1,677 research outputs found
Modulated Instability in Five-Dimensional U(1) Charged AdS Black Hole with R**2-term
We study the effect of R**2 term to the modulated instability in the U(1)
charged black hole in five-dimensional Anti-de Sitter space-time. We consider
the first-order corrections of R**2 term to the background and the linear order
perturbations in the equations of motion. From the analysis, we clarify the
effect of R**2 term in the modulated instability, and conclude that
fluctuations are stable in the whole bulk in the range of values the
coefficient of R**2 term can take.Comment: 19 pages, 1 figures; (v4) Published version in JHE
The Weak Gravity Conjecture and the Viscosity Bound with Six-Derivative Corrections
The weak gravity conjecture and the shear viscosity to entropy density bound
place constraints on low energy effective field theories that may help to
distinguish which theories can be UV completed. Recently, there have been
suggestions of a possible correlation between the two constraints. In some
interesting cases, the behavior was precisely such that the conjectures were
mutually exclusive. Motivated by these works, we study the mass to charge and
shear viscosity to entropy density ratios for charged AdS5 black branes, which
are holographically dual to four-dimensional CFTs at finite temperature. We
study a family of four-derivative and six-derivative perturbative corrections
to these backgrounds. We identify the region in parameter space where the two
constraints are satisfied and in particular find that the inclusion of the
next-to-leading perturbative correction introduces wider possibilities for the
satisfaction of both constraints.Comment: 24 pages, 6 figures, v2: published version, refs added, minor
clarificatio
The Rich Structure of Gauss-Bonnet Holographic Superconductors
We study fully backreacting, Gauss-Bonnet (GB) holographic superconductors in
5 bulk spacetime dimensions. We explore the system's dependence on the scalar
mass for both positive and negative GB coupling, . We find that when
the mass approaches the Breitenlohner-Freedman (BF) bound and
the effect of backreaction is to increase the
critical temperature, , of the system: the opposite of its effect in the
rest of parameter space. We also find that reducing below zero
increases and that the effect of backreaction is diminished. We study the
zero temperature limit, proving that this system does not permit regular
solutions for a non-trivial, tachyonic scalar field and constrain possible
solutions for fields with positive masses. We investigate singular, zero
temperature solutions in the Einstein limit but find them to be incompatible
with the concept of GB gravity being a perturbative expansion of Einstein
gravity. We study the conductivity of the system, finding that the inclusion of
backreaction hinders the development of poles in the conductivity that are
associated with quasi-normal modes approaching the real axis from elsewhere in
the complex plane.Comment: 26 pages, 11 figures, V3, Added discussion of non-tachyonic scalars,
alterations to figures and tex
Causality in AdS/CFT and Lovelock theory
We explore the constraints imposed on higher curvature corrections of the
Lovelock type due to causality restrictions in the boundary of asymptotically
AdS space-time. In the framework of AdS/CFT, this is related to positivity of
the energy constraints that arise in conformal collider physics. We present
explicit analytic results that fully address these issues for cubic Lovelock
gravity in arbitrary dimensions and give the formal analytic results that
comprehend general Lovelock theory. The computations can be performed in two
ways, both by considering a thermal setup in a black hole background and by
studying the scattering of gravitons with a shock wave in AdS. We show that
both computations coincide in Lovelock theory. The different helicities, as
expected, provide the boundaries defining the region of allowed couplings. We
generalize these results to arbitrary higher dimensions and discuss their
consequences on the shear viscosity to energy density ratio of CFT plasmas, the
possible existence of Boulware-Deser instabilities in Lovelock theory and the
extent to which the AdS/CFT correspondence might be valid for arbitrary
dimensions.Comment: 35 pages, 20 figures; v2: minor amendments and clarifications
include
Holographic Lovelock Gravities and Black Holes
We study holographic implications of Lovelock gravities in AdS spacetimes.
For a generic Lovelock gravity in arbitrary spacetime dimensions we formulate
the existence condition for asymptotically AdS black holes. We consider small
fluctuations around these black holes and determine the constraint on Lovelock
parameters by demanding causality of the boundary theory. For the case of cubic
Lovelock gravity in seven spacetime dimensions we compute the holographic Weyl
anomaly and determine the three point functions of the stress energy tensor in
the boundary CFT. Remarkably, these correlators happen to satisfy the same
relation as the one imposed by supersymmetry. We then compute the energy flux;
requiring it to be positive is shown to be completely equivalent to requiring
causality of the finite temperature CFT dual to the black hole. These
constraints are not stringent enough to place any positive lower bound on the
value of viscosity. Finally, we conjecture an expression for the energy flux
valid for any Lovelock theory in arbitrary dimensions.Comment: 31 pages, 1 figure, harvmac, references added, calculation of
viscosity/entropy ratio include
Supersymmetric Intersecting Branes on the Waves
We construct a general family of supersymmetric solutions in time- and
space-dependent wave backgrounds in general supergravity theories describing
single and intersecting p-branes embedded into time-dependent dilaton-gravity
plane waves of an arbitrary (isotropic) profile, with the brane world-volume
aligned parallel to the propagation direction of the wave. We discuss how many
degrees of freedom we have in the solutions. We also propose that these
solutions can be used to describe higher-dimensional time-dependent "black
holes", and discuss their property briefly.Comment: 12 pages, LaTe
Towards a large-scale quantum simulator on diamond surface at room temperature
Strongly-correlated quantum many-body systems exhibits a variety of exotic
phases with long-range quantum correlations, such as spin liquids and
supersolids. Despite the rapid increase in computational power of modern
computers, the numerical simulation of these complex systems becomes
intractable even for a few dozens of particles. Feynman's idea of quantum
simulators offers an innovative way to bypass this computational barrier.
However, the proposed realizations of such devices either require very low
temperatures (ultracold gases in optical lattices, trapped ions,
superconducting devices) and considerable technological effort, or are
extremely hard to scale in practice (NMR, linear optics). In this work, we
propose a new architecture for a scalable quantum simulator that can operate at
room temperature. It consists of strongly-interacting nuclear spins attached to
the diamond surface by its direct chemical treatment, or by means of a
functionalized graphene sheet. The initialization, control and read-out of this
quantum simulator can be accomplished with nitrogen-vacancy centers implanted
in diamond. The system can be engineered to simulate a wide variety of
interesting strongly-correlated models with long-range dipole-dipole
interactions. Due to the superior coherence time of nuclear spins and
nitrogen-vacancy centers in diamond, our proposal offers new opportunities
towards large-scale quantum simulation at room temperatures
30 inch Roll-Based Production of High-Quality Graphene Films for Flexible Transparent Electrodes
We report that 30-inch scale multiple roll-to-roll transfer and wet chemical
doping considerably enhance the electrical properties of the graphene films
grown on roll-type Cu substrates by chemical vapor deposition. The resulting
graphene films shows a sheet resistance as low as ~30 Ohm/sq at ~90 %
transparency which is superior to commercial transparent electrodes such as
indium tin oxides (ITO). The monolayer of graphene shows sheet resistances as
low as ~125 Ohm/sq with 97.4% optical transmittance and half-integer quantum
Hall effect, indicating the high-quality of these graphene films. As a
practical application, we also fabricated a touch screen panel device based on
the graphene transparent electrodes, showing extraordinary mechanical and
electrical performances
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