Precision Microwave Electrodynamic Measurements of K- and Co-doped BaFe${}_2$As${}_2$

Abstract

We have studied the microwave electrodynamics of single crystal iron-based superconductors Ba${}_{0.72}$K${}_{0.28}$Fe${}_2$As${}_2$ (hole-doped, $T_{\mathrm{c}}$ $\approx$30 K) and Ba(Fe${}_{0.95}$Co${}_{0.05}$)${}_2$As${}_2$ (electron-doped, $T_{\mathrm{c}}$ $\approx$20 K), by cavity perturbation and broadband spectroscopy. SQUID magnetometry was used to confirm the quality and homogeneity of the samples under study. Through cavity perturbation techniques, the temperature dependence of the in-plane London penetration depth $\Delta \lambda (T)$, and therefore the superfluid phase stiffness ${\lambda }^2(0)/{\lambda }^2(T)$ was measured. Down to 0.4 K, the data do not show the exponential saturation at low temperatures expected from a singly-, fully-gapped superconductor. Rather, both the electron- and the hole-doped systems seem to be best described by a power law behavior, with ${\lambda }^2(0)/{\lambda }^2(T)$ $\sim$ $T^n$ and \emph{n} $\approx$ 2.5. In the three samples we studied, a weak feature near the sensitivity limit of our measurements appears near $T/T_{\mathrm{c}}$ =~0.04, hinting at a corresponding low energy feature in the superconducting density of states. The data can also be relatively well-described by a simple two-gap s-wave model of the order parameter, but this yields parameters which seem unrealistic and dependent on the fit range. Broadband surface resistance measurements reveal a sample dependent residual loss whose origin is unclear. The data from the \FeAs samples can be made to scale as ${\omega }^2$ if the extrinsic loss is treated as an additive component, indicating large scattering rates. Finally, the temperature dependence of the surface resistance at 13 GHz obeys a power law very similar to those observed for $\Delta \lambda (T)$.

11 pages, 11 figures, to be published in Phys. Rev. B

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• Superconducting Properties of the $s^{\pm }$-wave state: Fe-based superconductors
• Anomalous Temperature Dependence of the Superfluid Density Caused by Dirty-to-Clean Crossover in FeSe${}_{0.4}$Te${}_{0.6}$ Single Crystals
• Using electron irradiation to probe iron - based superconductors
• Surface-resistance measurements using superconducting stripline resonators
• Superfluid density measurements of Ba(CoxFe1-x)2As2 films near optimal doping
• Anisotropic resistivity of Na${}_{1-\delta }$Fe${}_{1-x}$Co${}_x$As
• Using split-ring resonators to measure the electromagnetic properties of materials: An experiment for senior physics undergraduates
• Design and characterization of a novel toroidal split-ring resonator
• Direct measurement of the temperature dependence of the magnetic penetration depth in Ba(Fe${}_{1-x}$Ni${}_x$)${}_2$As${}_2$ superconductors
• Niobium stripline resonators for microwave studies on superconductors
• On the determination of the quasiparticle scattering rate in unconventional superconductors by microwave surface impedance