We report a detailed low-temperature thermodynamic investigation (heat capacity and magnetization) of the superconducting state of KFe2As2 for H || c axis. Our measurements reveal that the properties of KFe2As2 are dominated by a relatively large nodeless energy gap (Delta?0 = 1.9 kBTc) which excludes dx2-y2 symmetry. We prove the existence of several additional extremely small gaps (?Delta0 < 1.0 kBTc) that have a profound impact on the low-temperature and low-field behavior, similar to MgB2, CeCoIn5 and PrOs4Sb12. The zero-field heat capacity is analyzed in a realistic self-consistent 4-band BCS model which qualitatively reproduces the recent laser ARPES results of Okazaki et al. (Science 337 (2012) 1314). Our results show that extremely low-temperature measurements, i.e. T < 0.1 K, will be required in order to resolve the question of the existence of line nodes in this compound.
@article{arxiv.1309.5654,
title = {Multiband Superconductivity in KFe2As2: Evidence for one Isotropic and several Liliputian Energy Gaps},
author = {Frederic Hardy and Robert Eder and Martin Jackson and Dai Aoki and Carley Paulsen and Thomas Wolf and Philipp Burger and Anna Boehmer and Peter Schweiss and Peter Adelmann and Robert A. Fisher and Christoph Meingast},
journal= {arXiv preprint arXiv:1309.5654},
year = {2015}
}