Year

2018

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

Superconductivity is one of the most attractive research projects in condensed matter physics. Among various types of different superconductors, the family of ironbased superconductors has always been a lively topic since the discovery of the superconductivity in LaFeAsO1-xFx in 2008. The conflict between the necessity of Fe element in this type of superconductor and the superficial thinking on the detrimental effect of ferromagnetism on superconductivity makes iron-based superconductivity both intriguing and unconventionally elusive. In material science, iron selenide (FeSe) is a very promising candidate for exploring the mechanism of iron-based superconductivity due to its having the simplest binary composition and much less toxicity than iron-based superconductors containing As. Since the sign of a dramatically high superconducting transition temperature (Tc) over 77 K in a 1 unit-cell FeSe layer grown on SrTiO3 (STO) substrate was reported in 2012 by a group from China, a flourishing upsurge of FeSe thin films was triggered, and the popularity of superconducting research was rejuvenated. The veil between the scientists and the mystery of iron-based superconductivity has never been so thin. However, plenty of issues still remained to be solved, such as why the same thin film material, FeSe, shows totally different superconducting behaviors in the monolayer and the film with typical thicknesses. In this thesis work, the research subject is FeSe superconducting thin film fabricated by the pulsed laser deposition (PLD) method. The emphasis is on the interface effect and the influence of the external coating process on the superconductivity of FeSe thin films. Enhanced superconducting performance was obtained, and the corresponding mechanism was clarified, based on various characterization methods such as physical property measurements and high-resolution electron microscopy.

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