Evolution of spin excitations from bulk to monolayer FeSe

TL;DR

This study compares spin excitations in bulk and monolayer FeSe using RIXS and QMC, revealing significant differences that shed light on the pairing mechanism in FeSe superconductors.

Contribution

It provides the first detailed comparison of spin dynamics in bulk and monolayer FeSe, linking their evolution to Fermi surface changes and superconductivity.

Findings

Bulk FeSe shows dispersive magnetic excitations.

FeSe/STO exhibits gapped, dispersionless, and hardened spin excitations.

Spin excitations are reconfigured in monolayer FeSe, influencing pairing mechanisms.

Abstract

The discovery of enhanced superconductivity (SC) in FeSe films grown on SrTiO3 (FeSe/STO) has revitalized the field of Fe-based superconductors. In the ultrathin limit, the superconducting transition temperature Tc is increased by almost an order of magnitude, raising new questions on the pairing mechanism. As in other unconventional superconductors, antiferromagnetic spin fluctuations have been proposed as a candidate to mediate SC in this system. Thus, it is essential to study the evolution of the spin dynamics of FeSe in the ultrathin limit to elucidate their relationship with superconductivity. Here, we investigate and compare the spin excitations in bulk and monolayer FeSe grown on STO using high-resolution resonant inelastic x-ray scattering (RIXS) and quantum Monte Carlo (QMC) calculations. Despite the absence of long-range magnetic order, bulk FeSe displays dispersive magnetic excitations reminiscent of other Fe-pnictides. Conversely, the spin excitations in FeSe/STO are gapped, dispersionless, and significantly hardened relative to the bulk counterpart. By comparing our RIXS results with simulations of a bilayer Hubbard model, we connect the evolution of the spin excitations to the Fermiology of the two systems. The present study reveals a remarkable reconfiguration of spin excitations in FeSe/STO, which is essential to understand the role of spin fluctuations in the pairing mechanism.