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Precision electrodynamics of unconventional superconductors : microwave spectroscopy and penetration depth Bobowski, Jake Stanley


Precision measurements of the electrodynamics of unconventional superconductors are reported. Measurements of the magnetic penetration depth temperature dependence are made via cavity perturbation and a novel microwave spectrometer gives the surface resistance continuously from 0.05 to 26.5 GHz. The microwave conductivity of single crystals of YBa₂Cu₃O₆.₅₀ has been measured using the broadband spectrometer. Conductivity spectra were measured after preparing the crystal in the ortho-II phase in which the Cu-O chain oxygens are ordered into alternating full and empty chains. These spectra exhibit features expected for quasiparticle scattering from dilute weak impurities in a d-wave superconductor. The measurements were repeated after reducing the degree of oxygen order in the Cu-O chains. The disordered spectra retain the weak-limit scattering features, however, increased quasiparticle scattering broadens the widths. The conductivity of a new generation of samples, with an order of magnitude lower impurity concentrations, are unchanged from those of the older generation samples. In both generations of crystals the spectral widths are largely determined by residual disorder in the chains. The electrodynamics of single crystal Ba₀.₇₂K₀.₂₈Fe₂As₂ and Ba(Fe₀.₉₅Co₀.₀₅)₂As₂ have been investigated. Measurements of ∆⋌(T)are used to extract the superfluid density ⋌²(0)/⋌²(T) which is observed to approach Tc linearly indicative of mean-field behaviour. At low-temperatures, the superfluid density obeys the a power law ⋌²(0)/⋌²(T)= 1 - (T/T*)n with n varying from 2.1 to 2.7 for the three crystals studied. At T/Tc ≈ 0.04, all three samples exhibit a step-like feature of order 1 anstrom in the measured penetration depth that is of unknown origin. The surface resistance of two Ba₀.₇₂K0.₂₈Fe₂As₂ crystals reveal a sample-dependent extrinsic loss. The spectra, however, share a common temperature dependence and follow frequency squared when the extrinsic loss is removed. This frequency dependence translates to a flat quasiparticle conductivity and implies a high scattering rate(~ 200 GHz). The microwave spectroscopy of a Ba(Fe₀.₉₅Co₀.₀₅)₂As₂ sample reveals more anomalous behaviour. Even after subracting of the extrinsic loss, the surface resistance rapidly increases when the frequency scaled by Tc is above 0.04. The resistivity of the Ba(Fe₀.₉₅Co₀.₀₅)₂As₂ sample is an order of magnitude larger than that of the K-doped samples suggesting the anomalous behaviour may be activated by enhanced impurity concentrations.

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