The Study On The Surface Structures And Novel Properties Of Topological Crystalline Insulator Snte

As a new matter phase,topological crystalline insulator(TCI)is protected by the crystal symmetry,and has multiple Dirac surface states.Based on the first-principles and quantum transport calculations,we have systematically studied the novel properties of topological crystalline insulator SnTe on its(111)surface and thin film.Due to the polarity,the ideal(111)surface of SnTe is unstable in principle.So we first study the stability of SnTe(111)surface,and find that there are three stable surface structures under different growth conditions.The surface band calculations show that these structures support two qualitatively different types of topological surface states:the Te-terminated surface without reconstruction and the(?3 ×?3)-reconstructed surface possess the first type of surface states,with four Dirac points at four time-reversalinvariant points;the(2×1)-reconstructed surface folds the surface Brillouin zone,effectively induces interactions between the different Dirac valleys and produce a new type of surface states,with two Dirac points nearby but not at??.Besides selecting different surface orientations,our work suggests a promising alternative way to produce the different topological surface states of TCIs by controlling the growth conditions.Topological crystalline insulator has an even number of Dirac cones(i.e.,multiple valleys)in its surface band structure.We systematically investigate the strain-induced evolution of topological surface states on the SnTe(111)surface.Our results show that compressive strain can shift the Dirac cones at the?? and ?M valleys to different extents(even oppositely)in energy,while tensile strain can induce different band gaps at both valleys due to the enhanced intersurface coupling.Exploiting a strain-induced nanostructure with well-defined edges on the(111)surface,we demonstrate strong valley-selective filtering for massless Dirac fermions by dynamically applying local external pressure.Our findings may hold great promise for strain-engineered nanoelectronics and valleytronic applications in TCIs.The Dirac fermions in the thin films of topological materials have the helicity degree of freedom.Using topological crystalline insulator SnTe(111)thin films as an example,we find that giant helicity splitting in the band structures can be induced under moderate electric field.Based on this result,we perform the transport calculations of the Dirac fermions through a dual-gated nanostructure,and some helicity-resolved functionalities,including pronounced helicity-selective transmission,helicity switching,helical negative refraction and birefraction,are demonstrated,where the intra-helical scattering always dominates over the inter-helical one.Such intriguing control strategy for helical Dirac fermions may hold great promise for the applications of helicity-based electronics.