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.

Tunable Dirac semimetals with higher-order Fermi arcs in Kagome lattices Pd_(3)Pb_(2)X_(2)(X=S,Se)

Over the decade,Dirac semimetals(DSMs)have been extensively studied[1].However,the hallmarks of DSMs are still not clear[2,3].Recently,a generalized bulk-boundary correspondence,namely higher-order bulk-hinge correspondence,for DSMs[4–7]has been proposed,i.e.,one-dimensional(1D)higher-order Fermi arcs(HOFAs)are direct and topological consequences of 3D bulk Dirac points.The 3D bulk Dirac points lead to the nontrivial filling anomalyη[8,9]of the 2D insulating momentum-space plane away from them,which ensures the presence of gapless mid-gap states on 1D hinges.