R. T. Gordon, N. D. Zhigadlo, S. Weyeneth, S. Katrych, R. Prozorov
Single crystals of MgCNi$_3$, with areas sized up to 1 mm$^{2}$, were grown by the self flux method using a cubic anvil high pressure technique. In low applied fields, the dc magnetization exhibited a very narrow transition into the superconducting state, demonstrating good quality of the grown crystals. The low critical field H$_{c1}$, determined from a zero temperature extrapolation, is around 18 mT. The magnetic penetration depth for a single crystal sample has also been measured in dc fields up to 9 T using a tunnel diode resonator (TDR) circuit technique for two different sample orientations with respect to the direction of applied magnetic field. The superfluid density was constructed from the London penetration depth, which was measured in zero applied dc field, and it was found to follow the clean s-wave behavior predicted by the BCS theory quite well. The temperature dependence of the upper critical field, $H_{c2}$, was found to be isotropic with a slope at T$_c$ of -2.6 T/K and H$_{c2}$(0) $\approx$ 12.3 T at zero temperature. In sufficiently high enough applied dc magnetic fields, the TDR probes the vortex lattice response of superconductors in the mixed state. For this material, an irreversible feature has been observed in the TDR response when the sample is field-cooled and warmed versus zero-field-cooled and warmed. This feature possesses a non-monotonic field dependence and has commonly been referred to as the peak effect.
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http://arxiv.org/abs/1301.3561
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