4,754 research outputs found
Theory of magnetic field-induced metaelectric critical end point in BiMnO
A recent experiment on the multiferroic BiMnO compound under a strong
applied magnetic field revealed a rich phase diagram driven by the coupling of
magnetic and charge (dipolar) degrees of freedom. Based on the
exchange-striction mechanism, we propose here a theoretical model with the
intent to capture the interplay of the spin and dipolar moments in the presence
of a magnetic field in BiMnO. Experimentally observed behavior of the
dielectric constants, magnetic susceptibility, and the polarization is, for the
most part, reproduced by our model. The critical behavior observed near the
polarization reversal point in the phase diagram is interpreted as
arising from the proximity to the critical end point.Comment: Theory; relevant experiment uploaded as arXiv:0810.190
Matchmaker, Matchmaker, Make Me a Match: Migration of Populations via Marriages in the Past
The study of human mobility is both of fundamental importance and of great
potential value. For example, it can be leveraged to facilitate efficient city
planning and improve prevention strategies when faced with epidemics. The
newfound wealth of rich sources of data---including banknote flows, mobile
phone records, and transportation data---has led to an explosion of attempts to
characterize modern human mobility. Unfortunately, the dearth of comparable
historical data makes it much more difficult to study human mobility patterns
from the past. In this paper, we present an analysis of long-term human
migration, which is important for processes such as urbanization and the spread
of ideas. We demonstrate that the data record from Korean family books (called
"jokbo") can be used to estimate migration patterns via marriages from the past
750 years. We apply two generative models of long-term human mobility to
quantify the relevance of geographical information to human marriage records in
the data, and we find that the wide variety in the geographical distributions
of the clans poses interesting challenges for the direct application of these
models. Using the different geographical distributions of clans, we quantify
the "ergodicity" of clans in terms of how widely and uniformly they have spread
across Korea, and we compare these results to those obtained using surname data
from the Czech Republic. To examine population flow in more detail, we also
construct and examine a population-flow network between regions. Based on the
correlation between ergodicity and migration in Korea, we identify two
different types of migration patterns: diffusive and convective. We expect the
analysis of diffusive versus convective effects in population flows to be
widely applicable to the study of mobility and migration patterns across
different cultures.Comment: 24 pages, 23 figures, 5 table
Tunneling decay of false vortices
We consider the decay of vortices trapped in the false vacuum of a theory of
scalar electrodynamics in 2+1 dimensions. The potential is inspired by models
with intermediate symmetry breaking to a metastable vacuum that completely
breaks a U(1) symmetry, while in the true vacuum the symmetry is unbroken. The
false vacuum is unstable through the formation of true vacuum bubbles; however,
the rate of decay can be extremely long. On the other hand, the false vacuum
can contain metastable vortex solutions. These vortices contain the true vacuum
inside in addition to a unit of magnetic flux and the appropriate topologically
nontrivial false vacuum outside. We numerically establish the existence of
vortex solutions which are classically stable; however, they can decay via
tunneling. In general terms, they tunnel to a configuration which is a large,
thin-walled vortex configuration that is now classically unstable to the
expansion of its radius. We compute an estimate for the tunneling amplitude in
the semi-classical approximation. We believe our analysis would be relevant to
superconducting thin films or superfluids.Comment: 27 pages, 9 figure
The Battle of the Bulge: Decay of the Thin, False Cosmic String
We consider the decay of cosmic strings that are trapped in the false vacuum
in a theory of scalar electrodynamics in 3+1 dimensions. We restrict our
analysis to the case of thin-walled cosmic strings which occur when large
magnetic flux trapped inside the string. Thus the string looks like a tube of
fixed radius, at which it is classically stable. The core of the string
contains magnetic flux in the true vacuum, while outside the string, separated
by a thin wall, is the false vacuum. The string decays by tunnelling to a
configuration which is represented by a bulge, where the region of true vacuum
within, is ostensibly enlarged. The bulge can be described as the meeting, of a
kink soliton anti-soliton pair, along the length of the string. It can be
described as a bulge appearing in the initial string, starting from the string
of small, classically stable radius, expanding to a fat string of large,
classically unstable (to expansion) radius and then returning back to the
string of small radius along its length. This configuration is the bounce point
of a corresponding O(2) symmetric instanton, which we can determine
numerically. Once the bulge appears it explodes in real time. The kink soliton
anti-soliton pair recede from each other along the length of the string with a
velocity that quickly approaches the speed of light, leaving behind a fat tube.
At the same time the radius of the fat tube that is being formed, expands
(transversely) as it is no longer classically stable, converting false vacuum
to the true vacuum with ever diluting magnetic field within. The rate of this
expansion is determined by the energy difference between the true vacuum and
the false vacuum. Our analysis could be applied to a network, of cosmic strings
formed in the very early universe or vortex lines in a superheated
superconductor.Comment: 13 pages, 4 figure
Staggered Currents in the Vortex Core
We study the electronic structure of the vortex core in the cuprates using
the U(1) slave-boson mean-field wavefunctions and their Gutzwiller projection.
We conclude that there exists local orbital antiferromagnetic order in the core
near optimal doping. We compare the results with that of BCS theory and analyze
the spatial dependence of the local tunneling density of states.Comment: 4 pages, 3 figure
Instanton solutions mediating tunneling between the degenerate vacua in curved space
We investigate the instanton solution between the degenerate vacua in curved
space. We show that there exist -symmetric solutions not only in de
Sitter but also in both flat and anti-de Sitter space. The geometry of the new
type of solutions is finite and preserves the symmetry. The nontrivial
solution corresponding to the tunneling is possible only if gravity is taken
into account. The numerical solutions as well as the analytic computations
using the thin-wall approximation are presented. We expect that these solutions
do not have any negative mode as in the instanton solution.Comment: Some typos are corrected and references are added with respect to the
published version. 17pages, 11fi
Microscopic Theory of Rashba Interaction in Magnetic Metal
Theory of Rashba spin-orbit coupling in magnetic metals is worked out from
microscopic Hamiltonian describing d-orbitals. When structural inversion
symmetry is broken, electron hopping between -orbitals generates chiral
ordering of orbital angular momentum, which combines with atomic spin-orbit
coupling to result in the Rashba interaction. Rashba parameter characterizing
the interaction is band-specific, even reversing its sign from band to band.
Large enhancement of the Rashba parameter found in recent experiments is
attributed to the orbital mixing of 3d magnetic atoms with non-magnetic heavy
elements as we demonstrate by first-principles and tight-binding calculations.Comment: 5 pages, 2 figure
The false vacuum bubble nucleation due to a nonminimally coupled scalar field
We study the possibility of forming the false vacuum bubble nucleated within
the true vacuum background via the true-to-false vacuum phase transition in
curved spacetime. We consider a semiclassical Euclidean bubble in the Einstein
theory of gravity with a nonminimally coupled scalar field. In this paper we
present the numerical computations as well as the approximate analytical
computations. We mention the evolution of the false vacuum bubble after
nucleation.Comment: 23 pages, 12 figures, References added, minor correctio
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