The non-linear microwave surface impedance, Zs = Rs + iXs, of patterned YBCO thin films, was measured using a suspended line resonator in the presence of a perpendicular DC magnetic field, HDC, of magnitude comparable to that of the microwave field, H rf . Signature of the virgin state was found to be absent even for relatively low microwave power levels. The microwave loss was initially found to decrease for small applied HDC before increasing again. Also, non-linearities inherent in the sample were found to be substantially suppressed at low powers at these applied fields. These two features together can lead to significant improvement in device performance.

Measurements of the rf penetration depth λ(T, H, θ) are used to study the superconducting order parameter, vortex dynamics in the mixed state and delineate critical fields in the borocarbide superconductor YNi2B2C. The lower critical field has an anomalous T dependence, Hc1(T ) = 1.12 [1 − (T /Tc)] kOe, which is however consistent with independent superfluid density measurements at microwave frequencies. The vortex response is dominated by viscous flux flow, indicative of extremely weak pinning, and is parametrized by a field scale H c2, eff . The angular dependence of the vortex contribution λ(θ) is in good agreement with the Coffey-Clem model. Structure is seen in the depairing transition in the vicinity of the upper critical field, with the existence of well-defined critical fields Hc2a, H c2b and Hc2c, with the vortex field scale H c2, eff closest to H c2b . Overall the measurements indicate that YNi2B2C has a rich and unusual field dependence of its transport parameters.

We report the observation of novel behaviour in the radio frequency (rf ) and microwave response of DyNi2B2C over a wide range of temperature (T ) and magnetic field (H) in the antiferromagnetic (AFM) and superconducting (SC) states. At microwave frequencies of 10 GHz, the T dependence of the surface impedance Zs = Rs + iXs was measured which yields the T dependence of the complex conductivity σ1 − iσ2 in the SC and AFM states. At radio frequencies (4 MHz), the H and T dependence of the penetration depth λ(T, H) were measured.The establishment of antiferromagnetic order at TN = 10.3 K results in a marked decrease in the scattering of charge carriers, leading to sharp decreases in Rs and Xs. However, Rs and Xs differ from each other in the AFM state. We show that the results are consistent with conductivity relaxation whence the scattering rate becomes comparable to the microwave frequency.The rf measurements yield a rich dependence of the scattering on the magnetic field near and below TN . Anomalous decrease of scattering at moderate applied fields is observed at temperatures near and above TN , and arises due to a crossover from a negative magnetoresistance state, possibly associated with a loss of spin disorder scattering at low fields, to a positive magnetoresistance state associated with the metallic nature. The normal state magnetoresistance is positive at all temperatures for µ0H > 2T and at all fields for T > 15K. Several characteristic field and temperature scales associated with metamagnetic transitions (HM1(T ), HM2(T )) and onset of spin disorder HD(T ), in addition to Tc, TN and Hc2(T ) are observed in the rf measurements.74.25. Ha, 74.25.Nf, 74.72.Ny, 75.20.Hr, 75.40.Gb, 75.50.Ee, 75.90.+w

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