816 research outputs found
Nematicity as a route to a magnetic field-induced spin density wave order; application to the high temperature cuprates
The electronic nematic order characterized by broken rotational symmetry has
been suggested to play an important role in the phase diagram of the high
temperature cuprates. We study the interplay between the electronic nematic
order and a spin density wave order in the presence of a magnetic field. We
show that a cooperation of the nematicity and the magnetic field induces a
finite coupling between the spin density wave and spin-triplet staggered flux
orders. As a consequence of such a coupling, the magnon gap decreases as the
magnetic field increases, and it eventually condenses beyond a critical
magnetic field leading to a field-induced spin density wave order. Both
commensurate and incommensurate orders are studied, and the experimental
implications of our findings are discussed.Comment: 5 pages, 3 figure
Exploring AdS Waves Via Nonminimal Coupling
We consider nonminimally coupled scalar fields to explore the Siklos
spacetimes in three dimensions. Their interpretation as exact gravitational
waves propagating on AdS restrict the source to behave as a pure radiation
field. We show that the related pure radiation constraints single out a unique
self-interaction potential depending on one coupling constant. For a vanishing
coupling constant, this potential reduces to a mass term with a mass fixed in
terms of the nonminimal coupling parameter. This mass dependence allows the
existence of several free cases including massless and tachyonic sources. There
even exists a particular value of the nonminimal coupling parameter for which
the corresponding mass exactly compensates the contribution generated by the
negative scalar curvature, producing a genuinely massless field in this curved
background. The self-interacting case is studied in detail for the conformal
coupling. The resulting gravitational wave is formed by the superposition of
the free and the self-interaction contributions, except for a critical value of
the coupling constant where a non-perturbative effect relating the strong and
weak regimes of the source appears. We establish a correspondence between the
scalar source supporting an AdS wave and a pp wave by showing that their
respective pure radiation constraints are conformally related, while their
involved backgrounds are not. Finally, we consider the AdS waves for
topologically massive gravity and its limit to conformal gravity.Comment: 26 pages, 1 figure. Minor change
Superconducting quantum phase transitions tuned by magnetic impurity and magnetic field in ultrathin a-Pb films
Superconducting quantum phase transitions tuned by disorder (d), paramagnetic
impurity (MI) and perpendicular magnetic field (B) have been studied in
homogeneously disordered ultrathin a-Pb films. The MI-tuned transition is
characterized by progressive suppression of the critical temperature to zero
and a continuous transition to a weakly insulating normal state with increasing
MI density. In all important aspects, the d-tuned transition closely resembles
the MI-tuned transition and both appear to be fermionic in nature. The B-tuned
transition is qualitatively different and probably bosonic. In the critical
region it exhibits transport behavior that suggests a B-induced mesoscale phase
separation and presence of Cooper pairing in the insulating state.Comment: 17 pages, 4 figure
Chaotic dynamics in preheating after inflation
We study chaotic dynamics in preheating after inflation in which an inflaton
is coupled to another scalar field through an interaction
. We first estimate the size of the quasi-homogeneous
field at the beginning of reheating for large-field inflaton potentials
by evaluating the amplitude of the fluctuations on
scales larger than the Hubble radius at the end of inflation. Parametric
excitations of the field during preheating can give rise to chaos
between two dynamical scalar fields. For the quartic potential (,
) chaos actually occurs for in a
linear regime before which the backreaction of created particles becomes
important. This analysis is supported by several different criteria for the
existence of chaos. For the quadratic potential () the signature of chaos
is not found by the time at which the backreaction begins to work, similar to
the case of the quartic potential with .Comment: 12 pages, 10 figures, Version to appear in Classical and Quantum
Gravit
Pre-Hawking Radiation from a Collapsing Shell
We investigate the effect of induced massive radiation given off during the
time of collapse of a massive spherically symmetric domain wall in the context
of the functional Schr\"odinger formalism. Here we find that the introduction
of mass suppresses the occupation number in the infrared regime of the induced
radiation during the collapse. The suppression factor is found to be given by
, which is in agreement with the expected Planckian distribution
of induced radiation. Thus a massive collapsing domain wall will radiate mostly
(if not exclusively) massless scalar fields, making it difficult for the domain
