Adding Radio Frequency Capabilities to a millikelvin Scanning Tunneling Microscope

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.

Figures (12)

• Figure 1: (a) Schematic of the tank circuit integration into the mK-STM with relevant RF components as described in the main text. (b) Illustration of how capacitive variations (green) and resistive (blue) variations affect the amplitude associated with the reflectometry of the tank circuit.
• Figure 2: (a) Outline of RF scheme integrated into the DR. The majority of the components, described in the main text are mounted below the mixing chamber (30 mK) stage. The associated input/output ports are labeled via the blue and red arrows. The coaxial RF lines are shown parallel to the DR unit. Blue lines from room temperature to 4K stage represent thermocoax, the 4K to MXC lines are silver plated stainless steel coax, MXC to STM cold finger are copper coax. (b) Modified STM tip plate that houses the tank circuit situated within the STM walker assembly. Here, the inductor is hand wound around an aluminum oxide post, which is kept fixed to the plate via epoxy. Electrical contact to the tip holder and coax line is made via spot weld, while the contact to the center conductor of the coax is made via silver paste. The coax is ultimately fed into an SSMC port. (c) Room temperature reflectometry of the tank circuit, measured as S21 with the cryogenic amplifier only.
• Figure 3: (a) I(V) and (b) dI/dV with zero applied RF power to the tank circuit. The data are obtained by turning off the STM feedback control, fixing the tip to a constant height, and varying the tip-sample bias. Both sets of data consist of 20 averaged spectroscopy curves. The red curve is obtained from a fit to Eq. 3.
• Figure 4: (a) Stacked conductance data at $f_0$ = 312.032 MHz (black points) with each successive curve corresponding to an increase in the RF port power by increments of 3 dB. The overlaid red curves are formed from the convolution described in the main text, which is graphically summarized in (b). (c) Splitting of the modulated coherence curves as a function of power for four different frequencies: on resonance (312.032 MHz), 3dB down (312.111 MHz), far off resonance (313 MHz) and the secondary resonance (425.433 MHz). The splitting is measured via peak to peak distance, as labeled in the rightmost curve in (b).
• Figure 5: (a) Frequency traces of the tank circuit at base temperature with port powers 16 dBm (black) and -30 dBm (red). These applied powers correspond respectively to the normal and superconducting temperature regimes of the directional coupler. The inset shows a finer sampled frequency trace in the vicinity of the main resonance in the tank circuit. The colored dashed lines correspond to the frequencies sampled in the power dependence (b) Voltage positions of the modulated coherence peaks (see Fig. \ref{['Fig:pdep']}c) as a function of the output of the network analyzer. Note that the x axis is in mV, ranging from 0.71 mV (-50 dBm) to 15.83 mV (-23 dBm). The slopes of the red, blue, black and green linear traces are respectively 0.0886(12), 0.0894(12), 0.1038(12), 0.1404(8).