Surface and Interface Effects in Epitaxially Grown Niobium and Iron Pnictide Superconductors

Abstract

Superconductivity has been one of the most actively studied field in physics since it was discovered. In BCS superconductor, it is well explained microscopic phenomena by BCS theory, yet some BCS superconductor studies such as low dimensional superconductor or superconductor junction behaviors are still performed. For high TC superconductor such as iron pnictide compound, more phenological studies are needed since theory that fully describe the mechanism is not established. During my Ph.D course, I have performed the scanning tunneling microscopy and spectroscopy (STM/STS) studies for niobium and iron pnictide superconductors to extend the understanding of superconductivity. For this study, I constructed a cryogenic STM system and an STM combined molecular beam epitaxy (MBE) / pulsed deposition (PLD) growth system. I observed that the superconducting gap of niobium appears when film thickness is over 40 Å, and investigated the suppression of superconductivity in disordered niobium film correspond to Finkelsteins model. In epitaxially grown niobium film on W(110) substrate, I observed the oscillation of electron density around superconducting gap, which is analyzed to be an interface effect of superconducting and normal junction ? Tomasch effect. I synthesized Co doped BaFe2As2 film by PLD, and measured with STM/STS. I observed surface state effect by electronic screening of barium atoms on the surface, which made the superconducting gap be distorted. And I found some features which has not been reported with cleaved surface. For another iron pnictide superconductor, LiFeAs, I performed the STM measurement of epitaxially grown film by PLD.