Date of Award

Summer 8-2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Program/Concentration

Condensed Matter Physics and Materials Science

Committee Director

Alexander Gurevich

Committee Member

Gianluigi Ciovati

Committee Member

Jean R. Delayen

Committee Member

Gail E. Dodge

Committee Member

Hani Elsayed-Ali

Abstract

We investigated the performance limitations of superconducting radio-frequency (SRF) cavities and materials using multiple experimental techniques. In particular, this study focuses on understanding the surface properties of nitrogen-doped Nb cavities and superconducting thin films with higher Tc such as Nb3Sn. The main goal of this work is to use different techniques to better understand each aspect of the complex loss mechanism in superconductors to further improve the already highly efficient SRF cavities.

Nitrogen doping applied to a Nb SRF cavity significantly improves the quality factor Q0 compared to a conventional Nb cavity, at an expense of reduced maximum accelerating gradient. The early quench mechanism was analyzed by using temperature maps before and during the quenching. The temperature maps revealed insignificant heating before the quench, and we concluded that nitrogen doping reduces critical magnetic fields in local regions, leading to premature quenching.

To understand the origin of the increasing Q0 with the rf field, the density of states (DOS) of cold spots from nitrogen-doped and standard cavities were measured and analyzed using scanning tunneling microscopy. The results suggested that nitrogen doping reduces the spatial inhomogeneity of superconducting properties and shrinks the metallic suboxide layers, which tunes the DOS in such a way as to produce the field-induced reduction in the surface resistance.

To characterize SRF thin films, an experimental setup for measuring a coplanar waveguide (CPW) resonator was developed and tested. A surface impedance measurement of the Nb film showed good agreement with the BCS calculation. The preliminary results from measurements of Nb3Sn and NbTiN films are also presented here.

The nonlinear Meissner effect was investigated in Nb3Sn film CPW resonators by measuring the resonant frequency as a function of a parallel magnetic field. Contrary to a conventional quadratic dependence of the penetration depth λ(B) on the applied magnetic field B, as expected in s-wave superconductors, nearly a linear increase of λ(B) with B was observed. It was concluded that this behavior of λ(B) is due to weakly linked grain boundaries on the polycrystalline Nb3Sn films, which can mimic the NLME expected in a clean d-wave superconductor.

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DOI

10.25777/58jh-g352

ISBN

9798352694282

ORCID

0000-0002-6889-769X

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