Determining the temperature of a millikelvin scanning tunnelling microscope junction

Cooling the junction of a scanning tunneling microscope to millikelvin temperatures is fundamental for high-resolution scanning tunneling spectroscopy. However, accurately determining the junction temperature has proven elusive, due to the microscopic dimension of the junction and its continuous energy exchange with the surrounding environment. Here, we employ a millikelvin scanning tunnelling microscope cooled by an adiabatic demagnetization refrigerator. Using normal-metal and superconducting tips, we perform scanning tunnelling spectroscopy on an atomically clean surface of Al(100) in a superconducting state. By varying the refrigerator temperatures between 30 mK and 1.2 K, we show that the temperature of the junction is decoupled from the temperature of the surrounding environment. To corroborate our findings, we simulate the scanning tunnelling spectroscopy data with P(E) theory and determine that the junction has a temperature of 77 mK, despite its environment being at 1.5 K. Determining and controlling the junction temperature is one of the primary issues in millikelvin scanning tunneling microscopy. The authors show how to deduce this temperature from scanning tunneling spectroscopy experiments, demonstrating that their junction reaches 77 mK in spite of being exposed to a much hotter 1.5 K environment.