D. A. Zocco, K. Grube, F. Eilers, T. Wolf, H. v. Löhneysen
The upper critical field Hc2(T) of KFe2As2 has been studied via low-temperature thermal expansion and magnetostriction measurements. The observation of quantum oscillations of the sample length associated with orbits in its five hole-like Fermi surfaces suggest that the interaction of these bands strongly influences the superconductivity in KFe2As2 and manifests in the multigap shape of our thermal-expansion measurements. We present compelling evidence for Pauli-limiting effects dominating Hc2(T) for H || a, as revealed by a crossover from second- to first-order phase transitions to the superconducting state in the magnetostriction measurements down to 50 mK. Corresponding features were absent for H || c. To our knowledge, these results constitute the first confirmation of Pauli limiting of the Hc2(T) of a multiband superconductor. These results are supported by modeling Pauli limits for single-band and multiband cases.
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http://arxiv.org/abs/1305.5130
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