Superconductivity,charge Density Wave,and Topological State In Transition Metal Dichalcogenides Under Pressure

In 2004,scientists separated graphene from graphite by mechanical exfoliation.The exotic physical properties and high application value of graphene make scientists yearn for it.At the same time,a new branch:two-dimensional system gradually entered people's vision.Transition-metal dichalcogenides belong to 2-D material.Transition-metal dichalcogenides share the same formula,MX2,where M is a transition metal(for example,Ti,Zr,Hf,V,Nb,Ta,Mo,and W)and X is a chalcogenide atom(S,Se and Te).The M-X bonds are strongly covalent,but the sandwich layers are cou-pled only by weak van der Waals interactions.These compounds typically crystallize in many structures,including 2H-,IT-,1T'-and Td-type lattices.Depending on the composition and configuration.Bulk transition-metal dichalcogenides can be semicon-ductor,semimetal,and metal.The typical semiconductors(2H-MoS2),semi-metals(1T'-MoTe2)and metals(2H-NbSe2)in transition-metal dichalcogenides have been studied.The detailed contents are concluded as below:(1)2H-MoS2.We have reported on electrical transport and Raman-scattering measurements of MoS2 at high pressures and low temperatures.The temperature depen-dencies of the electrical conductivity show anomalies(humps).The Raman spectra also show anomalous behavior in the mixed state revealing a different peak at low tempera-tures along with the phonon mode softening and broadening.These behaviors suggest the presence of an additional order at low temperature,which creates a modification of the electronic structure(gapping of the Fermi surface)and structural instabilities(charge density wave).Given a common coexistence and competing of charge density wave and superconductivity in transition-metal dichalcogenides,an unusual superconductivity at very high pressures(>80 GPa)is justified.(2)2H-NbSe2.We have reported on electrical transport and Raman scattering measurements of 2H-NbSe2 at high pressures and low temperatures.As a precondition of the Higgs mode,CDW state is entirely investigated by the electrical transport prop-erties and the Raman scattering.The anomalies in the electrical transport around TCDW attribute to the formation of the open-orbit caused by CDW distortion.In the Raman spectra,the appearance of the CDW mode is driven by the softening of the two-phonon mode.We have also reported the experimental observation of a Higgs mode together with an additional Leggett mode and their evolution with pressure.The Leggett mode is first observed in 2H-NbSe2.Meanwhile,the spectral weights transfer between CDW mode and Higgs mode with changing temperature is observed at high pressures.Fur-thermore,the Higgs mode vanishes once CDW state is killed by pressure.The data we have gotten are fully consistent with the theory.Our work gives a clear understanding of the complex relations among the competition orders in multi-band superconductors and sheds light on the mechanism of superconductivity in unconventional superconductors.The conclusion of this work can be a reference for high-Tc superconductors.(3)17T'-MoTe2.The discovery of the topological superconductors has been a subject of much recent interest.One of the basic preconditions of the topological su-perconductor is the coexistence of superconductivity and topological state.MoTe2 has been identified to be a type-? weyl semimetal in the Td phase at low temperatures.By performing X-ray diffraction and Raman scattering measurements,we obtain the temperature-pressure phase diagram for the appearance of topological state.High-pressure resistivity measurements detect the emergence of superconductivity and its evolution with pressure in this material.The coexistence region for the topological or-der and superconductivity with temperature and pressure is thus established.This work suggests that MoTe2 is a good candidate for topological superconductivity at high pres-sures.