Identification of a Large Amount of Excess Fe in Superconducting Single-Layer FeSe/SrTiO3 Films

TL;DR

This study reveals a significant presence of excess Fe in superconducting single-layer FeSe/SrTiO3 films, challenging the assumption of stoichiometry and suggesting new avenues for understanding and enhancing high-temperature superconductivity.

Contribution

It is the first to identify and quantify large amounts of excess Fe in superconducting FeSe/SrTiO3 films using in situ STM during low-temperature annealing.

Findings

At least 20% excess Fe detected in the films.

Formation of second-layer FeSe islands at low annealing temperature (~150°C).

Excess Fe presence impacts understanding of high-Tc superconductivity.

Abstract

The single-layer FeSe films grown on SrTiO3 (STO) substrates have attracted much attention because of its record high superconducting critical temperature (Tc). It is usually believed that the composition of the epitaxially grown single-layer FeSe/STO films is stoichiometric, i.e., the ratio of Fe and Se is 1:1. Here we report the identification of a large amount of excess Fe in the superconducting single-layer FeSe/STO films. By depositing Se onto the superconducting single-layer FeSe/STO films, we find by in situ scanning tunneling microscopy (STM) the formation of the second-layer FeSe islands on the top of the first layer during the annealing process at a surprisingly low temperature ($\sim$150{\deg}C) which is much lower than the usual growth temperature ($\sim$490{\deg}C). This observation is used to detect excess Fe and estimate its quantity in the single-layer FeSe/STO films. The amount of excess Fe detected is at least 20% that is surprisingly high for the superconducting single-layer FeSe/STO films. The discovery of such a large amount of excess Fe should be taken into account in understanding the high-Tc superconductivity and points to a likely route to further enhance Tc in the superconducting single-layer FeSe/STO films.