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

TL;DR

This study measures heat capacity in FeAs-based compounds to explore their electronic and magnetic states, revealing entropy transfer, crystal field effects, and superconducting properties, with implications for understanding their thermodynamic behavior.

Contribution

It provides detailed heat capacity data on various FeAs-based compounds, identifying entropy transfer, crystal field levels, and superconducting features, advancing thermodynamic understanding of these materials.

Findings

Entropy transfer from magnetic ordering peaks with doping.

Identification of crystal field levels at 0, 23, and 56 meV.

Superconducting LiFeAs shows a rounded, smaller heat capacity feature at Tc.

Abstract

We report heat capacity measurements of the pnictide materials SmFeAsO$_{1-x}$F$_x$, NdFeAsO, LaFeAsO$_{1-x}$F$_x$ and LiFeAs. For SmFeAsO$_{1-x}$F$_x$, with x close to 0.1, we use 3 He measurements to demonstrate a transfer of entropy from the peak at TN to a previously unidentified ~2 K 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, 23, and 56 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 Tc, which is significantly rounded and much smaller than the BCS value.