Suppression and revival of superconducting phase coherence in monolayer FeSe/SrTiO$_3$

TL;DR

This study investigates the superconducting phase coherence in monolayer FeSe/SrTiO$_3$ using in-situ measurements, revealing suppression and revival phenomena linked to interface effects and external stimuli, with implications for understanding 2D superconductivity.

Contribution

It provides the first in-situ mutual inductance measurements of FeSe/STO, uncovering the anisotropic nature of interfacial superconducting coherence and effects of layer thickness and external light.

Findings

No diamagnetism observed in monolayer FeSe/STO down to 5 K.

Diamagnetism appears abruptly beyond 5 layers of FeSe.

Ultraviolet light enhances Tc at the interface regardless of FeSe thickness.

Abstract

Monolayer FeSe grown on SrTiO$_3$ (FeSe/STO) is an interfacial high temperature superconductor distinctively different from bulk FeSe. Due to the fragility of this two-dimensional system in the atmosphere, the investigation of its intrinsic superconductivity and intertwined orders has largely been limited to surface-sensitive charge probes compatible with ultra-high vacuum environment. However, the superconducting phase coherence of the interface is challenging to probe. Here, we perform in-situ mutual inductance in ultra-high vacuum on FeSe/STO in combination with band mapping by angle-resolved photoemission spectroscopy (ARPES). We find that even though the monolayer showed a gap-closing temperature above 50 K, surprisingly no diamagnetism is visible down to 5 K. This is the case for few-layer FeSe/STO until it exceeds a critical number of 5 layers where diamagnetism suddenly appears. But the superfluid density does not saturate down to the base temperature in these thick samples. On the other hand, the suppression of diamagnetism in the few-layer FeSe/STO can be lifted by depositing a FeTe layer on top. The superconducting transition is much sharper than that in the thick FeSe/STO. However, Tc and superfluid density both decrease with increasing FeTe thickness. Shining ultraviolet light on the FeTe/FeSe/STO heterostructure enhances Tc similarly independent of the FeSe thickness, showing that the diamagnetism originates at the FeSe/STO interface. Our observation may be understood by a scenario in which interfacial superconducting phase coherence is highly anisotropic.