Observation of a coherence peak in the microwave conductivity of optimally-doped Ba2Fe1.8Co0.2As2 single crystals with a critical temperature of 24 K

Abstract

We study the microwave surface impedance of 170-µm-thick BaFe1.8Co0.2As2 single crystals with a T C of 24 K by using an open-ended sapphire resonator. The surface resistance values measured at two different frequencies of 23.3 and 19.3 GHz revealed exponential behaviors at low temperatures when the residual surface resistance values were subtracted, which was attributed to quasiparticle excitations in the smaller energy gap, Δ1, having an s-wave nature. The measured Δ1 values were (2.1 ± 0.1) meV at 23.3 GHz and (2.3 ± 0.1) meV at 19.3 GHz. A coherence peak was observed at T/T C ∼ 0.85 at 23.3 GHz and at ∼0.71 at 19.3 GHz in the real part of the complex conductivity as a function of temperature, which was consistent with the s-wave nature of the energy gaps in the BaFe1.8Co0.2As2 single crystals. Our results support that the coherence peak, reported earlier for BaFe2−x Co x As2 films, is indeed due to the intrinsic properties of the BaFe2−x Co x As2 single crystals, and that, unlike the case in nuclear magnetic resonance experiments, the quasiparticle coherence in BaFe1.8Co0.2As2 single crystals can be detected in microwave experiments.