Vortex response and critical fields observed via RF penetration depth measurements on the superconductor YNi2B2C

Measurements of the RF penetration depth lambda(T, H, theta) 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, H-cl(T) = 1.12[1 - (T/T-c)]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 parameterized by a field scale H-c2,H-eff. The angular dependence of the vortex contribution lambda(theta) 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 H-c2 alpha, H-c2 beta and H-c2 gamma with the vortex field scale H-c2,H-eff closest to H-c2 beta. Overall the measurements indicate that YNi2B2C has a rich and unusual field dependence of its transport parameters.