A scanning tunneling microscope for spectroscopic imaging below 90 mK in magnetic fields up to 17.5 T

TL;DR

This paper presents an ultra-high vacuum scanning tunneling microscope capable of spectroscopic imaging at temperatures below 90 mK and magnetic fields up to 17.5 T, enabling detailed studies of superconductors.

Contribution

It introduces a novel design combining a dilution refrigerator and superconducting magnet for extended low-temperature, high-field spectroscopic imaging with improved thermal and noise filtering.

Findings

Achieved electron temperature below 90 mK.

Demonstrated long-term stability with 4.5 days operation.

Visualized superconducting vortices in cuprate superconductor.

Abstract

We describe the development and performance of an ultra-high vacuum scanning tunneling microscope working under combined extreme conditions of ultra-low temperatures and high magnetic fields. We combined a top-loading dilution refrigerator and a standard bucket dewar with a bottom-loading superconducting magnet to achieve 4.5 days operating time, which is long enough to perform various spectroscopic-imaging measurements. To bring the effective electron temperature closer to the mixing-chamber temperature, we paid particular attention to filtering out the radio-frequency noise, as well as enhancing the thermal link between the microscope unit and the mixing chamber. We estimated the lowest effective electron temperature to be below 90 mK by measuring the superconducting-gap spectrum of aluminum. We confirmed the long-term stability of the spectroscopic-imaging measurement by visualizing superconducting vortices in the cuprate superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$.