Specific heat and upper critical field in KFe2As2 single crystals

TL;DR

This study investigates the specific heat and upper critical field in high-quality KFe2As2 single crystals, revealing insights into its superconducting properties, anisotropy, and multiband effects through detailed experimental measurements.

Contribution

It provides the first comprehensive analysis of specific heat, critical fields, and anisotropy in KFe2As2, highlighting its weak to intermediate coupling nature and multiband effects.

Findings

Large Sommerfeld coefficient influenced by extrinsic effects

Superconductor with lambda_tot near 1 and weak electron-phonon coupling

High anisotropy of upper critical fields with multiband effects

Abstract

We report low-temperature specific heat measurements for high-quality single crystalline KFe2As2 (T_c about 3.5 K). The investigated zero-field specific heat data yields an unusually large nominal Sommerfeld coefficient gamma_n of 94(3) mJ/mol K^2 which is however significantly affected by extrinsic contributions as evidenced by a sizable residual linear specific heat and various theoretical considerations including also an analysis of Kadowaki-Woods relations. Then KFe2As2 should be classified as a weak to intermediately strong coupling superconductor with a total electron-boson coupling constant lambda_tot near 1 (including a calculated weak electron-phonon coupling constant of lambda_el-ph =0.17. From specific heat and ac susceptibility studies in external magnetic fields the magnetic phase diagram has been constructed. We confirm the high anisotropy of the upper critical fields B_c2(T) ranging from a factor of 5 near T_c to a slightly reduced value approaching T=0 for fields B || ab$ and || c and show that their ratio Gamma slightly exceeds the mass anisotropy of 4.35 derived from our full-relativistic LDA-band structure calculations. Its slight reduction when approaching T=0 is not a consequence of Pauli-limiting as in less perfect samples but point 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 || c indicates a peak-effect in the T-dependence of critical currents.