Coexistence of Clean- and Dirty-limit Superconductivity in LiFeAs

TL;DR

This study investigates the optical properties of LiFeAs, revealing the coexistence of clean- and dirty-limit superconductivity through analysis of two superconducting gaps and spectral weight transfer.

Contribution

It provides the first evidence of coexistence of clean- and dirty-limit superconductivity in LiFeAs based on optical conductivity measurements.

Findings

Two isotropic superconducting gaps identified

Spectral weight transfer confirms clean-limit superconductivity in one band

Coexistence of clean- and dirty-limit superconductivity demonstrated

Abstract

The optical properties of LiFeAs with $T_c \simeq$ 18 K have been determined in the normal and superconducting states. The superposition of two Drude components yields a good description of the low-frequency optical response in the normal state. Below $T_c$, the optical conductivity reveals two isotropic superconducting gaps with $\Delta_{1} \simeq 2.9$ $\pm$ 0.2 meV and $\Delta_{2} \simeq 5.5$ $\pm$ 0.4 meV. A comparison between the superconducting-state Mattis-Bardeen and the normal-state Drude components, in combination with a spectral weight analysis, indicates that the spectral weight associated with a band which has a very small scattering rate is fully transferred to the superfluid weight upon the superconducting condensate. These observations provide clear evidence for the coexistence of clean- and dirty-limit superconductivity in LiFeAs.