We developed a calorimeter with a vacuum container made of superconducting niobium (Nb) to study monolayers of helium adsorbed on graphite which are prototypical two-dimensional quantum matters below 1 K. Nb was chosen because of its small specific heat in the superconducting state. It is crucially important to reduce the addendum heat capacity (Cad) when the specific surface area of substrate is small. Here we show details of design, construction and results of Cad measurements of the Nb calorimeter down to 40 mK. The measured Cad was sufficiently small so that we can use it for heat capacity measurements on helium monolayers in a wide temperature range below 1 K. We found a relatively large excess heat capacity in Cad, which was successfully attributed to atomic tunneling of hydrogen (H) and deuterium (D) between trap centers near oxygen or nitrogen impurities in Nb. The tunnel frequencies of H and D deduced by fitting the data to the tunneling model are consistent with the previous experiments on Nb doped with H or D.
@article{arxiv.2010.15295,
title = {Superconducting Niobium Calorimeter for Studies of Adsorbed Helium Monolayers},
author = {Jun Usami and Koki Tokeshi and Tomohiro Matsui and Hiroshi Fukuyama},
journal= {arXiv preprint arXiv:2010.15295},
year = {2021}
}