518 research outputs found
DBI equations and holographic DC conductivity
We provide a simple method for writing the Dirac-Born-Infeld (DBI) equations
of a Dp-brane in an arbitrary static background whose metric depends only on
the holographic radial coordinate z. Using this method we revisit the
Karch-O'Bannon's procedure to calculate the DC conductivity in the presence of
constant electric and magnetic fields for backgrounds where the boundary is
four or three dimensional and satisfies homogeneity and isotropy. We find a
frame-independent expression for the DC conductivity tensor. For particular
backgrounds we recover previous results on holographic metals and strange
metals.Comment: 1+21 pages; v2 : references added, minor typos correcte
High-field irreversible moment reorientation in the antiferromagnet FeTe
Magnetization measurements have been performed on single-crystalline
FeTe in pulsed magnetic fields up to 53 T
and temperatures from 4.2 to 65 K. At K, a non-reversible reorientation
of the antiferromagnetic moments is observed at T as the pulsed
field is on the rise. No anomaly is observed at during the fall of the
field and, as long as the temperature is unchanged, during both rises and falls
of additional field pulses. The transition at is reactivated if the
sample is warmed up above the N\'{e}el temperature K and cooled
down again. The magnetic field-temperature phase diagram of FeTe in
is also investigated. We present the temperature
dependence of , as well as that of the antiferromagnetic-to-paramagnetic
borderline in temperatures above 40 K.Comment: 5 pages, 4 figure
Virtual Compton Scattering off a Spinless Target in AdS/QCD
We study the doubly virtual Compton scattering off a spinless target
within the Anti-de Sitter(AdS)/QCD formalism. We find
that the general structure allowed by the Lorentz invariance and gauge
invariance of the Compton amplitude is not easily reproduced with the standard
recipes of the AdS/QCD correspondence. In the soft-photon regime, where the
semi-classical approximation is supposed to apply best, we show that the
measurements of the electric and magnetic polarizabilities of a target like the
charged pion in real Compton scattering, can already serve as stringent tests.Comment: 21 pages, version to be published in JHEP
On a Holographic Model for Confinement/Deconfinement
We study the thermodynamics of the hard wall model, which consists in the
introduction of an infrared cut-off in asymptotically AdS spaces. This is a toy
model for confining backgrounds in the context of the gauge/gravity
correspondence. We use holographic renormalization and reproduce the existence
of a Hawking Page phase transition recently discussed by Herzog. We also show
that the entropy jumps from to , which reinforces the interpretation
of this transition as the gravity dual of confinement/deconfinement. We also
show that similar results hold for the phenomenologically motivated soft wall
model, underlining the potential universality of our analysis.Comment: 14 pages. V2: We included a new section discussing the soft wall
model and new references. V3: We clarified some points and updated the
references. Results unchanged. Version published in PR
Deep Inelastic Scattering in Conformal QCD
We consider the Regge limit of a CFT correlation function of two vector and
two scalar operators, as appropriate to study small-x deep inelastic scattering
in N=4 SYM or in QCD assuming approximate conformal symmetry. After clarifying
the nature of the Regge limit for a CFT correlator, we use its conformal
partial wave expansion to obtain an impact parameter representation encoding
the exchange of a spin j Reggeon for any value of the coupling constant. The
CFT impact parameter space is the three-dimensional hyperbolic space H3, which
is the impact parameter space for high energy scattering in the dual AdS space.
We determine the small-x structure functions associated to the exchange of a
Reggeon. We discuss unitarization from the point of view of scattering in AdS
and comment on the validity of the eikonal approximation.
We then focus on the weak coupling limit of the theory where the amplitude is
dominated by the exchange of the BFKL pomeron. Conformal invariance fixes the
form of the vector impact factor and its decomposition in transverse spin 0 and
spin 2 components. Our formalism reproduces exactly the general results predict
by the Regge theory, both for a scalar target and for gamma*-gamma* scattering.
We compute current impact factors for the specific examples of N=4 SYM and QCD,
obtaining very simple results. In the case of the R-current of N=4 SYM, we show
that the transverse spin 2 component vanishes. We conjecture that the impact
factors of all chiral primary operators of N=4 SYM only have components with 0
transverse spin.Comment: 44+16 pages, 7 figures. Some correction
Experimental study of a liquid Xenon PET prototype module
A detector using liquid Xenon in the scintillation mode is studied for
Positron Emission Tomography (PET). The specific design aims at taking full
advantage of the liquid Xenon properties. It does feature a promising
insensitive to any parallax effect. This work reports on the performances of
the first LXe prototype module, equipped with a position sensitive PMT
operating in the VUV range (178 nm).Comment: Proc. of the 7th International Workshops on Radiation Imaging
Detectors (IWORID-7), Grenoble, France 4-7 July 200
Polarized DIS in N=4 SYM: Where is spin at strong coupling?
Using the AdS/CFT correspondence, we calculate the polarized structure
functions in strongly coupled N=4 supersymmetric Yang-Mills theory deformed in
the infrared. We find that the flavor singlet contribution to the g_1 structure
function is vanishingly small, while the flavor non-singlet contribution shows
the Regge behavior at small-x with an intercept slightly less than 1. We
explicitly check that the latter satisfies the moment sum rule. We discuss the
`spin crisis' problem and suggest that at strong coupling the spin of a hadron
entirely comes from the orbital angular momentum.Comment: 24 page
Interplay of magnetism, Fermi surface reconstructions, and hidden-order in the heavy-fermion material URuSi
URuSi is surely one of the most mysterious of the heavy-fermion
compounds. Despite more than twenty years of experimental and theoretical
works, the order parameter of the transition at K is still
unknown. The state below remains called "hidden-order phase" and the
stakes are still to identify the energy scales driving the system to this
phase. We present new magnetoresistivity and magnetization measurements
performed on very-high-quality single crystals in pulsed magnetic fields up to
60 T. We show that the transition to the hidden-order state in URuSi is
initially driven by a high-temperature crossover at around 40-50 K, which is a
fingerprint of inter-site electronic correlations. In a magnetic field
applied along the easy-axis , the vanishing of this
high-temperature scale precedes the polarization of the magnetic moments, as
well as it drives the destabilization of the hidden-order phase. Strongly
impurity-dependent magnetoresistivity confirms that the Fermi surface is
reconstructed below and is strongly modified in a high magnetic field
applied along , i.e. at a sufficiently-high magnetic polarization.
The possibility of a sharp crossover in the hidden-order state controlled by a
field-induced change of the Fermi surface is pointed out.Comment: 10 pages, 6 figures, accepted in Physical Review
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