Interplay between band structure and Hund's correlation to increase T${}_c$ in FeSe

Abstract

FeSe is classed as a Hund's metal, with a multiplicity of $d$ bands near the Fermi level. Correlations in Hund's metals mostly originate from the exchange parameter \emph{J}, which can drive a strong orbital selectivity in the correlations. The Fe-chalcogens are the most strongly correlated of the Fe-based superconductors, with $d_{xy}$ the most correlated orbital. Yet little is understood whether and how such correlations directly affect the superconducting instability in Hund's systems. By applying a recently developed high-fidelity \emph{ab initio} theory, we show explicitly the connections between correlations in $d_{xy}$ and the superconducting critical temperature $T_c$. Starting from the \emph{ab initio} results as a reference, we consider various kinds of excursions in parameter space around the reference to determine what controls $T_c$. We show small excursions in $J$ can cause colossal changes in $T_c$. Additionally we consider changes in hopping by varying the Fe-Se bond length in bulk, in the free standing monolayer M-FeSe, and M-FeSe on a SrTiO${}_3$ substrate (M-FeSe/STO). The twin conditions of proximity of the $d_{xy}$ state to the Fermi energy, and the strength of $J$ emerge as the primary criteria for incoherent spectral response and enhanced single- and two-particle scattering that in turn controls $T_c$. Using constrained RPA, we show further that FeSe in monolayer form (M-FeSe) provides a natural mechanism to enhance $J$. We explain why M-FeSe/STO has a high $T_c$, whereas M-FeSe in isolation should not. Our study opens a paradigm for a unified understanding what controls $T_c$ in bulk, layers, and interfaces of Hund's metals by hole pocket and electron screening cloud engineering.

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• Direct evidence of a charge depletion region at the interface of Van der Waals monolayers and dielectric oxides: The case of superconducting FeSe/STO
• Disentangling the role of bond lengths and orbital symmetries in controlling $T_c$ in YBa${}_2$Cu${}_3$O${}_7$
• Symmetry-breaking-induced topology in FeSe