Probing the Effect of Superconductivity on the Casimir Force On-Chip with STM

Abstract

This thesis provides an overview of research focused on fabricating high-performance nanomechanical resonators from amorphous silicon carbide (a-SiC) and (super)conducting metallic niobium titanium nitride (NbTiN), and subsequently characterizing the superconducting NbTiN resonators using scanning tunneling microscopy (STM). The installation of on-chip nano mechanics with a minimally invasive STM detection technique enables the probing of subtle variations in the Casimir force between superconductors during their phase transition. This thesis consists of four parts.

With the aim of maximizing the coupling of the Casimir force to a large superconducting nanomembrane suspended over a sub-micron vacuum gap, we initially employed atomic layer deposition (ALD). Prior to using ALDto construct the high-aspect-ratio superconducting cavity, we investigated a novel amorphous silicon carbide (a-SiC) material in Chapter 2. Our study demonstrated that a-SiC exhibits high chemical inertness, remarkable ultimate tensile strength, and—most importantly—the capability to support nanomechanical resonators with high quality factors. Leveraging these excellent properties, we fabricated high-aspect-ratio a-SiC nanomembranes suspended over a sub-micron vacuum gap.

Subsequently, using the on-chip cavity formed by the strained a-SiC nanomembrane and the substrate with flat surface, we performed ALD to conformally coat all cavity surfaces with metallic NbTiN, thereby filling the vacuum gap atomically layer-by-layer. By optimizing the deposition conditions with the method described in Chapter 3, we achieved a high-aspect-ratio NbTiN cavity with a gap size of less than 100 nm. During this optimization, we also observed that metallic nanomechanical resonators fabricated via ALD can operate at room temperature with quality factors significantly higher than those of fully coated metallic resonators produced by other deposition techniques.

To measure the superconducting nanomembranes with minimal perturbation to their superconducting state, we installed the NbTiN nanomembrane fabricated by ALD into a scanning tunneling microscope (STM) and performed dynamic measurements in a cryogenic environment, as detailed in Chapter 4. In studying the tip–membrane interaction, we developed three measurement techniques to precisely determine the resonant frequency of the nanomembrane. One technique relies on the homodyne method, while the other two exploit the Van der Waals interaction between the STM tip and the nanomembrane.

Although the NbTiN nanomechanical resonators exhibit high performance, their superconducting properties are compromised by the absence of plasma bombardment on the inner cavity surfaces. Drawing on our experience with suspending large nanomembranes over small gap sizes in Chapter 3, we developed a new fabrication process in Chapter 5 to suspend a bare superconducting NbTiN nanomechanical membrane over another superconducting NbTiN film, with a sub-micron vacuum gap between them.

This configuration minimizes unwanted interactions that could otherwise affect the precision of measuring the variation in the Casimir force between superconductors during the phase transition. The high force sensitivity of the nanomembrane allows us to detect subtle force changes associated with the superconducting phase transition, including those arising from abrupt changes in the Casimir force.