Dispersionless orbital excitations in (Li,Fe)OHFeSe superconductors

TL;DR

This study uses resonant inelastic x-ray scattering to identify dispersionless orbital excitations in (Li,Fe)OHFeSe superconductors, emphasizing the role of orbital fluctuations in high-temperature superconductivity.

Contribution

It provides the first detailed characterization of orbital excitations in (Li,Fe)OHFeSe, linking orbital fluctuations to superconductivity enhancement.

Findings

Identified dispersionless Raman-like orbital excitations at 0.3 and 0.7 eV.

Orbital excitation intensities decrease with increasing temperature.

Orbital fluctuations are suggested to be crucial for high-$T_c$ superconductivity.

Abstract

The superconducting critical temperature $T_{\mathrm{c}}$ of intercalated iron-selenide superconductor (Li,Fe)OHFeSe (FeSe11111) can be increased to 42 K from 8 K of bulk FeSe. It shows remarkably similar electronic properties as the high-$T_{\mathrm{c}}$ monolayer FeSe and provides a bulk counterpart to investigate the origin of enhanced superconductivity. Unraveling the nature of excitations is crucial for understanding the pairing mechanism in high-$T_{\mathrm{c}}$ iron selenides. Here we use resonant inelastic x-ray scattering (RIXS) to investigate the excitations in FeSe11111. Our high-quality data exhibit several Raman-like excitations, which are dispersionless and isotropic in momentum transfer and robust against varying $T_{\mathrm{c}}$. Using atomic multiplet calculations, we assign the low-energy $\sim 0.3$ and 0.7 eV Raman peaks as local $e_g-e_g$ and $e_g-t_{2g}$ orbital excitations. The intensity of these two features decreases with increasing temperature, suggesting a primary contribution of the orbital fluctuations. Our results highlight the importance of orbital degree of freedom for high-$T_{\mathrm{c}}$ iron selenides.