Specific heat of K071Na0.29Fe2As2 at very low temperatures

Abstract

A commercially available calorimeter has been used to investigate the specific heat of a high-quality \kn\ single crystal. The addenda heat capacity of the calorimeter is determined in the temperature range $0.02\mathrm{K}\le T\le 0.54\mathrm{K}$. The data of the \kn\ crystal imply the presence of a large $T^2$ 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 $\hat{I}”T\approx 0.1\hat{\mathrm{I}}¼\mathrm{K}$ and an addenda heat capacity less than $200pJ/K$ at $T<100\mathrm{mK}$.

8 pages, 5 figures