Specific heat of K0.71Na0.29Fe2As 2 at very low temperatures

Abstract

A commercially available calorimeter has been used to investigate the specific heat of a high-quality K0.71Na0.29Fe 2As2 single crystal. The addenda heat capacity of the calorimeter is determined in the temperature range 0.02 K ≤ T ≤ 0.54 K. The data of the K0.71Na0.29Fe2As2 crystal imply the presence of a large T2 contribution to the specific heat which gives evidence of d-wave order parameter symmetry in the superconducting state. To improve the measurements, a novel design for a calorimeter with a paramagnetic temperature sensor is presented. It promises a temperature resolution of ΔT ≈ 0.1 μK and an addenda heat capacity less than 200 pJ/K at T < 100 mK. © 2014 Springer Science+Business Media New York.