Novel Phenomena in Modern Studies of Magnetism
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Date
2013-12-20
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Johns Hopkins University
Abstract
In this PhD Dissertation, we present investigation of contemporary problems in
magnetism. We focus on two important themes that have been active
research topics in condensed matter community: 1. Topological
defects in magnet and their dynamics 2. Exotic states
and critical phenomena in frustrated spin systems.
In the first topic, we consider the dynamics of topological defect
known as Skyrmion in thin film ferromagnet. We first discuss
the nontrivial dynamics exhibited by a Skyrmion bubble
confined in thin film disk as observed by numerical simulation. We
propose a phenomenological theory that can reproduce the
peculiar dynamics of the Skyrmion bubble. We show that, unlike previously studied topological defects, a Skyrmion bubble possesses inertia. We derive a theoretical description of the dynamics using standard theory
of ferromagnet. We discover the presence of two counter propagating
chiral edge modes. Most importantly, we derive the mass (inertia) from
the theory and express it in terms of microscopic
parameters.
In the second topic, a quantum phase transition in
$U(1)$ quantum spin liquid phase of 3-d pyrochlore quantum spin ice is investigated. Starting from
microscopic spin model, we map the spin to slave-boson,
derive continuum theory, and
finally arrive at a $U(1)$ gauge theory which takes the form of scalar quantum electrodynamics (QED).
The effective free energy for quantum spin liquid (QSL) to antiferromagnetic (AFM) phase transition mimics the one for Bardeen-Cooper-Schrieffer (BCS) superconductors classical transition under magnetic field. We show
that, provided Ginzburg criterion is satisfied, the gauge
field fluctuations drive the originally continuous QSL to AFM phase
transition at mean field level into discontinuous one. We
predict the location of quantum critical point which agrees well
with gauge mean field theory result. We calculate the size of phase transition and find that it is a weakly first order.
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Keywords
magnetism