Thickness dependence of superconductivity and superconductor-insulator transition in ultrathin FeSe films on SrTiO3(001) substrate

TL;DR

This study investigates how the superconducting transition temperature and electronic properties of ultrathin FeSe films on SrTiO3 vary with thickness, revealing an unconventional inverse relationship and an insulator-to-superconductor transition upon annealing.

Contribution

It provides new insights into thickness-dependent superconductivity and carrier type transition in ultrathin FeSe films on SrTiO3, highlighting an inverse Tc-thickness relationship and phase transition mechanisms.

Findings

Superconducting Tc decreases with increasing FeSe film thickness.

Post-annealing induces an insulator-to-superconductor transition.

Carrier type changes from holes to electrons at low temperatures.

Abstract

Interface-enhanced high-temperature superconductivity in one unit-cell (UC) FeSe film on SrTiO3(001) (STO) substrate has recently attracted much attention in condensed matter physics and material science. Here, by ex situ transport measurements, we report on the superconductivity in FeSe ultra-thin films with different thickness on STO substrate. We find that the onset superconducting transition temperature (Tc) decreases with increasing film thickness of FeSe, which is opposite to the behavior usually observed in traditional superconductor films. By systematic post-annealing of 5 UC FeSe films, we observe an insulator to superconductor transition, which is accompanied with a sign change of the dominated charge carriers from holes to electrons at low temperatures according to the corresponding Hall measurement.