Quartz Tuning Forks for SQUID-On-Tip Depth Detection

Abstract

Scanning probe microscopes are key in 2D materials research. One challenge in using them is that depth sensing cannot be done in a fully decoupled way using the same instrument used for measurement, and improper depth sensing that results in a crash can damage both tip and sample. For scanning SQUID-on-tip microscopes, a method was devised to use the phase of oscillation of quartz tuning forks in contact with the tip to sense variations in tip-sample interaction corresponding with depth. A low-noise amplification system is necessary to read out the small changes in the phase. In this paper, we describe the theory of tuning forks, amplifier noise, the system we use to characterize amplifiers and tuning forks. We conclude with potential upgrades to the system, regarding both lower-noise amplifiers and different geometries of the tip and tuning fork which may be more sensitive to depth.