Anisotropy of upper critical fields and interface superconductivity in FeSe/SrTiO₃grown by PLD

In this study, we grow FeSe/SrTiO₃with thicknesses of 4-19 nm using pulsed laser deposition and investigate their magneto-transport properties. The thinnest film (4 nm) exhibit negative Hall effect, indicating electron transfer into FeSe from the SrTiO₃substrate. This is in agreement with reports on ultrathin FeSe/SrTiO₃grown by molecular beam epitaxy. The upper critical field is found to exhibit large anisotropy (γ>11.9), estimated from the data near the transition temperature (Tc). In particular, the estimated coherence lengths in the perpendicular direction are 0.15-0.27 nm, which are smaller than thec-axis length of FeSe, and are found to be almost independent of the total thicknesses of the films. These results indicate that superconductivity is confined at the interface of FeSe/SrTiO₃.