Study On A Few Topological Materials And Related Interfacial Superconductivity

In the 1980 s,the discovery of the quantum Hall effect sparked intense research interest in searching and studying new quantum states in condensed matter physics.With the study of the integer quantum Hall effect and the fractional quantum Hall effect,the new quantum states finally has been explained by introducing the topological concept from mathematics.Subsequently,many attempts were made to seek for a quantum state with the properties of the quantum Hall effect in a zero magnetic field and then the existence of topological insulator state was confirmed by the combination of theory and experiment.The study of topological insulators makes people realize that some new quantum states distinct from all states of matter known before can be classified according to the topological invariant.So far,topological materials include topological insulators,topological semimetals and topological superconductors.Due to the fascinating topological-protected band structures,these materials usually present exotic physical phenomena.For example,the chiral anomaly-induced negative magnetoresistance,the upturn resistivity behavior,the extremely large magnetoresistance,topological nontrivial Berry phase,ultrahigh carrier mobility and unconventional superconducting pairing,and so on.The study of these materials will contribute to the development of new electronic devices,especially those based on spintronics and topological quantum computing.In this article,we systematically studied the electrical and magnetic properties of ZrTe at low temperature,the interfacial superconductivity induced by various metal thin films deposited on the surface of WC single crystal,and the interfacial superconductivity induced by the Ag film coated on the surface of TaAs.WC and ZrTe were predicted to be two topological semimetals with triply degenerate crossing points,which are different from the Weyl semimetal with two-fold degenerate Weyl point and Dirac semimetal with four-fold degenerate Dirac point.WC is one of the triply degenerate node topological semimetals confirmed by experiment,whose degenerate crossing points closer to Fermi level than that of others.WC is a superhard material widely used in high pressure equipment.We realized the interfacial superconductivity by deposited various nonmagnetic and ferromagnetic films on the surface of WC single crystals with magnetron sputtering.The well compatibility between the interfacial superconductivity and ferromagnetism indicates the unconventional nature of the induced interfacial superconductivity.However,ZrTe still has not been studied due to the difficulty in the preparation of single crystal and the difficulty in the measurements of angular resolution of photoelectron spectroscopy.Here,we have grown the single crystals of WC-type ZrTe and performed the magnetotransport and theory studies.TaAs is the firstly Weyl semimetal confirmed by experiment and theory.We observed an interfacial superconductivity induced by coating the Ag film on the surface of TaAs single crystals with magnetron sputtering.This finding suggests that the interfacial superconductivity may be of the properties of topological superconductor.This thesis contains the following four chapters:In chapter one,we firstly introduce the research progress of topological materials.It includes: people introduced the concept of topology into physics,classified topological materials according to the notion of topological order and studied the properties of topological materials experimentally.In chapter two,we firstly grew the single crystals of ZrTe and performed the measurements of magnetotransport and magnetization and the theoretical calculation to study the physical properties.The angular dependence of the magnetoresistance,Hall effect and the Shubnikov-de Haas(Sd H)oscillations indicate that ZrTe possesses a complex-three-dimensional Fermi surface and the hole-type carrier dominates the transport properties.The calculation demonstrates that there are four bands(including two bands form hole-type Fermi surfaces and two bands form electron-type Fermi surfaces)crossing the Fermi level on high symmetry lines.The measurements of magnetization show that ZrTe has strong anisotropic magnetism,which is paramagnetic for H //z and diamagnetic for H //x and y.Here,we still cannot give an explanation to this novel magnetization in view of theory and experiment.Therefore,further experimental and theoretical efforts will be strongly desired to get a comprehensive understanding of this issue.In chapter three,we have realized interfacial superconductivity up to 11.5 K at the interface of WC/metal by coating nonmagnetic and ferromagnetic normal metal thin flms on WC single crystals with magnetron sputtering.We performed conductance measurements on the flms using soft point-contact technique.We found that the superconducting transition temperatures are surprisingly insensitive to the magnetism of thin flms,which implies that the induced superconductivity maybe have a triplet-pairing component.We believe that the reconstruction of Fermi surface leaded by the interface coupling between metal film and single crystal surface might be the main reason of the existence of the interfacial superconductivity.However,the exact mechanism still requires further theoretical and experimental efforts.In chapter four,we have shown interfacial superconductivity up to 3.15 K on the surface of TaAs single crystals by coating Ag thin flms on TaAs crystals with magnetron sputtering.We performed the standard four probe measurements on large resistance TaAs sample obtained by processing the size of the sample.A sharp resistance drop at around 3 K was observed,but did not fall to zero.The bonding strength between Ag film and primitive surface of TaAs single crystal,were considered to prevent the interface from forming a connected superconducting path and lead to a non-zero resistance.Meanwhile,we performed the soft point contact measurements and observed a zero-bias conductance peak with two dips symmetric to zero bias.A possible topological superconductivity is expected for the induced superconductivity in TaAs/Ag interface.These findings based on TaAs and WC may provide an easy way to design topological superconductors.In the last part of this thesis,we summarize and prospect the research.