Study On The Superconductivity And Topological Surface State Of Type ? Dirac Semimetal NiTe₂

Topological superconductors have become an important approach to design new generation of quantum computers and spintronic devices.Topological semimetals have topological properties themselves,thus finding topological superconductors in topological semimetals with intrinsic superconductivity has attracted people's attention.NiTe₂ belongs to type ? Dirac semimetal.The Dirac point in NiTe₂is very close to the Fermi level,so it is a potential material for topological superconductors.In this paper,the electronic structure,phonon structure and electron-phonon interaction of NiTe₂ are studied via first-principles calculations.Furthermore,we will study the topological surface state on NiTe₂?001?cleavage plane,and tune it to approach the Fermi level by doping.Finally,possible topological superconductivity in type-? Dirac semimetal NiTe₂ will be discussed.The main research work of this article has two aspects:?1?The superconductivity of NiTe₂Our first-principles calculations indicate that the Dirac point is located very close to the Fermi level,there is a Dirac cone titled along the[001]direction in the bulk NiTe₂,which also indicates that NiTe₂ is a type ? Dirac semimetal.The linewidth around the Dirac point is obviously large,it means that the nontrivial bands forming the type-? Dirac point are strongly coupled with phonon modes,suggesting that the nontrivial bands play an important role in superconductivity.it provides various possibility of realizing topological superconductivity in NiTe₂.Besides linear dispersion bands forming the tilted Dirac cone,there are topology trivial bands crossing the Fermi level,the complicated Fermi surface around the Dirac point contains both electron and hole pockets.So we suggest that conventional and unconventional pairings coexist in NiTe₂ compound.The estimated electron-phonon coupling constant?is 0.51,and the superconducting transition temperature is 1.5 K within the conventional BCS theory.On the other hand,the Dirac points are much closer to the Fermi level in NiTe₂,unconventional odd-parity pairing states in connection with orbit-momentum may occur in it.We speculate that NiTe₂ is a promising material candidate for exploring topological superconductivity.?2?The topological surface state of NiTe₂We have studied the topological surface state on NiTe₂?001?surface,which originates from a nontrivial Z₂ invariant,but has no relation with the type ? Dirac point in the bulk NiTe₂.Our calculations indicate that this topological surface state is located at 1.2 e V below the Fermi level,and it possesses weak topological property.For this helical surface state,it is very similar to that of TI,its spin and momentum are locked and it is well separated from the bulk states.This nontrivial topological surface state can be tuned to near the Fermi level by adding two holes or by V substitution.For NiTe₂,the 2D topological superconductivity from this nontrivial topological surface state might be induced by the superconductivity in the bulk,as a result of the natural proximity effect between bulk and surface.