3,272 research outputs found
Sum Rules from an Extra Dimension
Using the gravity side of the AdS/CFT correspondence, we investigate the
analytic properties of thermal retarded Green's functions for scalars,
conserved currents, the stress tensor, and massless fermions. We provide some
results concerning their large and small frequency behavior and their pole
structure. From these results, it is straightforward to prove the validity of
various sum rules on the field theory side of the duality. We introduce a novel
contraction mapping we use to study the large frequency behavior of the Green's
functions.Comment: v2: 23 pages (plus appendix), revised presentation, discussion of
branch cuts moved to appendix, and some minor changes; v1: 24 pages (plus
appendix
Holographic Symmetry-Breaking Phases in AdS/CFT
In this note we study the symmetry-breaking phases of 3D gravity coupled to
matter. In particular, we consider black holes with scalar hair as a model of
symmetry-breaking phases of a strongly coupled 1+1 dimensional CFT. In the case
of a discrete symmetry, we show that these theories admit metastable phases of
broken symmetry and study the thermodynamics of these phases. We also
demonstrate that the 3D Einstein-Maxwell theory shows continuous symmetry
breaking at low temperature. The apparent contradiction with the
Coleman-Mermin-Wagner theorem is discussed.Comment: 15 pages, 7 figur
The Subleading Term of the Strong Coupling Expansion of the Heavy-Quark Potential in a Super Yang-Mills Plasma
Applying the AdS/CFT correspondence, the expansion of the heavy-quark
potential of the supersymmetric Yang-Mills theory at large is
carried out to the sub-leading term in the large 't Hooft coupling at nonzero
temperatures. The strong coupling corresponds to the semi-classical expansion
of the string-sigma model, the gravity dual of the Wilson loop operator, with
the sub-leading term expressed in terms of functional determinants of
fluctuations. The contributions of these determinants are evaluated
numerically.Comment: 17 pages in JHEP3, typos fixed, updated version to be published in
JHE
Non-Equilibrium Field Dynamics of an Honest Holographic Superconductor
Most holographic models of superconducting systems neglect the effects of
dynamical boundary gauge fields during the process of spontaneous
symmetry-breaking. Usually a global symmetry gets broken. This yields a
superfluid, which then is gauged "weakly" afterwards. In this work we build
(and probe the dynamics of) a holographic model in which a local boundary
symmetry is spontaneously broken instead. We compute two-point functions of
dynamical non-Abelian gauge fields in the normal and in the broken phase, and
find non-trivial gapless modes. Our AdS3 gravity dual realizes a p-wave
superconductor in (1+1) dimensions. The ground state of this model also breaks
(1+1)-dimensional parity spontaneously, while the Hamiltonian is
parity-invariant. We discuss possible implications of our results for a wider
class of holographic liquids.Comment: 32 pages, 12 figures; v3: string theory derivation of setup added
(section 3.1), improved presentation, version accepted by JHEP; v2: paragraph
added to discussion, figure added, references added, typos correcte
Boundary Conditions and Unitarity: the Maxwell-Chern-Simons System in AdS_3/CFT_2
We consider the holography of the Abelian Maxwell-Chern-Simons (MCS) system
in Lorentzian three-dimensional asymptotically-AdS spacetimes, and discuss a
broad class of boundary conditions consistent with conservation of the
symplectic structure. As is well-known, the MCS theory contains a massive
sector dual to a vector operator in the boundary theory, and a topological
sector consisting of flat connections dual to U(1) chiral currents; the
boundary conditions we examine include double-trace deformations in these two
sectors, as well as a class of boundary conditions that mix the vector
operators with the chiral currents. We carefully study the symplectic product
of bulk modes and show that almost all such boundary conditions induce
instabilities and/or ghost excitations, consistent with violations of unitarity
bounds in the dual theory.Comment: 50+1 pages, 6 figures, PDFLaTeX; v2: added references, corrected
typo
The Spin of Holographic Electrons at Nonzero Density and Temperature
We study the Green's function of a gauge invariant fermionic operator in a
strongly coupled field theory at nonzero temperature and density using a dual
gravity description. The gravity model contains a charged black hole in four
dimensional anti-de Sitter space and probe charged fermions. In particular, we
consider the effects of the spin of these probe fermions on the properties of
the Green's function. There exists a spin-orbit coupling between the spin of an
electron and the electric field of a Reissner-Nordstrom black hole. On the
field theory side, this coupling leads to a Rashba like dispersion relation. We
also study the effects of spin on the damping term in the dispersion relation
by considering how the spin affects the placement of the fermionic quasinormal
modes in the complex frequency plane in a WKB limit. An appendix contains some
exact solutions of the Dirac equation in terms of Heun polynomials.Comment: 27 pages, 11 figures; v2: minor changes, published versio
Neuroimaging Evidence of Major Morpho-Anatomical and Functional Abnormalities in the BTBR T+TF/J Mouse Model of Autism
