An ultra-high vacuum scanning tunneling microscope with pulse tube and Joule-Thomson cooling operating at sub-pm z-noise.

Abstract

Low-temperature scanning tunneling spectroscopy is a key method to probe electronic and magnetic properties down to the atomic scale, but suffers from extreme vibrational sensitivity. This makes it challenging to employ closed-cycle cooling with its required pulse-type vibrational excitations, albeit this is mandatory to avoid helium losses for counteracting the continuously raising helium prices. Here, we describe a compact ultra-high vacuum scanning tunneling microscope (STM) system with an integrated primary pulse tube cooler (PTC) for closed-cycle operation. It achieves temperatures down to 1.5 K via a secondary Joule-Thomson stage and a z-noise down to 300 fmRMS in the STM junction for the frequency range of 0.1 Hz-5 kHz (feedback loop off). This is better than many STMs cooled by an external supply of liquid helium. The challenge to combine an effective vibrational decoupling from the PTC with sufficient thermal conduction is tackled by using a multipartite approach including the concept of bellows with minimal stiffness to decouple the PTC vibrationally from the STM and an optimized STM design with minimal vibrational transfer to the STM junction. As important benchmarks, we could reduce the voltage noise in the tunnel junction down to 120 μV and supply radio frequency excitations up to 40 GHz with amplitudes up to 10 mV in the junction via a close-by antenna. The development principally enables other secondary cooling stages such that it opens the perspective for a helium conserving operation of STMs across the whole interesting temperature range.