Molecular Beam Epitaxy-scanning Tunneling Microscopy Of Superconducting Bismuthid Thin Films

Superconductivity in reduced dimensionality such as high temperature superconductivity in layered cuprates and iron-based superconductors has nowadays been the research subject of enormous studies in condensed matter physics and material science.Microscopic understanding of the superconducting properties,novel phenomena accompanying it,and possible interplay between them are of significant importance to fundamental research and future device applications.In this thesis,by using molecular beam epitaxy（MBE）technique,we have prepared two categories of bismuth–containing binary compounds（BaBi₃ and Ni-Bi）films and studied their exotic superconductivity and vortex matters by low temperature scanning tunneling microscopy（STM）operated in magnetic-field.Plenty of superconductors exhibit many structural or electronic symmetry breaking and phase transitions upon cooling,and are sometime subject to disorder scattering,since the occurrence of superconductivity relies mostly on chemical dopants or interface charge transfer that leads inevitably to disorder effects.Albeit with extensive theoretical and experimental investigations,the respective roles of these intertwined states and disorder played in superconductivity remains mysterious.Here we use MBE to grow structurally simple non-oxide perovskite superconductor BaBi₃ on SrTiO₃ substrates,and conduct the first tunneling experiments on the BaBi₃ crystalline films with fine control of symmetry breaking（caused by various stripe-like surface patterns）and disorders at varied film thickness.We have demonstrated that the stripes break the C₄ rotational symmetry and lead to the superconductivity with twofold symmetric order parameter.In addition,the tunneling spectra in disorder ed BaBi₃ thin films look more like V-shaped and are fairly accounted for by a model of fluctuating superconductivity.Our studies might help understand the tunneling spectra on superconductors with a rich of symmetry breaking states and disorders.The recently discovered quasi-one-dimensional（1D）superconductors,such as the chromium pnictides A₂Cr₃As₃（A=K,Rb,Cs）and Ta₄Pd₃Te₁₆,have attracted much interest and demand for a microscopic understanding how the electrons pair up.To shed insight into the novel properties in quasi-1D superconductors,we extend the MBE technique to prepare Ni-Bi bimetallic films by using the same SrTiO₃ substrates and identify two superconducting phases with quasi-1D crystal structure,to wit Bi-rich NiBi₃ and nonstoichiometric Ni_x Bi.This enables us to investigate the quasi-1D superconductivity and vortex properties on the nanometer scale,both of which intriguingly exhibit highly anisotropic behaviors with two-fold symmetry.These findings provide a useful information on electron pairing and vortex state in quasi-1D superconductors.