| Summary: | We have measured the reflectivity spectra of the LiFeAs ( T _c = 17.6 K) single crystal in the temperature range from 4 to 300 K. In the superconducting (SC) state ( T < T _c ), 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 > T _c ), 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 < T _c ) also suggests that two SC gaps best fit the data and their values were estimated as Δ _1 = 3.3 meV and Δ _2 = 1.59 meV, respectively. Using the Ferrell–Glover–Tinkham sum rule and dielectric function ϵ _1 ( ω ), the plasma frequency of the SC condensate ( ω _ps ) 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 .
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