Modulation doping of the FeSe monolayer on SrTiO$_3$

TL;DR

This paper develops a theoretical understanding of modulation doping in FeSe monolayers on SrTiO$_3$, explaining the universal 'magic' doping level and proposing experimental methods to control carrier density for enhanced superconductivity.

Contribution

It introduces a theory for modulation doping in FeSe/SrTiO$_3$ heterostructures, elucidating the origin of the universal doping level and offering pathways to tune carrier density.

Findings

Polarization change amplifies interface potential difference.

Titanate substrate acts as an infinite charge reservoir.

Theory explains the 'magic' doping phenomenon.

Abstract

The discovery of higher-temperature superconductivity in FeSe monolayers on SrTiO$_3$ (STO) substrates has sparked a surge of interest in the interface superconductivity. One point of the agreement reached to date is that modulation doping by impurities in the substrate is critical for the enhanced superconductivity. Remarkably, the universal doping of about 0.1 electrons per Fe, \textit{i.e.}, so-called ``magic'' doping, has been observed on a range of Ti oxide substrates, which concludes that there likely is some important interaction limiting the FeSe doping. Our study discovers that the polarization change at the interface Se because of the close proximity to the substrate from that in the free-standing FeSe film significantly amplifies the total potential difference at the interface above and beyond the work function difference for charge transfer. Additionally, the titanate substrate with a large number of free electrons basically serves as an ``infinite'' charge reservoir, which leads to the saturated FeSe doping with the complete removal of interface potential gradient. Our work has developed the theory for modulation doping in the Van der Waals materials/oxides heterostructure, providing a solution to the puzzle of ``magic'' doping in FeSe monolayers on titanates. The information also presents experimental pathways to accommodate a variable carrier density of FeSe monolayers via modulation doping.