Millimeter-wave surface impedance of optimally-doped Ba(Fe1-xCox)2As2 single crystals

• Published in: arXiv.org 2013-04
• Main Authors: Barannik, A, Cherpak, N T, Tanatar, M A, Vitusevich, S, Skresanov, V, Canfield, P C, Prozorov, R
• Format: Article
• Language: English
• Subjects: Cuprates; End plates; Extremely high frequencies; Impedance; Millimeter waves; Penetration depth; Power law; Q factors; Sapphire; Scattering; Single crystals; Superconductivity; Surface resistance; Temperature dependence; Two fluid models
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1304.0639

Precision measurements of active and reactive components of in-plane microwave surface impedance were performed in single crystals of optimally-doped Fe-based superconductor Ba(Fe1-xCox)2As2 (x = 0.074, Tc = 22.8 K). Measurements in a millimeter wavelength range (Ka band, 35-40 GHz) were performed using whispering gallery mode excitations in the ultrahigh quality factor quasioptical sapphire disk resonator with YBa2Cu2O7 superconducting (Tc = 90 K) end plates. The temperature variation of the London penetration depth is best described by a power-law function, delta {\lambda}(T) is proportional to T with the exponent n, n = 2.8, in reasonable agreement with radio-frequency measurements on crystals of the same batch. This power-law dependence is characteristic of a nodeless superconducting gap in the extended s-wave pairing scenario with a strong pair-breaking scattering. The quasiparticle conductivity of the samples, {\sigma}1(T), gradually increases with the decrease of temperature, showing no peak below or at Tc, in notable contrast with the behavior found in the cuprates. The temperature-dependent quasiparticle scattering rate was analyzed in a two-fluid model, assuming the validity of the Drude description of conductivity and generalized expression for the scattering rate. This analysis allows us to estimate the range of the values of a residual surface resistance from 3 to 6 mOhm.