How cold is the junction of a millikelvin scanning tunnelling microscope?

TL;DR

This study uses a millikelvin STM with an ADR to perform spectroscopy on Al(100), revealing that the junction temperature is significantly lower than the environment temperature, with implications for ultra-low temperature measurements.

Contribution

It demonstrates the decoupling of the STM junction temperature from the environment temperature and quantifies this difference using $P(E)$ theory and superconducting gap fitting.

Findings

Junction temperature can be as low as 77 mK.

Environmental temperature is approximately 1.5 K.

Junction temperature is decoupled from ambient conditions.

Abstract

We employ a scanning tunnelling microscope (STM) cooled to millikelvin temperatures by an adiabatic demagnetization refrigerator (ADR) to perform scanning tunnelling spectroscopy (STS) on an atomically clean surface of Al(100) in a superconducting state using normal-metal and superconducting STM tips. Varying the ADR temperatures between 30 mK and 1.2 K, we show that the temperature of the STM junction $T$ is decoupled from the temperature of the surrounding environment $T_{\mathrm{env}}$. Simulating the STS data with the $P(E)$ theory, we determine that $T_{\mathrm{env}} \approx 1.5$ K, while the fitting of the superconducting gap spectrum yields the lowest $T=77$ mK.