Ph.D. Dissertation Defense: Tim Kohler

Friday, April 16, 2021
2:00 p.m.
Zoom Meeting meeting link:
Maria Hoo
301 405 3681

ANNOUNCEMENT: Remote PhD Dissertation Defense

Name: Tim Kohler


Professor Steven Anlage (Chair)

Dr. Kevin Osborn (Co-Chair)

Professor Julius Goldhar

Professor Chris Monroe

Professor Ichiro Takeuchi, Dean's Representative

Date/time: Friday, April 16, 2021 at 2:00pm


Location: Zoom Meeting meeting link:





Superconducting films are inherently limited by losses due to two-level system (TLS) defects within the amorphous oxide layers surrounding them and from quasiparticles in the film. In this thesis I will discuss novel theoretical and experimental methods toward understanding superconducting resonator loss from deleterious surface TLS defects as well as a loss transition from non-equilibrium quasiparticles in granular TiN. I will show using finite element solver software that a resonator with submicron linewidth and linespacing can be used to better characterize and simulate surface TLS as part of a circuit QED system. I have observed individual surface TLS and found coupling values in the range of 50 kHz -280 kHz with p_{z,max}=4.5 Debye (.93 {Å}). I have found in experiment and in simulation that over 80% of the strongly coupled TLS reside within 50 nm of the Metal-Substrate(MS) and Substrate-Air (SA) interface. Additionally I have studied a loss transition from non-equilibrium quasiparticles in TiN films. These films exhibit an anomalous loss dependence on substrate treatment and film thickness. The films of interest are ones grown thin on oxidized substrates, which exhibit an order of magnitude decrease in internal quality factor (Qi) relative to either thicker films or films grown without the oxidized substrate. These films additionally exhibit a grain size on average of 7.5 nm, a higher inhomogeneous gap, a transition to tensile stress and a preference for the [111] crystal growth. The temperature dependence of the conductivity is fit and a factor of two difference in quasiparticle lifetime is found between the two films where the thinner film has a shorter lifetime. A two gap quasiparticle trapping model is fit to the temperature dependent loss data. The data is consistent with a model where non-equilibrium quasiparticles are trapped in low gapped grains on the inside of the films. From these works and others presented in my thesis the understanding of TLSs on surfaces and non-equilibrium quasiparticles in TiN has improved. This will help illuminate some of the most important absorption mechanisms plaguing superconducting qubits and resonators.



Audience: Graduate  Faculty 

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