Optical properties of the iron-based superconductor LiFeAs single crystal

Abstract

We have measured the reflectivity spectra of the LiFeAs (Tc = 17.6 K) single crystal in the temperature range from 4 to 300 K. In the superconducting (SC) state (T < Tc), the clean 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 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 compound. A theoretical analysis of the low-temperature data (T = 4 K < Tc) also suggests that two SC gaps best fit the data and their values were estimated as Δ1 = 3.3 meV and Δ2 = 1.59 meV, respectively. Using the Ferrell–Glover–Tinkham sum rule and dielectric function ϵ1(ω), the plasma frequency of the SC condensate (ωps) is consistently estimated to be 6665 cm−1, implying that about 65% of the free carriers of the normal state condense into the SC condensate. To investigate the various interband transition processes (for ω > 200 cm−1), we have also performed the local-density approximation band calculation and calculated the optical spectra of the interband transitions. This theoretical result provided qualitative agreement with the experimental data below 4000 cm−1 .