Membrane-based nanocalorimeter for high-resolution measurements of low-temperature specific heat

TL;DR

This paper introduces a highly sensitive, membrane-based nanocalorimeter capable of measuring the specific heat of extremely small samples across a wide temperature range with high resolution and accuracy.

Contribution

It presents a novel design of a differential nanocalorimeter with enhanced sensitivity, direct sample contact, and advanced data acquisition techniques for low-temperature specific heat measurements.

Findings

Successfully measured heat capacity of a small Au sample.

Determined specific heat of a 2.6 μg Pb sample in magnetic fields.

Achieved high resolution and accuracy in low-temperature specific heat data.

Abstract

A differential, membrane-based nanocalorimeter for general specific heat studies of very small samples, ranging from 0.5 mg to sub-{\mu}g in mass, is described. The calorimeter operates over the temperature range from above room temperature down to 0.5 K. It consists of a pair of cells, each of which is a stack of heaters and thermometer in the center of a silicon nitride membrane, in total giving a background heat capacity less than 100 nJ/K at 300 K, decreasing to 10 pJ/K at 1K. The device has several distinctive features: i) The resistive thermometer, made of a Ge_{1-x}Au_{x} alloy, displays a high dimensionless sensitivity |dlnR/dlnT | \geq 1 over the entire temperature range. ii) The sample is placed in direct contact with the thermometer, which is allowed to self-heat. The thermometer can thus be operated at high dc current to increase the resolution. iii) Data are acquired with a set of eight synchronized lock-in amplifiers measuring dc, 1st and 2nd harmonic signals of heaters and thermometer. This gives high resolution and allows continuous output adjustments without additional noise. iv) Absolute accuracy is achieved via a variable-frequency-fixed-phase technique in which the measurement frequency is automatically adjusted during the measurements to account for the temperature variation of the sample heat capacity and the device thermal conductance. The performance of the calorimeter is illustrated by studying the heat capacity of a small Au sample and the specific heat of a 2.6 {\mu}g piece of superconducting Pb in various magnetic fields.