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

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 $T_{\mathrm{e}\mathrm{n}\mathrm{v}}$. Simulating the STS data with the $P(E)$ theory, we determine that $T_{\mathrm{e}\mathrm{n}\mathrm{v}}\approx 1.5$ K, while the fitting of the superconducting gap spectrum yields the lowest $T=77$ mK.