775 research outputs found
Holographic Description of AdS Cosmologies
To gain insight in the quantum nature of the big bang, we study the dual
field theory description of asymptotically anti-de Sitter solutions of
supergravity that have cosmological singularities. The dual theories do not
appear to have a stable ground state. One regularization of the theory causes
the cosmological singularities in the bulk to turn into giant black holes with
scalar hair. We interpret these hairy black holes in the dual field theory and
use them to compute a finite temperature effective potential. In our study of
the field theory evolution, we find no evidence for a "bounce" from a big
crunch to a big bang. Instead, it appears that the big bang is a rare
fluctuation from a generic equilibrium quantum gravity state.Comment: 34 pages, 8 figures, v2: minor changes, references adde
Long Range Order at Low Temperature in Dipolar Spin Ice
Recently it has been suggested that long range magnetic dipolar interactions
are responsible for spin ice behavior in the Ising pyrochlore magnets and . We report here numerical
results on the low temperature properties of the dipolar spin ice model,
obtained via a new loop algorithm which greatly improves the dynamics at low
temperature. We recover the previously reported missing entropy in this model,
and find a first order transition to a long range ordered phase with zero total
magnetization at very low temperature. We discuss the relevance of these
results to and .Comment: New version of the manuscript. Now contains 3 POSTSCRIPT figures as
opposed to 2 figures. Manuscript contains a more detailed discussion of the
(i) nature of long-range ordered ground state, (ii) finite-size scaling
results of the 1st order transition into the ground state. Order of authors
has been changed. Resubmitted to Physical Review Letters Contact:
[email protected]
Asymptotically Anti-de Sitter spacetimes and scalar fields with a logarithmic branch
We consider a self-interacting scalar field whose mass saturates the
Breitenlohner-Freedman bound, minimally coupled to Einstein gravity with a
negative cosmological constant in D \geq 3 dimensions. It is shown that the
asymptotic behavior of the metric has a slower fall-off than that of pure
gravity with a localized distribution of matter, due to the back-reaction of
the scalar field, which has a logarithmic branch decreasing as r^{-(D-1)/2} ln
r for large radius r.
We find the asymptotic conditions on the fields which are invariant under the
same symmetry group as pure gravity with negative cosmological constant
(conformal group in D-1 dimensions). The generators of the asymptotic
symmetries are finite even when the logarithmic branch is considered but
acquire, however, a contribution from the scalar field.Comment: 7 pages, CECS style, references adde
Multitrace deformations, Gamow states, and Stability of AdS/CFT
We analyze the effect of multitrace deformations in conformal field theories
at leading order in a large N approximation. These theories admit a description
in terms of a weakly coupled gravity dual. We show how the deformations can be
mapped into boundary terms of the gravity theory and how to reproduce the RG
equations found in field theory. In the case of doubletrace deformations, and
for bulk scalars with masses in the range , the deformed
theory flows between two fixed points of the renormalization group, manifesting
a resonant behavior at the scale characterizing the transition between the two
CFT's. On the gravity side the resonance is mapped into an IR non-normalizable
mode (Gamow state) whose overlap with the UV region increases as the dual
operator approaches the free field limit. We argue that this resonant behavior
is a generic property of large N theories in the conformal window, and
associate it to a remnant of the Nambu-Goldstone mode of dilatation invariance.
We emphasize the role of nonminimal couplings to gravity and establish a
stability theorem for scalar/gravity systems with AdS boundary conditions in
the presence of arbitrary boundary potentials and nonminimal coupling.Comment: 14 pages, references added, introduction change
Type I collagen limits VEGFR-2 signaling by a SHP2 protein-tyrosine phosphatase-dependent mechanism 1.
