Superconductivity And Topological Properties Of NbC,TaC

The physicists has studied novel topological states for decades since the introducing of the concept of topology in condensed matter physics.As a result,various topological materials with nontrivial topological energy bands has been found and verified,such as topological insulators,topological semimetals and topological superconductors,which deepened and enriched the theoretical and experimental research of topological states.Meanwhile,lots of novel quantum states can be accomplished via topological system,such as quantum spin Hall effect and Majorana zero modes,which will promote the progress of scientific and technical revolution in the future.Finding and studing new topological materials have always been the main goal of the research for topological states.It can not make advances without discovery and synthesis of materials.In this dissertation,we introduce the synthesis and measurement of several single crystal materials with potential topological band structure,especially superconductors with topological properties.It is promising to find topological superconductivity in these materials,which is our focal point of work.The transition-metal carbides exhibit a large variety of band topologies,and some of them also show superconductivity.NbC is one of transition-metal carbides with nontrivial topological band structure and superconductivity.We grown single crystals of NbC by high temperature flux method and measured its superconducting properties via magnetization,resistivity and specific heat measurement,which confirmed that NbC is an s-wave superconductor with isotropic full energy gap and the superconduting transition temperature is 11.5 K.Our measurement also suggested strong electron-phonon interaction in NbC and theoretical research indicated that Fermi-surface nesting in NbC will lead to the emergence of the Kohn anomaly and enhance the electron-phonon coupling.ARPES measurements provided evidence of Fermi-surface nesting and topological surface states which corresponded with theoretical calculation.We also studied the superconducting gap using high-resolution laser ARPES,and we found the behavior of the gap deviates from that of conventional BCS theory.These intriguing properties mean that NbC is a material of fundamental interest for the exploration of exotic properties such as topological superconductivity and the Majorana bound states.We also synthesized single crystals of TaC,which shows similar properties with NbC.The physical measurements indicated that TaC is type-II superconductor with transition temperature of 10.3 K.And it can be well described by s-wave BCS theory with a single gap.Meanwhile,TaC exhibits some abnormal superconducting properties.The value of electron-phonon coupling constant is quite larger comparing with conventional BCS superconductors and isostructural NbC.And the upper critical field of TaC shows linear temperature dependence,which is similar to some iron-based superconductors with multiband superconductivity.Considering that previous studies suggested nontrivial topological band structure in TaC,it is valuable to carry out the ARPES research for TaC in the futrue.We grown single crystals of MnSb₂Te₄ and investigated its structure,magnetization,electrical transport and heat specific heat properties.We found Mn-Sb intermixing in MnSb₂Te₄,which leads to anisotropy and ferrimagnetic magnetic state with transition temperature of 31 K.Electrical transport measurements showed negative magnetoresistance near 30 K dominated by the evolution of magnetic polaron driven by temperature and magnetic field.The anomalous Hall effect connected with magnetic behavior was also found at low temperature.These behaviors in MnSb₂Te₄ provides an opportunity to investigate the interactions between magnetic structure and topological properties.Furthermore,we introduce the discovery of directional massless Dirac fermions in single crystals of NbSi_0.45Te₂ and direct evidence for charge compensation-induced large magnetoresistance in thin WTe₂.We synthesized high quality single crystals and provided samples for these researches.Finally,we list the single crystals I grown during my PhD study as a summary of my work.Nowadays,the study of topological states is in the ascendent.It is necessary to promote the research of topological quantum states based on finding and synthesizing new materials.Especially the exploring of topological supercondutors is valuable for the development of theory and application in the future.With the efforts of researchers in this field,there will be new progress and breakthrough in the study of topological materials.