Fully suspended nano-beams for quantum fluids

TL;DR

This paper introduces fully suspended silicon nitride nano-beams as non-invasive, highly sensitive probes for quantum fluids, demonstrating their fabrication, cryogenic performance, and application in studying helium gas boundary layers.

Contribution

It presents the design, fabrication, and cryogenic testing of fully suspended nano-beams as novel probes for quantum fluids, enabling measurements at the coherence length scale.

Findings

Nano-beams achieve quality factors up to 10^5.

Probes successfully investigated helium gas boundary layers.

No significant rarefaction effect from nearby surfaces observed.

Abstract

Non-invasive probes are keystones of fundamental research. Their size, and maneuverability (in terms of e.g. speed, dissipated power) define their applicability range for a specific use. As such, solid state physics possesses e.g. Atomic Force Microscopy (AFM), Scanning Tunneling Microscopy (STM), or Scanning SQUID Microscopy. In comparison, quantum fluids (superfluid $^3$He, $^4$He) are still lacking probes able to sense them (in a fully controllable manner) down to their smallest relevant lengthscales, namely the coherence length $\xi_0$. In this work we report on the fabrication and cryogenic characterization of fully suspended (hanging over an open window, with no substrate underneath) Si$_3$N$_4$ nano-beams, of width down to 50 nm and quality factor up to $10^5$. As a benchmark experiment we used them to investigate the Knudsen boundary layer of a rarefied gas: $^4$He at very low pressures. The absence of the rarefaction effect due to the nearby chip surface discussed in Gazizulin et al. [1] is attested, while we report on the effect of the probe size itself.