Specific heat and upper critical fields in KFe2As2 single crystals

We report low-temperature specific heat measurements for high-quality single crystalline KFe2As2 (T-c approximate to 3.5 K). The investigated zero-field specific heat data yields an unusually large nominal Sommerfeld coefficient, gamma(n) = 94(3) mJ/mol K-2, which is, however, affected by extrinsic contributions as evidenced by a sizable residual linear specific heat and various theoretical considerations, including an analysis of Kadowaki-Woods relations. These results indicate that KFe2As2 should be classified as a weakly or intermediately coupled superconductor with a total electron-boson coupling constant lambda(tot) similar to 1 (including a calculated weak electron-phonon coupling lambda(ph) = 0.17). From specific heat and ac susceptibility studies in external magnetic fields the magnetic phase diagram is also constructed. We confirm the high anisotropy of the upper critical fields mu H-0(c2)(T), ranging from a factor of 5 near T-c to a slightly reduced value around 4.5 approaching T = 0 for fields B parallel to ab and parallel to c and show that their ratio slightly exceeds the mean mass anisotropy of 4.4 derived from our full-relativistic local-density-approximation band structure calculations, also in accord with recent preliminary penetration depth data by Eskildsen et al. [Eskildsen, Forgan, and Kawano-Furukawa, Rep. Prog. Phys. 74, 124504 (2011)] near 4. Its slight reduction when approaching T = 0 is not a consequence of Pauli limiting as in less perfect samples but points likely to a multiband effect. We also report irreversibility field data obtained from ac susceptibility measurements. The double-maximum in the T dependence of its imaginary part for fields B parallel to c indicates a peak effect in the T dependence of critical currents.