4,365 research outputs found
An electric-field representation of the harmonic XY model
The two-dimensional harmonic XY (HXY) model is a spin model in which the
classical spins interact via a piecewise parabolic potential. We argue that the
HXY model should be regarded as the canonical classical lattice spin model of
phase fluctuations in two-dimensional condensates, as it is the simplest model
that guarantees the modular symmetry of the experimental systems. Here we
formulate a lattice electric-field representation of the HXY model and contrast
this with an analogous representation of the Villain model and the
two-dimensional Coulomb gas with a purely rotational auxiliary field. We find
that the HXY model is a spin-model analogue of a lattice electric-field model
of the Coulomb gas with an auxiliary field, but with a temperature-dependent
vacuum (electric) permittivity that encodes the coupling of the spin vortices
to their background spin-wave medium. The spin vortices map to the Coulomb
charges, while the spin-wave fluctuations correspond to auxiliary-field
fluctuations. The coupling explains the striking differences in the
high-temperature asymptotes of the specific heats of the HXY model and the
Coulomb gas with an auxiliary field. Our results elucidate the propagation of
effective long-range interactions throughout the HXY model (whose interactions
are purely local) by the lattice electric fields. They also imply that global
spin-twist excitations (topological-sector fluctuations) generated by local
spin dynamics are ergodically excluded in the low-temperature phase. We discuss
the relevance of these results to condensate physics.Comment: 13 pages, 10 figure
Topological-sector fluctuations and ergodicity breaking at the Berezinskii-Kosterlitz-Thouless transition
The Berezinskii-Kosterlitz-Thouless (BKT) phase transition drives the
unbinding of topological defects in many two-dimensional systems. In the
two-dimensional Coulomb gas, it corresponds to an insulator-conductor
transition driven by charge deconfinement. We investigate the global
topological properties of this transition, both analytically and by numerical
simulation, using a lattice-field description of the two-dimensional Coulomb
gas on a torus. The BKT transition is shown to be an ergodicity breaking
between the topological sectors of the electric field, which implies a
definition of topological order in terms of broken ergodicity. The breakdown of
local topological order at the BKT transition leads to the excitation of global
topological defects in the electric field, corresponding to different
topological sectors. The quantized nature of these classical excitations, and
their strict suppression by ergodicity breaking in the low-temperature phase,
afford striking global signatures of topological-sector fluctuations at the BKT
transition. We discuss how these signatures could be detected in experiments
on, for example, magnetic films and cold-atom systems.Comment: 11 pages, 6 figure
Phase order in superfluid helium films
Classic experimental data on helium films are transformed to estimate a
finite-size phase order parameter that measures the thermal degradation of the
condensate fraction in the two-dimensional superfluid. The order parameter is
found to evolve thermally with the exponent , a
characteristic, in analogous magnetic systems, of the
Berezinskii-Kosterlitz-Thouless (BKT) phase transition. Universal scaling near
the BKT fixed point generates a collapse of experimental data on helium and
ferromagnetic films, and implies new experiments and theoretical protocols to
explore the phase order. These results give a striking example of experimental
finite-size scaling in a critical system that is broadly relevant to
two-dimensional Bose fluids.Comment: 6 pages, 2 figure
Holographic Superconductors in a Cohesive Phase
We consider a four-dimensional N=2 gauged supergravity coupled to matter
fields. The model is obtained by a U(1) gauging of a charged hypermultiplet and
therefore it is suitable for the study of holographic superconductivity. The
potential has a topologically flat direction and the parameter running on this
"moduli space" labels the new superconducting black holes. Zero temperature
solutions are constructed and the phase diagram of the theory is studied. The
model has rich dynamics. The retrograde condensate is just a special case in
the new class of black holes. The calculation of the entanglement entropy makes
manifest the properties of a generic solution and the superconductor at zero
temperature is in a confined cohesive phase. The parameter running on the
topologically flat direction is a marginal coupling in the dual field theory.
We prove this statement by considering the way double trace deformations are
treated in the AdS/CFT correspondence. Finally, we comment on a possible
connection, in the context of gauge/gravity dualities, between the geometry of
the scalar manifold in N=2 supergravity models and the space of marginal
deformations of the dual field theory.Comment: 32 pages, 11 figures. Introduction rewritten and clarified, comments
and details on section 4 added, acknowledgements rectified. To appear in JHE
Lattice potentials and fermions in holographic non Fermi-liquids: hybridizing local quantum criticality
We study lattice effects in strongly coupled systems of fermions at a finite
density described by a holographic dual consisting of fermions in
Anti-de-Sitter space in the presence of a Reissner-Nordstrom black hole. The
lattice effect is encoded by a periodic modulation of the chemical potential
with a wavelength of order of the intrinsic length scales of the system. This
corresponds with a highly complicated "band structure" problem in AdS, which we
only manage to solve in the weak potential limit. The "domain wall" fermions in
AdS encoding for the Fermi surfaces in the boundary field theory diffract as
usually against the periodic lattice, giving rise to band gaps. However, the
deep infrared of the field theory as encoded by the near horizon AdS2 geometry
in the bulk reacts in a surprising way to the weak potential. The hybridization
of the fermions bulk dualizes into a linear combination of CFT1 "local quantum
critical" propagators in the bulk, characterized by momentum dependent
exponents displaced by lattice Umklapp vectors. This has the consequence that
the metals showing quasi-Fermi surfaces cannot be localized in band insulators.
