Microscopic Description of Unconventional Nodal Superconductivity in FeSe

TL;DR

This paper proposes a new mechanism for unconventional nodal superconductivity in FeSe, linking it to a ferro-quadrupolar order and incoherent metallic states, supported by first-principles calculations and symmetry analysis.

Contribution

It introduces a novel theoretical framework connecting ferro-quadrupolar order and incoherent metallic states to unconventional superconductivity in FeSe.

Findings

Ferro-quadrupolar order enhances orbital-selective Mottness in FeSe.

Unconventional nodal superconductivity with $s_{}$-pair symmetry is a direct instability of an incoherent bad-metal.

Strain and pressure effects on $T_{c}$ are explained by competition between superconductivity and ferro-quadrupolar order.

Abstract

Finding of unconventional superconductivity (USC) in FeSe in an electronic "normal" state with broken $C_{4v}$ rotational symmetry testifies to the diversity of pairing states in Fe-based superconductors. Moreover, such USC emerges as a direct instability of a normal state without Landau Fermi liquid quasiparticles, increasingly dubbed a `strange' metal. Here, we combine inputs from a first-principles correlated electronic structure method (LDA+DMFT) and symmetry analyses to propose a novel mechanism for unconventional nodal superconductivity as a direct instability of an incoherent bad-metal without Landau Fermi-liquid quasiparticles. We find that a ferro-quadrupolar order, with novel spin quadrupoar correlations enhances orbital-selective Mottness in FeSe, and competes with unconventional nodal superconductivity with $s_{\pm}$-pair symmetry. We support our proposal by demonstrating good accord with spectral and magnetic fluctuation data, and rationalize the strain and pressure dependence of $T_{c}$ by appealing to competition between superconductivity and electronic ferro-quadrupolar order.