Point Contact Andreev Reflection Spectroscopy Of Transition-Metal Dichalcogenides 2H-Nb_1-xTa_xSe₂

The microscopic coexistence or competition of multiple ordered phases and superconductors in the ground state has become a common phenomenon of unconventional superconductors.Studying the relationship between these ordered phases and superconductors is of great significance for understanding the mechanism of unconventional superconductivity.Among them,the transition-metal dichalcogenides2H-Nb Se₂charge density wave(CDW)and superconducting microscopic coexistence provide an ideal model for studying the two ordered states of CDW and superconductivity.Point-contact Andreev reflectance spectroscopy(PCAR)is an effective tool for directly detecting the density of states,which can distinguish multi-band superconducting energy gaps,detect the symmetry of superconducting sequence parameter pairings,and distinguish energy gap anisotropy,so it is favored by some people.This thesis is dedicated to the study of the relationship between the two ordered phases of CDW and superconductivity in 2H-Nb_xTa_1-xSe₂(x=0?0.1)single crystals using PCAR technology.The main content and results of the paper are as follows:Chapter 1 is an introduction,briefly explaining the basic properties and theory of superconductivity,the size and anisotropy of the superconducting energy gap,and an unconventional superconductor with charge density waves.The basic methods used in unconventional superconductivity research are introduced in detail,including plane tunnel spectra and point contact Andreev reflection spectra.Finally,the BTK theory used for point contact experimental data fitting is introduced.Chapter 2 is part of the work involved in the construction of the laboratory at the graduate level,including the basic testing principles and use methods of experimental instruments.It focuses on the core self-designed components of the extreme test system,combined with the most characteristic research method of this experimental group-the high-pressure tunnel spectrum.A comprehensive introduction to how to achieve high voltage in the low temperature and strong magnetic test,how to perform the point contact tunnel spectrum of the high voltage device,and the two methods and contact mechanism of point contact.Chapter 3 introduces the measurement work on 2H-Nb_xTa_1-xSe₂ single crystal in detail,including resistivity measurement,transport measurement and Andreev reflection spectroscopy measurement,the measurement data is well fitted according to the extended BTK theory.The fitting results show that the symmetry of superconducting pairing is more important than the anisotropic s-wave.The increase of Ta doping concentration(x=0?0.1)effectively inhibits CDW,and the superconducting pairing strength 2?/6)₍(8)has no significant change during this process.The conclusion is that there is no correlation between superconductivity and CDW in 2H-Nb_xTa_1-xSe₂.Finally,in the chapter 4,this work is summarized and prospected.