During angiogenesis, a combined action between newly secreted extracellular matrix proteins and the repertoire of integrins expressed by endothelial cells contributes in the regulation of their biological functions. Extracellular matrix-engaged integrins influence tyrosine kinase receptors, thus promoting a regulatory cross-talk between adhesive and soluble stimuli. For instance, vitronectin has been reported to positively regulate VEGFR-2. Here, we show that collagen I downregulates VEGF-A-mediated VEGFR-2 activation. This activity requires the tyrosine phosphatase SHP2, which is recruited to the activated VEGFR-2 when cells are plated on collagen I, but not on vitronectin. Constitutive expression of SHP2(C459S) mutant inhibits the negative role of collagen I on VEGFR-2 phosphorylation. VEGFR-2 undergoes internalisation, which is associated with dynamin II phosphorylation. Expression of SHP2(C459S) impairs receptor internalisation suggesting that SHP2-dependent dephosphorylation regulates this process. These findings demonstrate that collagen I in provisional extracellular matrix surrounding nascent capillaries triggers a signaling pathway that negatively regulates angiogenesis
Application of the Cluster Variation Method to Spin Ice Systems on the Pyrochlore Lattice
The cactus approximation in the cluster variation method is applied to the
spin ice system with nearest neighbor ferromagnetic coupling. The temperature
dependences of the entropy and the specific heat show qualitatively good
agreement with those observed by Monte Carlo simulations and experiments, and
the Pauling value is reproduced for the residual entropy. The analytic
expression of the q-dependent magnetic susceptibility is obtained, from which
the absence of magnetic phase transition is confirmed. The neutron scattering
pattern is also evaluated and found to be consistent with that obtained from
Monte Carlo simulations.Comment: 8 pages, 7 figure
Asymptotic generators of fermionic charges and boundary conditions preserving supersymmetry
We use a covariant phase space formalism to give a general prescription for
defining Hamiltonian generators of bosonic and fermionic symmetries in
diffeomorphism invariant theories, such as supergravities. A simple and general
criterion is derived for a choice of boundary condition to lead to conserved
generators of the symmetries on the phase space. In particular, this provides a
criterion for the preservation of supersymmetries. For bosonic symmetries
corresponding to diffeomorphisms, our prescription coincides with the method of
Wald et al.
We then illustrate these methods in the case of certain supergravity theories
in . In minimal AdS supergravity, the boundary conditions such that the
supercharges exist as Hamiltonian generators of supersymmetry transformations
are unique within the usual framework in which the boundary metric is fixed. In
extended AdS supergravity, or more generally in the presence
of chiral matter superfields, we find that there exist many boundary conditions
preserving supersymmetry for which corresponding generators
exist. These choices are shown to correspond to a choice of certain arbitrary
boundary ``superpotentials,'' for suitably defined ``boundary superfields.'' We
also derive corresponding formulae for the conserved bosonic charges, such as
energy, in those theories, and we argue that energy is always positive, for any
supersymmetry-preserving boundary conditions. We finally comment on the
relevance and interpretation of our results within the AdS-CFT correspondence.Comment: 45 pages, Latex, no figures, v2: extended discussion of positive
energy theorem and explicit form of fermionic generators, references adde
Ordered Phase of the Dipolar Spin Ice under [110]-Magnetic Fields
We find that the true ground state of the dipolar spin ice system under
[110]-magnetic fields is the ``Q=X'' structure, which is consistent with both
experiments and Monte Carlo simulations. We then perform a Monte Carlo
simulation to confirm that there exists a first order phase transition under
the [110]-field. In particular this result indicates the existence of the first
order phase transition to the ``Q=X'' phase in the field above 0.35 T for
Dy2Ti2O7. We also show the magnetic field-temperature phase diagram to
summarize the ordered states of this system.Comment: 4 pages, 5 figures, in RevTex4, submitted to J. Phys. Soc. Jp
New stability results for Einstein scalar gravity
We consider asymptotically anti de Sitter gravity coupled to a scalar field
with mass slightly above the Breitenlohner-Freedman bound. This theory admits a
large class of consistent boundary conditions characterized by an arbitrary
function . An important open question is to determine which admit stable
ground states. It has previously been shown that the total energy is bounded
from below if is bounded from below and the bulk scalar potential
admits a suitable superpotential. We extend this result and show that the
energy remains bounded even in some cases where can become arbitrarily
negative. As one application, this leads to the possibility that in
gauge/gravity duality, one can add a double trace operator with negative
coefficient to the dual field theory and still have a stable vacuum
Local moment formation in zinc doped cuprates
We suggest that when zinc is substituted for copper in the copper oxide
planes of high superconductors, it does not necessarily have a valency
of 2+. Rather, the valency of a zinc impurity should be determined by its
surrounding medium. In order to study this hypothesis, we examine the effect of
static impurities inducing diagonal disorder within a one band Hubbard model
coupled to a localised state. We use this model to discuss the physics of zinc
doping in the cuprates. Specifically, we discuss the formation of local moments
near impurity sites and the modification of the transverse spin susceptibility
in the antiferromagnetic state.Comment: 7 pages RevTex, includes 4 figure
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