘体.本工作为这些独特的狄拉克半金属以及其相关拓扑相变的研究提供了可行的平台.