224 research outputs found
Holographic Kondo Model in Various Dimensions
We study the addition of localised impurities to U(N) Supersymmetric
Yang-Mills theories in (p+1)-dimensions by using the gauge/gravity
correspondence. From the gravity side, the impurities are introduced by
considering probe D(8-p)-branes extendingalong the time and radial directions
and wrapping an (7-p)-dimensional submanifold of the internal (8-p)-sphere, so
that the degrees of freedom are point-like from the gauge theory perspective.
We analyse both the configuration in which the branes generate straight flux
tubes -corresponding to actual single impurities - and the one in which
connected flux tubes are created- corresponding to dimers. We discuss the
thermodynamics of both the configurations and the related phase transition. In
particular, the specific heat of the straight flux-tube configuration is
negative for p<3, while it is never the case for the connected one. We study
the stability of the system by looking at the impurity fluctuations. Finally,
we characterise the theory by computing one- and two-point correlators of the
gauge theory operators dual to the impurity fluctuations. Because of the
underlying generalised conformal structure, such correlators can be expressed
in terms of an effective coupling constant (which runs because of its
dimensionality) and a generalised conformal dimension.Comment: 56 pages, 3 figures; v2: typos correcte
A Non-relativistic Logarithmic Conformal Field Theory from a Holographic Point of View
We study a fourth-order derivative scalar field configuration in a fixed
Lifshitz background. Using an auxiliary field we rewrite the equations of
motion as two coupled second order equations. We specialize to the limit that
the mass of the scalar field degenerates with that of the auxiliary field and
show that logarithmic modes appear. Using non-relativistic holographic methods
we calculate the two-point correlation functions of the boundary operators in
this limit and find evidence for a non-relativistic logarithmic conformal field
theory at the boundary.Comment: 17 pages, v2 : refs. adde
Vacuum Ambiguity in de Sitter Space at Strong Coupling
It is well known that in the weak coupling regime, quantum field theories in
de Sitter space do not have a unique vacuum, but a class of vacua parametrized
by a complex parameter , i.e., the so-called -vacua. In this
article, using gauge/gravity duality, we calculate the symmetric two-point
function of strongly coupled supersymmetric Yang-Mills theory on
. We find that there is a class of de Sitter invariant vacua,
parametrized by a set of complex parameters .Comment: 17 pages in JHEP style, references adde
Probes on D3-D7 Quark-Gluon Plasmas
We study the holographic dual model of quenched flavors immersed in a
quark-gluon plasma with massless dynamical quarks in the Veneziano limit. This
is modeled by embedding a probe D7 brane in a background where the backreaction
of massless D7 branes has been taken into account. The background, and hence
the effects, are perturbative in the Veneziano parameter N_f/N_c, therefore
giving small shifts of all magnitudes like the constituent mass, the quark
condensate, and several transport coefficients. We provide qualitative results
for the effect of flavor degrees of freedom on the probes. For example, the
meson melting temperature is enhanced, while the screening length is
diminished. The drag force is also enhanced.Comment: 31 pages, 17 figure
Zero Sound in Strange Metallic Holography
One way to model the strange metal phase of certain materials is via a
holographic description in terms of probe D-branes in a Lifshitz spacetime,
characterised by a dynamical exponent z. The background geometry is dual to a
strongly-interacting quantum critical theory while the probe D-branes are dual
to a finite density of charge carriers that can exhibit the characteristic
properties of strange metals. We compute holographically the low-frequency and
low-momentum form of the charge density and current retarded Green's functions
in these systems for massless charge carriers. The results reveal a
quasi-particle excitation when z<2, which in analogy with Landau Fermi liquids
we call zero sound. The real part of the dispersion relation depends on
momentum k linearly, while the imaginary part goes as k^2/z. When z is greater
than or equal to 2 the zero sound is not a well-defined quasi-particle. We also
compute the frequency-dependent conductivity in arbitrary spacetime dimensions.
Using that as a measure of the charge current spectral function, we find that
the zero sound appears only when the spectral function consists of a single
delta function at zero frequency.Comment: 20 pages, v2 minor corrections, extended discussion in sections 5 and
6, added one footnote and four references, version published in JHE
Fermion correlators in non-abelian holographic superconductors
We consider fermion correlators in non-abelian holographic superconductors.
