Comment on “Microwave vortex dissipation of superconducting Nd−Ce−Cu−O epitaxial films in high magnetic fields”

Abstract

Experimental and theoretical results contained in a paper and in an Erratum by Yeh et al. [Phys. Rev. B 48, 9861 (1993); Phys. Rev. B 56, 5683(E) (1997)] discussing experiments employing a novel sapphire ring resonator to investigate microwave frequency vortex dissipation in thin film Nd1.85Ce0.15CuOx as a function of applied field and temperature are discussed. These authors also claim to have extended the Coffey-Clem [Phys. Rev. Lett. 67, 386 (1991); Physica C 185-189, 1915 (1991); Phys. Rev. B 45, 10527 (1992)] model to include thin-films. The authors’ theoretical results for the surface resistance as a function of applied field and temperature, which as illustrated in their paper were in detailed agreement with the data, are only approximately able to describe the data by scaling the theoretical expressions by unspecified multiplicative factors ∼50. The given theory contains no free scaling parameter. In the Erratum, the theory is still in detailed agreement with the NdCe214 data, although these data have changed by roughly a factor of ∼1∕50 due to a claimed experimental recalibration. No explanation was offered to account for the change in the theory, by ∼1∕50. In addition, the expressions provided for the surface resistance are not correct with obvious nonphysical limits: for example, the normal state surface resistance is independent of the resistivity. The recalibrated data are inconsistent with published results for the surface resistance for superconductors and thin film conductors. Equations for the B=0 surface resistance treatment, purportedly appropriate to a finite grain-size limit, were dimensionally inconsistent in the original paper, and an associated fitting parameter was not specified. A value for this “missing” parameter was given in the Erratum, but evaluation of the revised B=0 surface resistance expression with this parameter fails even to approximate the published B=0 curves. Several results from the work of others were misquoted.

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