wall to shed any global quantum numbers and evaporate before the horizon is
formed.Comment: 10 pages, 3 figures. We updated the acknowledgments as well as added
a statement clarifying that we are following the methods first laid out in
Phys. Rev. D 76, 024005 (2007
Translational Symmetry Breaking in the Superconducting State of the Cuprates: Analysis of the Quasiparticle Density of States
Motivated by the recent STM experiments of J.E. Hoffman et.al. and C. Howald
et.al., we study the effects of weak translational symmetry breaking on the
quasiparticle spectrum of a d-wave superconductor. We develop a general
formalism to discuss periodic charge order, as well as quasiparticle scattering
off localized defects. We argue that the STM experiments in
cannot be explained using a simple charge density
wave order parameter, but are consistent with the presence of a periodic
modulation in the electron hopping or pairing amplitude. We review the effects
of randomness and pinning of the charge order and compare it to the impurity
scattering of quasiparticles. We also discuss implications of weak
translational symmetry breaking for ARPES experiments.Comment: 12 pages, 9 figs; (v2) minor corrections to formalism, discussions of
dispersion, structure factors and sum rules added; (v3) discussion of
space-dependent normalization added. To be published in PR
Solutions of Minimal Four Dimensional de Sitter Supergravity
Pseudo-supersymmetric solutions of minimal , de Sitter
supergravity are classified using spinorial geometry techniques. We find three
classes of solutions. The first class of solution consists of geometries which
are fibrations over a 3-dimensional manifold equipped with a Gauduchon-Tod
structure. The second class of solution is the cosmological Majumdar-Papapetrou
solution of Kastor and Traschen, and the third corresponds to gravitational
waves propagating in the Nariai cosmology.Comment: 17 Pages. Minor correction to section 4; equation (4.21) corrected
and (old) equation (4.26) deleted; the final result is unchange
On the divergences of inflationary superhorizon perturbations
We discuss the infrared divergences that appear to plague cosmological
perturbation theory. We show that within the stochastic framework they are
regulated by eternal inflation so that the theory predicts finite fluctuations.
Using the formalism to one loop, we demonstrate that the infrared
modes can be absorbed into additive constants and the coefficients of the
diagrammatic expansion for the connected parts of two and three-point functions
of the curvature perturbation. As a result, the use of any infrared cutoff
below the scale of eternal inflation is permitted, provided that the background
fields are appropriately redefined. The natural choice for the infrared cutoff
would of course be the present horizon; other choices manifest themselves in
the running of the correlators. We also demonstrate that it is possible to
define observables that are renormalization group invariant. As an example, we
derive a non-perturbative, infrared finite and renormalization point
independent relation between the two-point correlators of the curvature
perturbation for the case of the free single field.Comment: 12 page
SO(4) Theory of Competition between Triplet Superconductivity and Antiferromagnetism in Bechgaard Salts
Motivated by recent experiments with Bechgaard salts, we investigate the
competition between antiferromagnetism and triplet superconductivity in quasi
one-dimensional electron systems. We unify the two orders in an SO(4) symmetric
framework, and demonstrate the existence of such symmetry in one-dimensional
Luttinger liquids. SO(4) symmetry, which strongly constrains the phase diagram,
can explain coexistence regions between antiferromagnetic, superconducting, and
normal phases, as observed in (TMTSF)PF. We predict a sharp neutron
scattering resonance in superconducting samples.Comment: 5 pages, 3 figures; Added discussion of applicability of SO(4)
symmetry for strongly anisotropic Fermi liquids; Added reference
On Spacetimes with Constant Scalar Invariants
We study Lorentzian spacetimes for which all scalar invariants constructed
from the Riemann tensor and its covariant derivatives are constant (
spacetimes). We obtain a number of general results in arbitrary dimensions. We
study and construct warped product spacetimes and higher-dimensional
Kundt spacetimes. We show how these spacetimes can be constructed from
locally homogeneous spaces and spacetimes. The results suggest a number
of conjectures. In particular, it is plausible that for spacetimes that
are not locally homogeneous the Weyl type is , , or , with any
boost weight zero components being constant. We then consider the
four-dimensional spacetimes in more detail. We show that there are severe
constraints on these spacetimes, and we argue that it is plausible that they
are either locally homogeneous or that the spacetime necessarily belongs to the
Kundt class of spacetimes, all of which are constructed. The
four-dimensional results lend support to the conjectures in higher dimensions.Comment: 25 pages, 1 figure, v2: minor changes throughou
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