Point Contact Tunneling Spectroscopy Studies of Quasi-one Dimensional Superconductor RbCr3As3 and Topological Insulator Bi1.08Sn0.02Sb0.9Te2S

Author:Liu Zhi Xin

Supervisor:wen hai hu yang huan


Degree Year:2019





With the emergence of new superconductors,the superconducting mechanism of these new materials has become an important research direction of superconductivity.The unconventional superconducting mechanism means that the superconductivity is not induced by the electron-phonon coupling,this now becomes an important scientific issue in the research of superconductivity.Based on the tunneling effect of quantum mechanics,the tunneling junctions are used to get the information of the density of electronic states near the Fermi energy.As an important tool for detecting the superconducting energy gap structure,the point-contact Andreev reflection technique is widely used to study the superconducting gap function and the related pairing mechanism in unconventional superconductors.In this thesis,we mainly use the experimental technique of point contact tunneling to study the superconducting gap structure of unconventional superconductor RbCr3As3,and also to study the properties of s-wave superconductor/topological insulator tunneling junction.Some important results are obtained.The first chapter of this thesis introduces the background of research on superconductivity,and then introduces the newly discovered Cr-based superconductors.Afterwards,the background of topological insulator and the relatively ideal topological insulator Sn-doped Bi1.1Sb0.9Te2S are briefly introduced.The second chapter focuses on the experimental principles and methods of point-contact Andreev reflection measurements.And then we introduce the understanding of the point contact tunneling spectra by the theoretical model proposed by Blonder,Tinkham and Klapwijk(BTK).Afterwards,the preparation processes of the point contact junctions and the home-made experimental setup for the experiments are presented.The third chapter introduces the results of the point-contact tunneling spectra and the scanning tunneling spectra(single-electron tunneling spectra)measured on the Cr-based superconductor RbCr3As3.For the STM/STS measurements we used the needle-like RbCr3As3 as the tip and measured the tunneling effect between the sample and a gold plate.We find that this material has two superconducting gaps based on the measured point-contact tunneling spectra,however,only the smaller superconducting energy gap was observed on the scanning tunneling spectra.Further analysis of the scanning tunneling spectra reveals that there is a large anisotropy or even gap node for the smaller gap.The difference between the two sets of experimental results may be related to the direction of the injecting current.The fourth chapter introduces the results of the point-contact tunneling spectra measured on the topological insulator Bi1.08Sn0.02Sb0.9Te2S by using the superconducting Nb tip.We observed the suppression of differential conductance near zero bias,instead of the enhancement due to Andreev reflection on the spectra.The fitting to the measured spectrum results in a superconducting gap much larger than the superconducting gap of the bulk Nb.The gaped feature exists at temperatures even above the critical temperature of bulk Nb,and is visible when the magnetic field is stronger than the upper critical magnetic field of bulk Nb.We argue that such behaviors may be related to the pressure induced superconductivity by the tip in the junction area,or just some novel phenomena arising from the junction between an s-wave superconductor and an ideal topological insulator.In the fifth chapter of this thesis,we introduce the ionic liquid gating manipulation technology and the application to some unconventional superconductors.We use this technique to the FeSe single crystal and the critical temperature(Tc)is increased from 8K to 30K.We also use this technique and try to change Tc of other superconductors including Tl2Ba2CaCu208+δ film and FeS single crystal.Finally,we make a summary of the thesis.