High-Field Magnetotransport and Superconductor-Insulator Transition of FeSe Thin Films

The interfacial superconductivity of FeSe on SrTiO3 has been a subject of considerable focus in recent years due to a significantly enhanced critical temperature, compared to bulk FeSe, in the monolayer limit. Here we investigate the temperature dependence of both the in-plane and out-of-plane upper critical fields (H_c2) and the high-field magnetoresistance behavior up to 65 T in few-layer FeSe thin films grown by molecular beam epitaxy on undoped SrTiO3 substrates. This data demonstrates a clear violation of the weak-coupling Pauli limit and provides insight into the pairing mechanisms responsible for high-Tc superconductivity in our FeSe thin films. Additionally, we investigate the carrier density driven superconductor-insulator transition of FeSe by alternating between Fe depositions and both in-situ electronic transport measurements and band structure mappings using angle-resolved photoemission spectroscopy.