Heat capacity measurements on FeAs-based compounds: a thermodynamic probe of electronic and magnetic states

We report heat capacity measurements of the pnictide materials SmFeAsO1-xFx, NdFeAsO, LaFeAsO1-xFx and LiFeAs. For SmFeAsO1-xFx, with x close to 0.1, we use He-3 measurements to demonstrate a transfer of entropy from the peak at T-N to a previously unidentified similar to 2K feature, which grows with increasing doping. Our results on the Sm samples are compared with a similarly doped La sample to elucidate the crystal field levels of the Sm3+ ion at 0, 20 and 45 meV, which lead to a Schottky-like anomaly, and also show that there is a significant increase in the Sommerfeld coefficient gamma when La is replaced by Sm or Nd. The lattice contribution to the heat capacity of the superconducting oxypnictides is found to vary negligibly with chemical substitution. We also present a heat capacity measurement of LiFeAs showing the feature at T-c, which is significantly rounded and much smaller than the BCS value.