Electron-boson spectral density of LiFeAs obtained from optical data

TL;DR

This study extracts the electron-boson spectral density of LiFeAs from optical data using maximum entropy, revealing a consistent peak at 8 meV linked to other spectroscopic observations, and discusses the impact of impurity scattering.

Contribution

It introduces a method to accurately recover the electron-boson spectral density from optical data considering impurity effects in LiFeAs.

Findings

Identified a robust peak at ~8 meV in the spectral density.

The peak aligns with features in STS and INS data.

Superconducting gap shows evidence of anisotropy.

Abstract

We analyze existing optical data in the superconducting state of LiFeAs at $T =$ 4 K, to recover its electron-boson spectral density. A maximum entropy technique is employed to extract the spectral density $I^2\chi(\omega)$ from the optical scattering rate. Care is taken to properly account for elastic impurity scattering which can importantly affect the optics in an $s$-wave superconductor, but does not eliminate the boson structure. We find a robust peak in $I^2\chi(\omega)$ centered about $\Omega_R \cong$ 8.0 meV or 5.3 $k_B T_c$ (with $T_c =$ 17.6 K). Its position in energy agrees well with a similar structure seen in scanning tunneling spectroscopy (STS). There is also a peak in the inelastic neutron scattering (INS) data at this same energy. This peak is found to persist in the normal state at $T =$ 23 K. There is evidence that the superconducting gap is anisotropic as was also found in low temperature angular resolved photoemission (ARPES) data.