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.

Topological Properties in Strained Monolayer Antimony Iodide

Two-dimensional(2D)topological insulators present a special phase of matter manifesting unique electronic properties.Till now,many monolayer binary compounds of Sb element,mainly with a honeycomb lattice,have been reported as 2D topological insulators.However,research of the topological insulating properties of the monolayer Sb compounds with square lattice is still lacking.Here,by means of the first-principles calculations,a monolayer SbI with square lattice is proposed to exhibit the tunable topological properties by applying strain.At different levels of the strain,the monolayer SbI shows two different structural phases:buckled square structure and buckled rectangular structure,exhibiting attracting topological properties.We find that in the buckled rectangular phase,when the strain is greater than 3.78%,the system experiences a topological phase transition from a nontrivial topological insulator to a trivial insulator,and the structure at the transition point actually is a Dirac semimetal possessing two type-ⅠDirac points.In addition,the system can achieve the maximum global energy gap of 72.5 meV in the topological insulator phase,implying its promising application at room temperature.This study extends the scope of 2D topological physics and provides a platform for exploring the low-dissipation quantum electronics devices.

Highly in-plane anisotropic optical properties of fullerene monolayers

Both the intrinsic anisotropic optical materials and fullerene-assembled 2D materials have attracted much interest in fundamental science and potential applications.The synthesis of a monolayer(ML)fullerene makes the combination of these two features plausible.In this work,using first-principles calculations,we systematically study the electronic structure and optical properties of quasi-hexagonal phase(qHP)ML and quasi-tetragonal phase(qTP)ML fullerenes.The calculations of q HP ML show that it is a semiconductor with small anisotropic optical absorption,which agrees with the recent experimental measurements.However,the results for qTP ML reveal that it is a semimetal with highly in-plane anisotropic absorption.The dichroic ratio,namely the absorption ratio of x-and y-polarized lightα_(xx)/α_(yy),is around 12 at photon energy of 0.29 eV.This anisotropy is much more pronounced when the photon energy is between 0.7 and 1.4 eV,whereα_(xx)becomes nearly zero whileα_(yy)is more than two orders of magnitude larger.This indicates qTP ML as a candidate for long-pursuit lossless metal and a potential material for atomically thin polarizer.We hope this will stimulate further experimental eforts in the study of qTP ML and other fullerene-assembled 2D materials.

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.