High-temperature superconductivity in FeSe monolayers

This paper reviews the basic experimental and theoretical aspects of high-temperature superconductivity in intercalated FeSe compounds and FeSe monolayer films on ${\rm SrTiO}_3$ and similar substrates. The paper examines in detail the electronic structure of these systems, how it is calculated, and how the calculated results compare with ARPES experiments. It is emphasized that the reviewed systems have qualitatively different electronic spectra from the typical pattern of well-studied FeAs superconductors and explores the implications of these differences for a theoretical description of how these spectra form. Possible mechanisms of Cooper pairing in FeSe monolayers are discussed and the associated problems are examined. Because FeSe monolayer films on ${\rm SrTiO}_3$ are typical Ginzburg ‘sandwiches’, the possibility of increasing their $T_{\rm c}$ via ‘excitonic’ superconductivity mechanisms is considered. It is shown that, while the classical version of this mechanism (as proposed for such systems by Allender, Bray, and Bardeen) fails to explain the observed values of $T_{\rm c}$, the situation changes when optical phonons in ${\rm SrTiO}_3$ (with energy of about 100 meV) are considered to be ‘excitons’. Both the simplest possible model of $T_{\rm c}$ enhancement due to interaction with such phonons and more complex ones with dominant ‘forward’ scattering that explain successfully the increase in $T_{\rm c}$ compared to bulk FeSe and intercalated FeSe systems are verified. Problems related to the antiadiabatic nature of this superconductivity mechanism are also discussed.