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Title:Phase transitions and vortex dynamics in superconducting island arrays
Author(s):Durkin, Malcolm Shaw
Director of Research:Mason, Nadya
Doctoral Committee Chair(s):Bezryadin, Alexey
Doctoral Committee Member(s):Hughes, Taylor L.; DeMarco, Brian
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Condensed matter
Abstract:In this thesis, we use superconducting island arrays as a platform for studying vortex motion and quantum phase transitions. We investigate superconducting vortex dynamics and lattice structures in superconducting arrays by performing electrical transport measurements on Nb island arrays on Au at milli-kelvin temperatures and finite fields. At low fillings, we observe anomalous vortex dynamics that we attribute to a history dependent dissipative force as the vortex moves through the lattice. At higher fillings, vortex-vortex interaction becomes significant and is dominated by collective vortex motion. We find that the transition from pinned to vortex lattice flow is split into two transitions as the filling is shifted from the commensurate filling regime, where the vortex lattice has strong crystalline order, to an incommensurate filling, where the vortex lattice no longer matches potential wells of the SNS array. We find that this behavior is consistent with domain wall motion in a polycrystalline vortex lattice at commensurate fillings. Superconducting island arrays can also be used to study phase transitions. Previous work in our group found that the onset of superconductivity in Nb islands was strongly dependent on the island spacing in the array. Performing follow up measurements, we find that the critical island temperature increases as the underlying Au is made thicker, indicating that this effect is dependent on the strength of electrical interactions between islands and is not due to normal metal suppression. Performing measurements on individual islands, we find that the vast majority of 260nm islands undergo a transition at temperatures far lower than those in island arrays, to the extent that they cannot be observed in a Helium 4 cryostat, and that there is a broad distribution of island critical temperatures observed. This suggests that the onset of superconductivity in rare ordered regions plays a significant role the onset of superconductivity in both the arrays samples and single islands. Lastly, we present work studying the superconductor to insulator transition in Sn island arrays on graphene as well as the technical difficulties involved.
Issue Date:2017-05-02
Rights Information:Copyright 2017 Malcolm Durkin
Date Available in IDEALS:2017-09-29
Date Deposited:2017-08

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