In the AdS2 metal regime, where the conformal dimension of the fermionic
operator is large and no Fermi surfaces are present at low T/\mu, the lattice
gives rise to a characteristic dependence of the energy scaling as a function
of momentum. We predict crossovers from a high energy standard momentum AdS2
scaling to a low energy regime where exponents found associated with momenta
"backscattered" to a lower Brillioun zone in the extended zone scheme. We
comment on how these findings can be used as a unique fingerprint for the
detection of AdS2 like "pseudogap metals" in the laboratory.Comment: 42 pages, 5 figures; v2, minor correction, to appear in JHE
Pregnancy experiences of Western Australian women attending a specialist childbirth and mental illness antenatal clinic
Our purpose was to explore the pregnancy experiences of Australian women attending a specialized Childbirth and Mental Illness (CAMI) antenatal clinic. A qualitative exploratory design was selected to give voice to women with a severe mental illness receiving antenatal care. Telephone interviews with 41 women, 24 primiparous and 17 multiparous, were analysed using thematic analysis. Three themes emerged: ‘Building relationships’, ‘Acknowledged me as a person with special needs’ and ‘Respect and understanding without stigma’. Findings offer insight into care experiences possible within a multidisciplinary model developed to addresses psychiatric and obstetric needs of pregnant women with severe mental illness
A soliton menagerie in AdS
We explore the behaviour of charged scalar solitons in asymptotically global
AdS4 spacetimes. This is motivated in part by attempting to identify under what
circumstances such objects can become large relative to the AdS length scale.
We demonstrate that such solitons generically do get large and in fact in the
planar limit smoothly connect up with the zero temperature limit of planar
scalar hair black holes. In particular, for given Lagrangian parameters we
encounter multiple branches of solitons: some which are perturbatively
connected to the AdS vacuum and surprisingly, some which are not. We explore
the phase space of solutions by tuning the charge of the scalar field and
changing scalar boundary conditions at AdS asymptopia, finding intriguing
critical behaviour as a function of these parameters. We demonstrate these
features not only for phenomenologically motivated gravitational Abelian-Higgs
models, but also for models that can be consistently embedded into eleven
dimensional supergravity.Comment: 62 pages, 21 figures. v2: added refs and comments and updated
appendice
Virtual-crystal approximation that works: Locating a composition phase boundary in Pb(Zr_{1-x}Ti_3)O_3
We present a new method for modeling disordered solid solutions, based on the
virtual crystal approximation (VCA). The VCA is a tractable way of studying
configurationally disordered systems; traditionally, the potentials which
represent atoms of two or more elements are averaged into a composite atomic
potential. We have overcome significant shortcomings of the standard VCA by
developing a potential which yields averaged atomic properties. We perform the
VCA on a ferroelectric oxide, determining the energy differences between the
high-temperature rhombohedral, low-temperature rhombohedral and tetragonal
phases of Pb(Zr_{1-x}Ti_x)O_3 at x=0.5 and comparing these results to
superlattice calculations and experiment. We then use our new method to
determine the preferred structural phase at x=0.4. We find that the
low-temperature rhombohedral phase becomes the ground state at x=0.4, in
agreement with experimental findings.Comment: 5 pages, no figure
RNA-MATE: a recursive mapping strategy for high-throughput RNA-sequencing data
Summary: Mapping of next-generation sequencing data derived from RNA samples (RNAseq) presents different genome mapping challenges than data derived from DNA. For example, tags that cross exon-junction boundaries will often not map to a reference genome, and the strand specificity of the data needs to be retained. Here we present RNA-MATE, a computational pipeline based on a recursive mapping strategy for placing strand specific RNAseq data onto a reference genome. Maximizing the mappable tags can provide significant savings in the cost of sequencing experiments. This pipeline provides an automatic and integrated way to align color-space sequencing data, collate this information and generate files for examining gene-expression data in a genomic context
Semi-local quantum liquids
Gauge/gravity duality applied to strongly interacting systems at finite
density predicts a universal intermediate energy phase to which we refer as a
semi-local quantum liquid. Such a phase is characterized by a finite spatial
correlation length, but an infinite correlation time and associated nontrivial
scaling behavior in the time direction, as well as a nonzero entropy density.
For a holographic system at a nonzero chemical potential, this unstable phase
sets in at an energy scale of order of the chemical potential, and orders at
lower energies into other phases; examples include superconductors and
antiferromagnetic-type states. In this paper we give examples in which it also
orders into Fermi liquids of "heavy" fermions. While the precise nature of the
lower energy state depends on the specific dynamics of the individual system,
we argue that the semi-local quantum liquid emerges universally at intermediate
energies through deconfinement (or equivalently fractionalization). We also
discuss the possible relevance of such a semi-local quantum liquid to heavy
electron systems and the strange metal phase of high temperature cuprate
superconductors.Comment: 31 pages, 7 figure
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