High-pressure Study On Several Typical Layered Chalcogenides

Due to the novel crystal and electronic band structure,layered chalcogenides exhibit rich and unique physical properties and have attracted a lot of research interest in condensed matter physics.Various crystal structures can be formed when chalcogens combine with different elements,and thus lead to different physical properties.Among them,M2X3(M=Bi,Sb;X=Se,Te)are the widest studied 3D topological insulators.Differing from an ordinary insulator,topological insulators are characterized by insulating bulk energy gap but gapless surface states protected by time-reversal symmetry,and therefore exhibit novel quantum phenomena,such as weak antilocalization effect,quantum spin Hall effect,etc.Except for topological insulators,metal also can be classified based on topology,i.e.,topological semimetal.For example,2D MX2(M=Pt,Pd;X=Se,Te)are new discovered type-II Dirac semimetals.Differing from type-I Dirac semimetal,they have the titled Dirac cone,which exhibit strongly anisotropy.For instance,negative magnetoresistance is expected only along certain momentum direction.In addition to 3D and 2D materials,MX3(M=Ti,Zr,Nb,Ta;X=S,Se,Te)crystallizes in a quasi-1D chain-like structure,which exhibits strong in-plane anisotropy in optical absorption,mobility and thermoelectric properties.Layered chalcogenides might have wide potential applications in optoelectronics device,sensors and quantum computing due to these unique properties.As one of the fundamental state parameters,high pressure is an effective tool to tune the crystal lattice and electronic states.Many novel physical phenomena can be induced in layered chalcogenides under high pressure,such as superconductivity,electronic topological transition and structure transition.These studies under high pressure will help us to get a deeper insight into the physical origin of novel properties.Here,we conducted a joint study on several typical layered chalcogenides through high-pressure electrical transport,synchrotron X-ray diffraction,Raman scattering experiments as well as first-principles calculations.The main contents are included as follows:The first chapter is an introduction to the high-pressure technology,topological electronic materials and research progress of MX3 system.The second chapter,pressure-induced topological insulator-to-metal transition and superconductivity in Sn-doped Bi1.1Sb0.9Te2S(Sn-BSTS).With a surface state Dirac point energy well isolated from the bulk valence and conduction bands,topological insulator Sn-BSTS is an ideal platform for studying intrinsic properties of topological surface states and bulk electrons,separately.Here,we present high-pressure transport studies on single crystal Sn-BSTS,combined with Raman scattering and synchrotron x-ray diffraction measurements.With increasing pressure,we find that the conductivity of topological surface state changes marginally while that of bulk state increases quickly as the energy bandgap decreases rapidly and reaches to a closure around Pc=9 GPa.Meanwhile,a kink in the pressure dependences of Raman frequencies and a minimum in c/a are observed within the pristine structural phase.We argue that the metallization-related origin is responsible for there anomalies.At higher pressures,the emergence of superconductivity and decrease in Tc are related to sequential structural transitions in Sn-BSTS,similar to those usually observed in other tetradymite-type TIs;but the Tc-maximum of?12 K achieved here is the highest value in this TIs family studied so far.The third chapter,high pressure effect on the type-II Dirac semimetal PtSe2.Recently,transition metal dichalcogenide PtSe2 is theoretically predicted and experimentally confirmed to be a type-? Dirac semimetal.Here,we present a study of the electrical conductivity and Raman spectroscopy on single crystal PtSe2 at high pressures up to 42.3 GPa.While no trace of structural phase transition is found from the Raman data,the pressure dependence of the in-plane(Eg)Raman mode shows an abrupt increase at PC?22 GPa.Meanwhile,we find that the transport behavior of PtSe2 changes from a non-Fermi liquid to Fermi liquid behavior around Pc from fitting to the resistance between 1.8 and 100 K by the power law of R(T)=R0+ATn at various pressures.In addition,R5K/R250K increases gradually upon compression and saturates above PC.We argue that a possible pressure-induced disruption of the topological state of PtSe2 may be responsible for this anomalous behavior.The fourth chapter,pressure-induced anomalous enhancement of insulating state and novel iso-symmetric structural transition in TiS3.At ambient pressure,TiS3 is a direct bandgap(Eg?1.0 eV)semiconductor,which possesses a quasi-one dimensional chain-like structure.We present in situ high-pressure synchrotron X-ray diffraction(XRD)and electrical transport measurements on quasi-one-dimensional single crystal TiS3 up to 29.9-39.0 GPa in diamond anvil cells,coupled with first-principles calculations.We find that the conductive behavior of semiconductor TiS3 becomes increasingly insulating with pressure till PC1?12 GPa,where extremes in all the three axial ratios are observed.Upon further compression,the resistance decreases rapidly but remains the semiconducting behavior.At around PC2?22 GPa,the XRD data evidences a structural phase transition accompanied by the appearance of metalization.Based on our theoretical calculations,this structural transition is determined to be iso-symmetric,i.e.without change of the structural symmetry(P21/m),mainly resulted from rearrangement of the dangling S2 pair along the a axis.The fifth chapter,summary and prospect.