Optical properties of iron-based superconductor LiFeAs single crystal

TL;DR

This study investigates the optical properties of LiFeAs single crystals, revealing a multiband superconducting gap structure and providing insights into the superconductor's electronic behavior through experimental and theoretical analysis.

Contribution

The paper presents the first detailed optical spectroscopy analysis of LiFeAs, identifying two superconducting gaps and confirming its multiband nature with theoretical support.

Findings

Observation of an isotropic full gap below 25 cm-1 in the superconducting state

Identification of two superconducting gaps with values 1.59 meV and 3.15 meV

Estimation that 59% of free carriers condense into the superconducting state

Abstract

We have measured the reflectivity spectra of the iron based superconductor LiFeAs (Tc = 17.6 K) in the temperature range from 4 to 300 K. In the superconducting state (T < Tc), the clear opening of the optical absorption gap was observed below 25 cm-1, indicating an isotropic full gap formation. In the normal state (T > Tc), the optical conductivity spectra display a typical metallic behavior with the Drude type spectra at low frequencies, but we found that the introduction of the two Drude components best fits the data, indicating the multiband nature of this superconductor. A theoretical analysis of the low temperature data (T=4K < Tc) also suggests that two superconducting gaps best fit the data and their values were estimated as {\Delta}1 = 1.59 meV and {\Delta}2 = 3.15 meV, respectively. Using the Ferrell-Glover-Tinkham (FGT) sum rule and dielectric function {\epsilon}1({\omega}), the superconducting plasma frequency ({\omega}ps) is consistently estimated to be 6,665 cm-1, implying that about 59 % of the free carriers in the normal state condenses into the SC condensate. To investigate the various interband transition processes (for {\omega} > 200 cm-1), we have also performed the local-density approximation (LDA) band calculation and calculated the optical spectra of the interband transitions. The theoretical results provided a qualitative agreement with the experimental data below 4000 cm-1