The spectral function of the fermions exhibits several interesting features
such as support in displaced Dirac cones and an asymmetric distribution of
normal modes. These features are compared to similar ones observed in angle
resolved photoemission experiments on high T_c superconductors. Along the way
we elucidate some properties of p-wave superconductors in AdS_4 and discuss the
construction of SO(4) superconductors.Comment: 49 pages, 11 figure
Kerr/CFT, dipole theories and nonrelativistic CFTs
We study solutions of type IIB supergravity which are SL(2,R) x SU(2) x
U(1)^2 invariant deformations of AdS_3 x S^3 x K3 and take the form of products
of self-dual spacelike warped AdS_3 and a deformed three-sphere. One of these
backgrounds has been recently argued to be relevant for a derivation of
Kerr/CFT from string theory, whereas the remaining ones are holographic duals
of two-dimensional dipole theories and their S-duals. We show that each of
these backgrounds is holographically dual to a deformation of the DLCQ of the
D1-D5 CFT by a specific supersymmetric (1,2) operator, which we write down
explicitly in terms of twist operators at the free orbifold point. The
deforming operator is argued to be exactly marginal with respect to the
zero-dimensional nonrelativistic conformal (or Schroedinger) group - which is
simply SL(2,R)_L x U(1)_R. Moreover, in the supergravity limit of large N and
strong coupling, no other single-trace operators are turned on. We thus propose
that the field theory duals to the backgrounds of interest are nonrelativistic
CFTs defined by adding the single Schroedinger-invariant (1,2) operator
mentioned above to the original CFT action. Our analysis indicates that the
rotating extremal black holes we study are best thought of as finite
right-moving temperature (non-supersymmetric) states in the above-defined
supersymmetric nonrelativistic CFT and hints towards a more general connection
between Kerr/CFT and two-dimensional non-relativistic CFTs.Comment: 48+8 pages, 4 figures; minor corrections and references adde
The effects of melatonin versus placebo on delirium in hip fracture patients: study protocol of a randomised, placebo-controlled, double blind trial
<p>Abstract</p> <p>Background</p> <p>With an ageing population, older persons become a larger part of the hospital population. The incidence of delirium is high in this group, and experiencing delirium has major short- and long-term sequelae, which makes prevention crucial. During delirium, a disruption of the sleep-wake cycle is frequently observed. Melatonin plays an important role in the regulation of the sleep-wake cycle, so this raised the hypothesis that alterations in the metabolism of melatonin might play an important role in the development of delirium. The aim of this article is to describe the design of a randomised, placebo controlled double-blind trial that is currently in progress and that investigates the effects of melatonin versus placebo on delirium in older, postoperative hip fracture patients.</p> <p>Methods/Design</p> <p>Acutely hospitalised patients aged 65 years or older admitted for surgical repair of hip fracture are randomised (n = 452) into a treatment or placebo group. Prophylactic treatment consists of orally administered melatonin (3 mg) at 21:00 h on five consecutive days. The primary outcome is the occurrence of delirium, to be diagnosed according to the Confusion Assessment Method, within eight days after start of the study medication. Secondary outcomes are delirium severity, measured by the Delirium Rating Scale; duration of delirium; differences in subtypes of delirium; differences in total length of hospital stay; total dose of antipsychotics and/or benzodiazepine use during delirium; and in-hospital complications. In the twelve-month follow up visit, cognitive function is measured by a Mini-Mental state examination and the Informant Questionnaire on Cognitive Decline in the Elderly. Functional status is assessed with the Katz ADL index score (patient and family version) and grip strength measurement. The outcomes of these assessments are compared to the outcomes that were obtained during admission.</p> <p>Discussion</p> <p>The proposed study will contribute to our knowledge because studies on the prophylactic treatment of delirium with long term follow up remain scarce. The results may lead to a prophylactic treatment for frail older persons at high risk for delirium that is safe, effective, and easily implementable in daily practice.</p> <p>Trial registration</p> <p>Dutch Clinical Trial Registry: <a href="http://www.trialregister.nl/trialreg/admin/rctview.asp?TC=1576">NTR1576</a></p
Collective Excitations of Holographic Quantum Liquids in a Magnetic Field
We use holography to study N=4 supersymmetric SU(Nc) Yang-Mills theory in the
large-Nc and large-coupling limits coupled to a number Nf << Nc of
(n+1)-dimensional massless supersymmetric hypermultiplets in the Nc
representation of SU(Nc), with n=2,3. We introduce a temperature T, a baryon
number chemical potential mu, and a baryon number magnetic field B, and work in
a regime with mu >> T,\sqrt{B}. We study the collective excitations of these
holographic quantum liquids by computing the poles in the retarded Green's
function of the baryon number charge density operator and the associated peaks
in the spectral function. We focus on the evolution of the collective
excitations as we increase the frequency relative to T, i.e. the
hydrodynamic/collisionless crossover. We find that for all B, at low
frequencies the tallest peak in the spectral function is associated with
hydrodynamic charge diffusion. At high frequencies the tallest peak is
associated with a sound mode similar to the zero sound mode in the
collisionless regime of a Landau Fermi liquid. The sound mode has a gap
proportional to B, and as a result for intermediate frequencies and for B
sufficiently large compared to T the spectral function is strongly suppressed.
We find that the hydrodynamic/collisionless crossover occurs at a frequency
that is approximately B-independent.Comment: 45 pages, 8 png and 47 pdf images in 22 figure
Irrelevant deformations and the holographic Callan-Symanzik equation
We discuss the systematics of obtaining the Callan-Symanzik equation within
the framework of the gauge/gravity dualities. We present a completely general
formula which in particular takes into account the new holographic
renormalization results of arXiv:1102.2239. Non-trivial beta functions are
obtained from new logarithmic terms in the radial expansion of the fields. The
appearance of multi-trace counterterms is also discussed in detail and we show
that mixing between single- and multi-trace operators leads to very specific
non-linearities in the Callan-Symanzik equation. Additionally, we compute the
conformal anomaly for a scalar three-point function in a CFT.Comment: 40 page
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