Multiband superconductivity in KFe₂As₂: Evidence for one isotropic and several lilliputian energy gaps

We report a detailed low-temperature thermodynamic investigation (heat capacity and magnetization) of the superconducting state of KFe ₂As₂ for H||c axis. Our measurements reveal that the properties of KFe₂As₂ are dominated by a relatively large nodeless energy gap (δ₀ = 1.9 κ_BT_c) which excludes dx²-y² symmetry. We prove the existence of several additional extremely small gaps (δ₀ < 1.0 k _BT_c) that have a profound impact on the low-temperature and lowfield behavior, similar to MgB₂, CeCoIn₅, and PrOs₄Sb₁₂. The zero-field heat capacity is analyzed in a realistic selfconsistent 4-band BCS model which reproduces only qualitatively the recent laser ARPES results of Okazaki et al. [Science 337, 1314 (2012)]. Our results show that extremely low-temperature measurements, i.e., T < 0.1 K, are required in order to resolve the question of the existence of line nodes in this compound.