Based On The Janus Functionalization Scheme,Design The Intrinsically Spin-valley-coupled Nodle And Nodal-loop Topological Semimetals

In recent years,two-dimensional(2D)V-V binary materials have attracted much attention,which is mainly due to the combination of different V-V elements can produce many ideal physical properties,such as moderate energy gap,high carrier mobility,excellent thermoelectric properties,large Rashba spin splitting,and various novel topological quantum states.More importantly,some of the binary materials formed between the V group elements have been experimentally confirmed.In 2019,a new class of 2D Dirac quantum states,spin-valley-coupled Dirac semimetals(svc-DSMs),which are different from graphene,was discovered in a functionalized V-V binary material Sb As X2 monolayer(ML).The results show that the linear dispersion of svc-DSMs is dependent on spin and valley,which can produce the Dirac spin-valley Hall effect,and has potential application value in the future quantum devices with low energy consumption and ultra-high speed.However,the existing real material is not the nature,the need for field regulation.Based on this,this paper focuses on the problem of ‘how to realize the intrinsic svc-DSM,and whether the cascade effect of the intrinsic system can generate new quantum states?'to carry out in-depth research.It's specific contents and conclusions are as follows:(1)In view of the influence of electronegativity of functional groups on the built in electric field in the case of asymmetric central inversion,a Janus functionalization scheme is proposed to form a balance between the built in electric field and the spin-orbit coupling and obtain the intrinsic svc-DSM state.With the aid of first-principles calculations,we found that Br Bi As Cl ML has the intrinsic properties of the svc-DSM quantum state by screening the electronic structure of 24 possible Janus functionalized structures of Bi X(X = Sb,As)ML.Due to the strong SOC action,the highest and lowest conductivity bands of Br Bi As Cl ML have a large spin splitting(?h = 0.306 and ?e = 1.272 e V),resulting in the band gap closure and the formation of a spin-valley dependent Dirac cone.More importantly,the svc-DSM state can still be well maintained in Br Bi As Cl ML under the action of suitable substrates,which provides a guarantee for the application of svc-DSMs state devices.(2)Based on the discovery of the first intrinsically svc-DSM material,we further explored the cascading effect of Br Bi As Cl ML.By considering a variety of possible stacking methods,we obtained nine double-layer Br Bi As Cl structures.It is found that the Str3 structure with the upper Br atom perpendicular to the lower hexagonal vacancy is the ground state of energy and has the highest binding energy(27.93 e V).The electronic structure calculation shows that the double-layer Br Bi As Cl Str3 form a spin-valley-coupled quasi nodal-loop semimetallic which is different from the single-node svc-DSM due to the interaction between the layers.The corresponding quasi-Dirac states are contributed by the upper As-pxy orbitals and the lower Bi-pxy orbitals.The analysis of spin magnetic moment and Berry curvature shows that the low-energy Dirac quasi fermions at K and-K have both opposite spin magnetic moment and Berry curvature,so the node lines are spin and valley dependent.This discovery further enriches the categories of nodal semimetals,and is expected to provide theoretical reference for the future application of multifunctional quantum states(spin,valleys and nodal-line).