The Theoretical Study Of Superconductivity And Topological Properties Of Transitition Metal Dichalcogenides By High Pressure And Charge Carrier Doping

Transition metal dichalcogenides(TMD)1T-TiTe2,1T-TaS2 and PdTe2 have abundant superconductivity,charge density wave(CDW)and topological porperties,and are extensively studied in condense matter physics and matter physics.High pressure and charge carrier doping can manipulate the physical properties of materials very cleanly,becoming an important method in materials research.This thesis sysmatically investigates the superconductivity,CDW and topological porperties of these materials under high pressure and carrier doping.Recnetly,1T-TiTe2 was predicted to experience a serial of topological phase transitions under the uniaxial pressure.If superconductivity can also exist in it,1T-TiTe2 should be a very promising topological superconductivity material.However,there is no report on it before.Here,we investgate the superconductivity of 1T-TiTe2 under high pressure by first-principles calculations.Our results show that the superconductivity of 1 T-TiTe2 exhibits very different behaviors under hydrostatic and uniaxial pressure.The hydrostatic pressure is harmful to the superconductivity,while the uniaxial pressure is beneficial to the superconductivity.The superconducting transition temperature Tc at ambient pressure is 0.73 K,and it reduces monotonously under the hydrostatic pressure to 0.32 K at 30 GPa,while Tc increases dramatically under the uniaxial pressure along the c axis.The established Tc under the uniaxial pressure of 17 GPa,below which the structural stability is maintained,is 6.34 K.The increase of the density of states at the Fermi level,and the softening of the acoustic modes with pressure are considered as the main reasons that lead to the enhanced superconductivity under uniaxial pressure.PtSe2 family materials(PtSe2,PtTe2 and PdTe2)are the type-II Dirac semimetals,besidese PdTe2 is also a superconductor,making PdTe2 the first material that processes both superconductivity and type-II Dirac fermions.Taking PdTe2 as a representative,we study the evolution of Dirac cones in PtSe2 family materials under hydrostatic pressure.Our results show that the pair of type-? Dirac points of PdTe2 disappears at 6.1 GPa.Interestingly,a new pair of type-I Dirac points from the same two bands emerges at 4.7 GPa.Due to the distinctive band structures compared with those of PtSe2 and PtTe2,the two types of Dirac points can coexist in PdTe2 under proper pressure(4.7-6.1 GPa).The emergence of type-I Dirac cones and the disappearance of type-?Dirac ones are attributed to an increase/decrease of the energy of the states at the T and A points,which have antibonding/bonding characters of the interlayer Te-Te atoms.On the other hand,we find that the superconductivity of PdTe2 slightly decreases with pressure.We also predict that PtSe2 family materials should undergo topological transitions if the inversion symmetry is broken,i.e.the Dirac fermions in PtSe2 family materials split into triply degenerate points in PtSeTe family materials(PtSSe,PtSeTe,and PdSeTe)with orderly arranged S/Se(Se/Te)atoms.1T-TaS2 exhibits a rich set of CDW orders.Recent investigations suggested that using light or an electric field can manipulate the commensurate CDW(CCDW)ground state.These manipulations are considered to be determined by charge-carrier doping.However,the mechanisms of such transitions are not fully clear yet.Here we use first-principles calculations to simulate the carrier-doping effect on the CCDW in 1 T-TaS2.We investigate the charge-doping effects on the electronic structures and phonon instabilities of the 1T structure,and we analyze the doping-induced energy and distortion ratio variations in the CCDW structure.We found that both in bulk and monolayer 1T-TaS2,the CCDW is stable upon electron doping,while hole doping can significantly suppress the CCDW,implying different mechanisms of such reported manipulations.Light or positive perpendicular electric-field-induced hole doping increases the energy of the CCDW,so that the system transforms to a nearly commensurate CDW or a similar metastable state.On the other hand,even though the CCDW distortion is more stable upon in-plane electric-field-induced electron injection,some accompanied effects can drive the system to cross over the energy barrier from the CCDW to a nearly commensurate CDW or a similar metastable state.We also estimate that hole doping can introduce potential superconductivity with a Tc of 6-7 K.In addition,the recent synthesised borophene may have very high superconductivity.The study of borophene can provide some reference for TMD,and it is also the research content of this paper.The borophene with vacancy-free structure was estimated to have the highest Tc compared with those with vacancy structures.Can its superconductivity be enhanced further?We predict that the superconductivity of borophene with vacancy-free structure can be significantly enhanced by strain and charge carrier doping.The intrinsic metallic ground state with high density of states at Fermi energy and strong Fermi surface nesting lead to sizeable electron-phonon coupling,making the freestanding borophene superconduct with Tc close to 19.0 K.The tensile strain can increase the Tc to 27.4 K,while the hole doping can notably increase Tc to 34.8 K.The results indicate that the borophene grown on substrates with large lattice parameters or under photoexcitation can show enhanced superconductivity with Tc far above the liquid hydrogen temperature of 20.3 K,which will largely broaden the applications of such promising material.This thesis systematically investagetes the superconductivity,CDW and topological properties of these materials,and tries to find their relationships.This thesis may provide some clue for the research of other materials.