Reed Beverstock Dissertation Defense
Location: ISC 3 Room 0280
When: Dec 6th, 2024 @ 10:30 am
Title: Superconducting Niobium Titanium Nitride Films and Structures for Accelerator Applications
This work presents the characterization of the surface, material, and superconducting properties of high-quality reactive direct current magnetron sputtering (DCMS) NbTiN thin films. Developing specialized films is a promising method to surpass the limitations of bulk Nb superconducting radio frequency (SRF) cavities. The performance of the cavities is limited by magnetic flux entering the surface at first critical field, Hc1. Although many compounds exhibit higher superconducting transition temperatures (Tc) than Nb, their lower Hc1 has limited their practical application. Depositing higher Tc compounds in multilayer superconductor/insulator/superconductor (SIS) structures on bulk Nb can delay magnetic flux penetration. The higher Tc can reduce operational expenses of cooling, and a stronger Hc1 allows for a higher acceleration gradient through an SRF cavity.
NbTiN is a promising candidate for SIS structures. Deposited with reactive DCMS, these epitaxial monocrystalline films exhibited the high Tc ≥16 K δ-phase NbTiN above 10 nm thick on MgO substrates. The interface quality was assessed by depositing bilayers of ≈3 nm NbTiN with ≈1 nm aluminum nitride (AlN), repeated up to 32 times without in-creased roughness. These stacked layers demonstrated metamaterial properties, including epsilon-near-zero and hyperbolic dispersion, with Tc exceeding those of individual NbTiN layers.
Surface characterization involved atomic force microscopy (AFM) and scanning electron microscopy (SEM) of representative samples. The microstructure was analyzed using electron backscatter diffraction, transmission electron microscopy, and x-ray diffraction techniques. Film thicknesses were assessed with X-ray reflectivity, stylus profilometry, ellipsometry, and AFM. Superconducting properties were tested using DC and AC techniques of single and multilayers. Using a multi-sample Kelvin Method, DC measurements assessed the residual resistivity ratio and Tc for the films. AC measurements utilized The Surface Impedance Characterization (SIC) system, Quadrupole Resonator (QPR) at Helmholtz-Zentrum Berlin as well as 1 kHz measurements with a newly built 3rd harmonic magnetometer to evaluate sample’s magnetic screening of the underlying bulk by measuring the first flux penetration (Hfp).
This work characterizes the surface, material, and superconducting properties of high-quality NbTiN thin films produced by reactive DC magnetron sputtering. Implementing thin film structures on accelerating cavities can reduce operational and construction expenses. Additionally, this research relates to detectors and metamaterial technologies in a parallel strategy. Multilayer thin NbTiN films exhibited hyperbolic metamaterial properties and demonstrated an increase in Tc compared to single layers. SIS structures of thick NbTiN with AlN interlayers can increase the screening of magnetic fields to the bulk superconductor.
