Superconductivity and Nematic Fluctuations in a model of FeSe monolayers: A Determinant Quantum Monte Carlo Study

TL;DR

This study uses determinant quantum Monte Carlo to explore how nematic fluctuations influence superconductivity in a minimal FeSe model, revealing enhanced pairing without nematic long-range order and discovering an antiferro-orbital insulator at strong coupling.

Contribution

It introduces an efficient DQMC algorithm with parallel tempering for studying a sign problem free FeSe model, revealing new phases and the role of nematic fluctuations in superconductivity.

Findings

Superconductivity with extended s-wave order is observed at intermediate couplings.

Nematic long-range order is not detected in the studied parameter range.

An antiferro-orbital insulating phase appears at strong coupling.

Abstract

In contrast to bulk FeSe, which exhibits nematic order and low temperature superconductivity, atomic layers of FeSe reverse the situation, having high temperature superconductivity appearing alongside a suppression of nematic order. To investigate this phenomenon, we study a minimal electronic model of FeSe, with interactions that enhance nematic fluctuations. This model is sign problem free, and is simulated using determinant quantum Monte Carlo (DQMC). We developed a DQMC algorithm with parallel tempering, which proves to be an efficient source of global updates and allows us to access the region of strong interactions. Over a wide range of intermediate couplings, we observe superconductivity with an extended s-wave order parameter, along with enhanced, but short ranged, $q=(0,0)$ ferro-orbital (nematic) order. These results are consistent with approximate weak coupling treatments that predict that nematic fluctuations lead to superconducting pairing. Surprisingly, in the parameter range under study, we do not observe nematic long range order. Instead, at stronger coupling an unusual insulating phase with $q=(\pi,\pi)$ antiferro-orbital order appears, which is missed by weak coupling approximations.