Raman Study of Cooper Pairing Instabilities in (Li$_{1-x}$Fe$_x$)OHFeSe

TL;DR

This study uses electronic Raman spectroscopy to investigate superconducting pairing instabilities in (Li$_{1-x}$Fe$_x$)OHFeSe, revealing collective modes and insights into pairing mechanisms related to band structure effects.

Contribution

It provides the first detailed Raman analysis of pairing modes in (Li$_{1-x}$Fe$_x$)OHFeSe, highlighting the role of band structure and collective excitations in its superconductivity.

Findings

Observation of two collective modes below T_c.

Identification of a nearly resolution-limited peak at 110 cm$^{-1}$ as a collective mode.

Absence of gap features in A$_{1g}$ and B$_{2g}$ symmetries suggests specific pairing mechanisms.

Abstract

We studied the electronic Raman spectra of (Li$_{1-x}$Fe$_x$)OHFeSe as a function of light polarization and temperature. In the B$_{1g}$ spectra alone we observe the redistribution of spectral weight expected for a superconductor and two well-resolved peaks below T$_c$. The nearly resolution-limited peak at 110 cm$^{-1}$ (13.6 meV) is identified as a collective mode. The peak at 190 cm$^{-1}$ (23.6 meV) is presumably another collective mode since the line is symmetric and its energy is significantly below the gap energy observed by single-particle spectroscopies. Given the experimental band structure of (Li$_{1-x}$Fe$_x$)OHFeSe, the most plausible explanations include conventional spin-fluctuation pairing between the electron bands and the incipient hole band and pairing between the hybridized electron bands. The absence of gap features in A$_{1g}$ and B$_{2g}$ symmetry favors the second case. Thus, in spite of various differences between the pnictides and chalcogenides, this Letter demonstrates the proximity of pairing states and the importance of band structure effects in the Fe-based compounds.