THz electrodynamics of BaFe_1.91Ni_0.09As₂ film analyzed in the framework of multiband Eliashberg theory

Abstract

The temperature dependences of the plasma frequency, superfluid density and London penetration depth were determined from terahertz spectra of conductivity and dielectric permittivity of BaFe 1.91 Ni 0.09 As 2 film with critical temperature T c = 19.6 K. Part of experimental data were analyzed within a simple three-band Eliashberg model where the mechanism of superconducting coupling is mediated by antiferromagnetic spin fluctuations, whose characteristic energy Ω 0 scales with T c according to the empirical law Ω 0 = 4.65k B T c , and with a total electron-boson coupling strength λ tot = 2.17.

Optical and hidden transport properties of BaFe1.91Ni0.09As2 film

“…These values are consistent with the SC gaps of BaFe 1.9 Ni 0.1 As 2 single crystals determined from specific heat measurements as well as directly by intrinsic multiple Andreev reflections effect (IMARE) spectroscopy [15,16]. A three-band Eliashberg model used for the analysis of our previous THz spectroscopy data for the similar BaFe 1.91 Ni 0.09 As 2 film (T c = 19.6 K) [17] produces quite different values of the SC gaps, 2Δ 1 /k B T c = 5.1, 2Δ 2 /k B T c = 3.3, and 2Δ 2 /k B T c = 7.0. Such a difference can be related to the slightly different T c of our films as well as the coupling constants for Ba(Fe, Co) 2 As 2 used in previous calculations.…”

“…In our recent paper [17], we studied the optical properties of the BaFe 1.91 Ni 0.09 As 2 film by THz spectroscopy within the range of 10-50 cm −1 . The experimental data was analyzed within a simple three-band Eliashberg model, where the mechanism of superconducting coupling is mediated by antiferromagnetic spin fluctuations, whose characteristic energy Ω 0 scales with T c according to the empirical law Ω 0 = 4.65k B T c , and with a total electron-boson coupling strength λ tot = 2.17.…”

Eliashberg Theory of a Multiband Non-Phononic Spin Glass Superconductor

“…To calculate the critical temperature and the gaps, it is necessary to solve 4 coupled equations: 2 for the renormalization functions Z j (iω n ) and 2 for the gaps ∆ j (iω n ), where j, k are band index (that range between 1 and 2) and ω n are the Matsubara frequencies. The imaginary-axis equations [41][42][43][44][45][46] read as follows:…”