The Study On The Transport Property Of Transition Metal Dichalcogenide T_d-MoTe₂ Under High Magnetic Field

Transition metal dichalcogenide T_d-MoTe₂ has numorous physical properties including superconductivity,extremely large magnetoresistance（XMR）and topological semimetal properties.Recently,Quantum Spin Hall（QSH）insulator has been predicted to exist in monolayer T_d-MoTe₂,leading this material one of the hot topicsin condensed matter physics.In this thesis,we focus on T_d-MoTe₂ and investigate the transport property under low temperature and high magnetic field.Evidence extracted from the transport experiments for the existence of Weyl semimetal state was reported.In addition,we investigated the influence on the superconductivity,XMR and topological property and the tunning effect of ground state under pressure,chemical doping and exfoliation.The main research contents of the thesis are as follows:1.We used flux method to synthesize high quality single crystal and conduct quantum transport studies under low temperature and high magnetic fields.Unsaturated XMR has been observed at H=33 T.After analyzing the high magnetic field Shubnikov-De Haas oscillation,the relevant information of T_d-MoTe₂ Fermi surface and the non-trivial Berry phase were obtained.In addition,after substracting the interferencesignal caused by the misalignment of Hall bar,we extracted the intrinsic planar Hall effects,which confirmed the existence of chiral charge of T_d-MoTe₂.Thus we provide the evidence of Weyl semimetal state of T_d-MoTe₂ through transportmeasurement.2.We investigate the origin of XMR effect at low temperature of T_d-MoTe₂ single crystal.Bycombining the angle dependent magnetoresistance,Hall resistance,thermopower and theoretical calculations,we found that the temperature-induced Lifshitz phase occurs at T=60 K,which drive the system to become compensated at low temperatures andthe electron-hole compensated band structure is the main cause of the XMR effect.We fabricated device of MoTe₂ flake up to nano-scale by exfoliation.When the thickness of the sample is gradually reduced,the inversion symmetry breaking orthorhombic phase exists over the entire measured temperature range.At the same time,the compensated band structure is destroyed.Compared to the electron-hole carrier density mismatch,the carrier mobility reduction can greatly suppress the XMR effect.Combined with the results of the bulk single crystals,weproposed that not only the compensated band structure,the high mobility of the material,but also the important reason for observing XMR at low temperatures.3.We investigate the transport property of MoTe₂ under pressure.As the pressure increases to 1 GPa,XMR is almost suppressed and the electron-hole compensated band structure is destroyed.In addition,the enhancement of superconductivity of MoTe₂ at 0-1 GPa may be related to the suppression of structural phase transition leading to the reconstruction of Fermi surface and the enhanced superconductivity at higher pressures may be related to the pressure-induced Fermi surface reconstruction.4.We in troduced chemical pressure by partial substitution of the Te ions by the S ions.We found that S-doping can greatly enhance the superconductivity of the MoTe₂ single crystal.The superconducting transition temperature Tc of the optimum doping ratio MoTe_1.8S_0.2 single crystal is 1.3 K.Compared with the MoTe₂（TC=0.1K）,the Tc increased by nearly 13 times.The type II superconductivity has been verified through the resistivity and magnetization measurements.Combined with the specific heat results and theoretical calculations,we found that the MoTe_2-xSxsingle crystal belongs to weak coupling superconductors and the enhancement of the superconductivity may be related to the enhanced electron-phonon coupling induced by the S doping.A dome-shape superconducting phase diagram is obtained in the S-doped MoTe₂ single crystals.