Specific heat of K071Na0.29Fe2As2 at very low temperatures

TL;DR

This study measures the specific heat of K071Na0.29Fe2As2 at very low temperatures, revealing evidence for d-wave superconducting order and introducing a novel calorimeter design with enhanced sensitivity.

Contribution

It provides detailed low-temperature specific heat data for K071Na0.29Fe2As2 and introduces a new calorimeter design with improved temperature resolution.

Findings

Large T^2 contribution to specific heat indicating d-wave symmetry

Calorimeter with temperature resolution of 0.1 μK

Addenda heat capacity less than 200 pJ/K at T < 100 mK

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 $0.02 \, \mathrm{K} \leq T \leq 0.54 \, \mathrm{K}$. 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 $\Delta T \approx 0.1 \, \mathrm{\mu K}$ and an addenda heat capacity less than $200 \, \mathrm{pJ/K}$ at $ T < 100 \, \mathrm{mK}$.