Temperature and frequency of complex conductivity, penetration depth and superfluid density of Ba(Fe_1−xCo_x)₂As₂-superconductor in one- and two-gap models

Abstract

The temperature and frequency dependence of complex dynamical conductivity $\sigma = \sigma_{1} + i\sigma_{2}$, penetration depth, and superfluid density of Ba(Fe_1−xCo_x)₂As₂ are calculated in the framework of one- and two-gap models at terahertz frequencies for a temperature range of 2 K < T < T_C (T_C = 22 K). In the single-gap model, an isotropic s-wave gap without any node and a d-wave gap with possible nodes are considered. In the two-gap model, one of the gaps was considered isotropic s-wave gap with amplitude $\varDelta_{A} = 3\;{\text{meV}}$, while the other gap was supposed to be an anisotropic d-wave gap with possible nodes, and its rms amplitude was $\varDelta_{0} = 8\;{\text{meV}}$. The interaction between the gaps is waived, since it is small, and its description is problematic. Comparing the result with the experimental data proves that the two-gap model consistently describes the optical characteristics of Ba(Fe_1−xCo_x)₂As₂. In the two-gap model, the temperature dependence of $\sigma_{1}$ demonstrated a coherence peak at frequencies below 15 cm⁻¹. The temperature dependence of the penetration depth calculated from $\sigma_{2}$ shows power-law behavior at low temperatures, and the superfluid density varies steeply near T_C.