Adding Radio Frequency Capabilities to a millikelvin Scanning Tunneling Microscope

TL;DR

This paper introduces a straightforward method to incorporate RF reflectometry into a millikelvin STM, enabling high-sensitivity, in-situ measurements of nanoscale features and electron properties at ultra-low temperatures.

Contribution

The work demonstrates the integration of RF reflectometry with a mk-STM using best practices from QIS, including impedance matching and tip-sample capacitance tuning, to enhance measurement sensitivity.

Findings

RF reflectometry can be successfully integrated into mk-STM.

Impedance matching significantly improves measurement sensitivity.

Nanoscale features as small as 5 nm$^2$ can be imaged using this technique.

Abstract

We present a simple home made solution enabling in-situ RF reflectometry measurements with a millikelvin scanning tunneling microscope (mk-STM). The additions described below were made using RF best practices following similar detection schemes commonly employed in the quantum information science (QIS) community. Using a Niobium STM tip to form a superconductor-insulator-normal metal (SIN) tunnel junction, the evolution of coherence peaks at the SC-gap edge are carefully measured to characterize the RF losses and electron temperature. We further identify impedance matching as a crucial factor to achieve high sensitivity in the reflectometry by tuning the tip-sample capacitance as a function of approach distance. As a demonstration of this capability, we measure a 50x50 nm$^2$ area of island features that have been condensed onto the surface of a gold single crystal. Position dependent reflectometry losses allow us to image island sizes down to a total surface area of 5 nm$^2$ given our current sensitivity.