Thermodynamic properties of Ba1-xKxFe2As2 and Ca1-xNaxFe2As2

The specific heat C(T) of a BaFe2As2 single crystal and hole-doped superconducting Ba0.5K0.5Fe2As2 and Ca0.5Na0.5Fe2As2 polycrystals were measured. For the undoped BaFe2As2 single crystal, a very sharp specific heat peak was observed at 136 K. This is attributed to the structural and antiferromagnetic transitions occurring at the same temperature. For the superconducting Ba0.5K0.5Fe2As2 polycrystal, a clear peak of C/T was observed at T-c = 36 K, which is the highest peak seen at a superconducting transition for iron-based high-T-c superconductors so far. For the superconducting Ca0.5Na0.5Fe2As2 sample with lower T-c = 18 K, there is no obvious anomaly in C/T below T-c, which may be due to its low superfluid density. The electronic specific heat coefficients gamma and Debye temperatures Theta(D) of these compounds were obtained from the low-temperature data.