BTBR T+tf/J (BTBR) mice display prominent behavioural deficits analogous to the defining symptoms of autism, a feature that has prompted a widespread use of the model in preclinical autism research. Because neuro-behavioural traits are described with respect to reference populations, multiple investigators have examined and described the behaviour of BTBR mice against that exhibited by C57BL/6J (B6), a mouse line characterised by high sociability and low self-grooming. In an attempt to probe the translational relevance of this comparison for autism research, we used Magnetic Resonance Imaging (MRI) to map in both strain multiple morpho-anatomical and functional neuroimaging readouts that have been extensively used in patient populations. Diffusion tensor tractography confirmed previous reports of callosal agenesis and lack of hippocampal commissure in BTBR mice, and revealed a concomitant rostro-caudal reorganisation of major cortical white matter bundles. Intact inter-hemispheric tracts were found in the anterior commissure, ventro-medial thalamus, and in a strain-specific white matter formation located above the third ventricle. BTBR also exhibited decreased fronto-cortical, occipital and thalamic gray matter volume and widespread reductions in cortical thickness with respect to control B6 mice. Foci of increased gray matter volume and thickness were observed in the medial prefrontal and insular cortex. Mapping of resting-state brain activity using cerebral blood volume weighted fMRI revealed reduced cortico-thalamic function together with foci of increased activity in the hypothalamus and dorsal hippocampus of BTBR mice. Collectively, our results show pronounced functional and structural abnormalities in the brain of BTBR mice with respect to control B6 mice. The large and widespread white and gray matter abnormalities observed do not appear to be representative of the neuroanatomical alterations typically observed in autistic patients. The presence of reduced fronto-cortical metabolism is of potential translational relevance, as this feature recapitulates previously-reported clinical observations
High Speed Cycle-Approximate Simulation of Embedded Cache-Incoherent and Coherent Chip-Multiprocessors
Planar Hall effect from the surface of topological insulators
A prominent feature of topological insulators (TIs) is the surface states comprising of spin-nondegenerate massless Dirac fermions. Recent technical advances have made it possible to address the surface transport properties of TI thin films by tuning the Fermi levels of both top and bottom surfaces. Here we report our discovery of a novel planar Hall effect (PHE) from the TI surface, which results from a hitherto-unknown resistivity anisotropy induced by an in-plane magnetic field. This effect is observed in dual-gated devices of bulk-insulating Bi2−x Sb x Te3 thin films, where the field-induced anisotropy presents a strong dependence on the gate voltage with a characteristic two-peak structure near the Dirac point. The origin of PHE is the peculiar time-reversal-breaking effect of an in-plane magnetic field, which anisotropically lifts the protection of surface Dirac fermions from backscattering. The observed PHE provides a useful tool to analyze and manipulate the topological protection of the TI surface
Bid can mediate a pro-apoptotic response to etoposide and ionizing radiation without cleavage in its unstructured loop and in the absence of p53
BH3-only protein Bid is a key player in death receptor-induced apoptosis, because it provides the link with the mitochondrial route for caspase activation. In this pathway, Bid is activated upon cleavage by caspase-8. Its BH3 domain-containing carboxy-terminal fragment subsequently provokes mitochondrial outer membrane permeabilization by Bak/Bax activation. Bid has also been implicated in the apoptotic response to ionizing radiation (IR) and the topoisomerase inhibitor etoposide, anti-cancer regimens that cause double-strand (ds)DNA breaks. We confirm the existence of this pathway and show that it is p53-independent. However, the degree of Bid participation in the apoptotic response to dsDNA breaks depends on the nature of cell transformation. We used Bid-deficient mouse embryonic fibroblast (MEF) lines that were reconstituted with Bid to control the cellular background and demonstrated that the Bid-dependent apoptotic pathway induced by IR and etoposide operates in MEFs that are transformed by SV40, but is not evident in E1A/Ras-transformed MEFs. The Bid-dependent apoptotic response in p53-deficient SV40-transformed MEFs contributed to clonogenic execution of the cells, implying relevance for treatment outcome. In these cells, Bid acted in a conventional manner in that it required its BH3 domain to mediate apoptosis in response to IR and etoposide, and triggered apoptotic execution by indirect activation of Bak/Bax, mitochondrial permeabilization and caspase-9 activation. However, the mechanism of Bid activation was unconventional, because elimination of all known or suspected cleavage sites for caspases or other proteolytic enzymes and even complete elimination of its unstructured cleavage loop left Bid's pro-apoptotic role in the response to IR and etoposide unaffected
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