Dissertation/Thesis Abstract

Nano-SQUID susceptometry and fluctuation effects in superconducting rings
by Koshnick, Nicholas C., Ph.D., Stanford University, 2009, 102; 3351494
Abstract (Summary)

This thesis is broken down into two main components: the design and implementation of two generations of Scanning Superconducting Quantum Interference Device (SQUID) sensors, and the use of these tools to describe several kinds of fluctuations that can significantly affect the properties of micron sized aluminum rings.

The first chapter describes the susceptometer that was used for the bulk of the ring experiments. In it, I outline the advantages of a multi-layer design with local field coils and integrated feedback loops. To reach the fundamental sensitivity limits set by Johnson noise in the device itself, I employed piezo electric positioners to measure background magnetic fields, in addition to taking images of the magnetic field. The second chapter describes a generation of SQUIDs with FIB-patterned pickup loops that are connected to a similar base design. The coupling to ultra-small objects is significantly enhanced by the pickup loop's small diameter (down to ∼ 600 nm) and reduced scan height ([special characters omitted] 300 nm) enabled by lithographically patterned terraces. When combined with our device's low flux noise, this enhanced coupling brings SQUIDs close to the point where they could distinguish the field from a single electron spin.

Fluctuations are important for superconductors when multiple wave-function configurations need to be considered to describe the overall behavior of the system. This study includes several experimental regimes where physically distinct energy scales play an important role. In relatively dirty rings, I show how fluxoid transitions can significantly reduce the ring's ability to screen magnetic field. In smaller, cleaner rings, the contribution from fluxoid modes plays a smaller role, and non-Gaussian and Gaussian fluctuations induce superconductivity at and above the superconducting critical temperature, Tc. In this case, I also focus on rings that are biased with half of a flux quantum of field, which enables two flux modes to contribute to the ring's response. I specify the point where thermal fluctuations smear out the Little-Parks effect, where Tc, is reduced due to the Aharonov-Bohm phase winding energy. In the final section, I report on our efforts to fabricate and measure ultra clean rings where the Thouless energy is approximately equal to Tc.

Indexing (document details)
School: Stanford University
School Location: United States -- California
Source: DAI-B 70/03, Dissertation Abstracts International
Subjects: Condensed matter physics
Keywords: Fluctuation effects, Superconducting rings, Superconductivity, Susceptometry
Publication Number: 3351494
ISBN: 978-1-109-07729-2
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