Pressure-induced Structural Phase Transition And Superconductivity In Typical Transition Metal Dichalcogenides 1T-TaSe₂ And 2H-MoTe₂

Transition metal dichalcogenides（TMDCs）are highly important layered materials systems in condensed matter physics due to their simple chemical composition,abundant crystal polytypes,and rich physicochemical properties.Among the three thermodynamic parameters-temperature,chemical composition,and pressure-pressure can effectively modulate the interatomic distance,and greatly influence crystal structure and electronic energy band structure of a substance,leading to the emergence of novel physical phenomena that don’t exist under ambient pressure.In layered TMDCs,the interlayers are bonded by the Van der Waals force,and the covalent bond connects intralayer atoms.As result of the difference between the weak Van der Waals force and strong covalent bond,anisotropic physical properties commonly exist.Significantly,pressure could rapidly tune the interlayer distance and tremendously influence the TMDCs,and that makes TMDCs ideal for high-pressure research.For example,apllying pressure could close the bandgap and lead to the transiton from insulator/semiconductor to metal,and even induce superconductivity at ultrahigh pressure.In addition,the pressure-suppressed charge density wave by a broken superstructure could be another example.Therefore,pressure can be a clean tool to explore the relationship between charge density wave（CDW）and superconductivity because both always appear closely in TMDCs.In this thesis,we focus on studying the effects of high pressure on the electronic and structural properties of 1T-Ta Se₂ and 2H-Mo Te₂.We employ in-situ diamond anvil cell（DAC）technology to perform high-pressure electrical transport measurements,high-pressure synchrotron X-ray diffraction（XRD）,and high-pressure Raman spectroscopy.In addition,we use theoretical calculations to support our experimental findings.Specifically,we comprehensively investigate two main research topics:（1）the relationship between CDW and superconductivity,and the influence of structural phase transition on superconductivity in 1T-Ta Se₂ under pressure;（2）the evolution of the electronic energy band structure under different pressure environments,and the mechanism of induced superconductivity under non-hydrostatic pressure in 2H-Mo Te₂.The results and conclusions of our research are presented in the following sections:1、Using high-pressure electrical transport and synchrotron XRD measurements,we studied the evolution of CDW,superconductivity,and structural phase transition in compressed 1T-Ta Se₂.The suppression of CDW is observed on the R-T curves.The transition temperature of CDW decreases gradually as pressure increases,which finally disappears on the R-T curves above 9.4 GPa.Superconductivity occurs at 2.6 GPa with an initial T_c of～2 K.Upon further compression,the T_c linearly increases and reaches a maximum of 5.9 K below 21.8 GPa.According to the Hall effect measurements,we suggest that possibly the CDW domain wall,which was presented in the pressurized1T-Ta S₂ and 1T-Ti Se₂,occurs in the 1T-Ta Se₂ above 9.4 GPa and coexists with the superconductivity.Above 21.8 GPa,T_c turns to a downward trend under high pressure.The XRD results show a structural phase transition at 19.0 GPa,resulting in the decrease of the T_c.The new high-pressure structure was confirmed as a monoclinic phase with space group C2/m.The theoretic calculations were performed to clarify the nature of the superconductivity in the C2/m structure.Upon decompression,we found that the superconductivity is detained in the released sample at ambient pressure.The XRD results denote that the process of structural phase transition is irreversible,and partial C2/m phase is detained in the released sample,which captures the superconductivity at ambient pressure.In this work,by enhancing the pressure to a higher value,we acquire the complete phase diagram of T_c under pressure in 1T-Ta Se₂and suggest a new route to consider the relationship between CDW and superconductivity.Meaningfully,we harvest a new superconducting structure at the ambient pressure in Ta Se₂ polytypes.2、Using high-pressure electrical transport,synchrotron XRD and Raman spectra measurements,we studied the evolution of superconductivity and crystal structure in2H-Mo Te₂ under pressure,and presented an anisotropic compression model,which expresses the way to apply pressure under non-hydrostatic pressure environment.And then,we performed theoretical calculations to figure out the evolution of electronic energy band structure and superconductivity under the non-hydrostatic pressure environment with that model.High-pressure XRD and Raman spectra measurements denote no structural phase transition in 2H-Mo Te₂ up to 54.0 GPa,except for the different compression behavior of the c axis in a non-hydrostatic pressure environment.Herein,we presented an anisotropic compression model.The metallization begins at around 6.0 GPa and finishes at approximately 12.0 GPa.High-pressure Raman spectra measurements can well explain the metallization in 2H-Mo Te₂.And the evolution of the carrier concentration obtained from Hall effect measurements also clearly reflect the metallization.As pressure increases,superconductivity is observed at higher pressures.According to theoretical calculations,anisotropic compression favors the increase in the density of states at the Fermi level and the softening of the phonon modes.Furthermore,the enhanced electron-phonon coupling may intrigue superconductivity in 2H-Mo Te₂.In our theoretical calculations,a kind of Lifshitz electronic phase transformation is observed,and the larger anisotropic parameter will reduce the pressure of phase transformation.In summary,the anisotropic compression produced by the non-hydrostatic pressure environment will reduce the pressure of metallization and intrigue superconductivity in 2H-Mo Te₂.This research demonstrates how pressure environments can influence the electronic properties of materials,further advancing our understanding of pressure-induced superconductivity in layered compounds.