68 research outputs found
Inverse magnetic catalysis in dense holographic matter
We study the chiral phase transition in a magnetic field at finite
temperature and chemical potential within the Sakai-Sugimoto model, a
holographic top-down approach to (large-N_c) QCD. We consider the limit of a
small separation of the flavor D8-branes, which corresponds to a dual field
theory comparable to a Nambu-Jona Lasinio (NJL) model. Mapping out the surface
of the chiral phase transition in the parameter space of magnetic field
strength, quark chemical potential, and temperature, we find that for small
temperatures the addition of a magnetic field decreases the critical chemical
potential for chiral symmetry restoration - in contrast to the case of
vanishing chemical potential where, in accordance with the familiar phenomenon
of magnetic catalysis, the magnetic field favors the chirally broken phase.
This "inverse magnetic catalysis" (IMC) appears to be associated with a
previously found magnetic phase transition within the chirally symmetric phase
that shows an intriguing similarity to a transition into the lowest Landau
level. We estimate IMC to persist up to 10^{19} G at low temperatures.Comment: 42 pages, 11 figures, v3: extended discussion; new appendix D;
references added; version to appear in JHE
Towards Unquenched Holographic Magnetic Catalysis
We propose a string dual to the SU(Nc) N=4 SYM coupled to Nf massless
fundamental flavors in an external magnetic field. The flavors are introduced
by homogeneously smeared Nf D7-branes and the external magnetic field via a
non-trivial Kalb-Ramond B-field. Our solution is perturbative in a parameter
that counts the number of internal flavor loops. In the limit of vanishing
B-field the background reduces to the supersymmetric one obtained in
hep-th/0612118. We introduce an additional probe D7--brane and in the
supersymmetric limit of vanishing B-field perform a holographic renormalization
of its "on-shell" action. We consider also non-supersymmetric probes with fixed
worldvolume gauge field corresponding to a magnetic field coupled only to the
fundamental fields of the probe brane. We study the influence of the
backreacted flavors on the effect of dynamical mass generation. Qualitatively
the physical picture remains unchanged. In the next step we consider the case
when the magnetic field couples to both the backreacted and the probe
fundamental degrees of freedom. At sufficiently strong magnetic field the meson
spectrum signals an instability of the probe D7-brane, which we interpret as
reflecting an instability of the supergravity background.Comment: 33 pages, 5 figures, v2: minor corrections and references adde
Magnetic Catalysis and Quantum Hall Ferromagnetism in Weakly Coupled Graphene
We study the realization in a model of graphene of the phenomenon whereby the
tendency of gauge-field mediated interactions to break chiral symmetry
spontaneously is greatly enhanced in an external magnetic field. We prove that,
in the weak coupling limit, and where the electron-electron interaction
satisfies certain mild conditions, the ground state of charge neutral graphene
in an external magnetic field is a quantum Hall ferromagnet which spontaneously
breaks the emergent U(4) symmetry to U(2)XU(2).
We argue that, due to a residual CP symmetry, the quantum Hall ferromagnet
order parameter is given exactly by the leading order in perturbation theory.
On the other hand, the chiral condensate which is the order parameter for
chiral symmetry breaking generically obtains contributions at all orders. We
compute the leading correction to the chiral condensate. We argue that the
ensuing fermion spectrum resembles that of massive fermions with a vanishing
U(4)-valued chemical potential. We discuss the realization of parity and charge
conjugation symmetries and argue that, in the context of our model, the charge
neutral quantum Hall state in graphene is a bulk insulator, with vanishing
longitudinal conductivity due to a charge gap and Hall conductivity vanishing
due to a residual discrete particle-hole symmetry.Comment: 35 page
Chiral Symmetry Breaking and External Fields in the Kuperstein-Sonnenschein Model
A novel holographic model of chiral symmetry breaking has been proposed by
Kuperstein and Sonnenschein by embedding non-supersymmetric probe D7 and
anti-D7 branes in the Klebanov-Witten background. We study the dynamics of the
probe flavours in this model in the presence of finite temperature and a
constant electromagnetic field. In keeping with the weakly coupled field theory
intuition, we find the magnetic field promotes spontaneous breaking of chiral
symmetry whereas the electric field restores it. The former effect is
universally known as the "magnetic catalysis" in chiral symmetry breaking. In
the presence of an electric field such a condensation is inhibited and a
current flows. Thus we are faced with a steady-state situation rather than a
system in equilibrium. We conjecture a definition of thermodynamic free energy
for this steady-state phase and using this proposal we study the detailed phase
structure when both electric and magnetic fields are present in two
representative configurations: mutually perpendicular and parallel.Comment: 50 pages, multiple figures, minor typo fixed, references adde
Quark Number Susceptibility with Finite Chemical Potential in Holographic QCD
We study the quark number susceptibility in holographic QCD with a finite
chemical potential or under an external magnetic field at finite temperature.
We first consider the quark number susceptibility with the chemical potential.
We observe that approaching the critical temperature from high temperature
regime, the quark number susceptibility divided by temperature square develops
a peak as we increase the chemical potential, which confirms recent lattice QCD
results. We discuss this behavior in connection with the existence of the
critical end point in the QCD phase diagram. We also consider the quark number
susceptibility under the external magnetic field. We predict that the quark
number susceptibility exhibits a blow-up behavior at low temperature as we
raise the value of the magnetic field. We finally spell out some limitations of
our study.Comment: 25 pages, 3 figures, published versio
Gross-Neveu Models, Nonlinear Dirac Equations, Surfaces and Strings
Recent studies of the thermodynamic phase diagrams of the Gross-Neveu model
(GN2), and its chiral cousin, the NJL2 model, have shown that there are phases
with inhomogeneous crystalline condensates. These (static) condensates can be
found analytically because the relevant Hartree-Fock and gap equations can be
reduced to the nonlinear Schr\"odinger equation, whose deformations are
governed by the mKdV and AKNS integrable hierarchies, respectively. Recently,
Thies et al have shown that time-dependent Hartree-Fock solutions describing
baryon scattering in the massless GN2 model satisfy the Sinh-Gordon equation,
and can be mapped directly to classical string solutions in AdS3. Here we
propose a geometric perspective for this result, based on the generalized
Weierstrass spinor representation for the embedding of 2d surfaces into 3d
spaces, which explains why these well-known integrable systems underlie these
various Gross-Neveu gap equations, and why there should be a connection to
classical string theory solutions. This geometric viewpoint may be useful for
higher dimensional models, where the relevant integrable hierarchies include
the Davey-Stewartson and Novikov-Veselov systems.Comment: 27 pages, 1 figur
Physics, Astrophysics and Cosmology with Gravitational Waves
Gravitational wave detectors are already operating at interesting sensitivity
levels, and they have an upgrade path that should result in secure detections
by 2014. We review the physics of gravitational waves, how they interact with
detectors (bars and interferometers), and how these detectors operate. We study
the most likely sources of gravitational waves and review the data analysis
methods that are used to extract their signals from detector noise. Then we
consider the consequences of gravitational wave detections and observations for
physics, astrophysics, and cosmology.Comment: 137 pages, 16 figures, Published version
<http://www.livingreviews.org/lrr-2009-2
A Gapless Hard Wall: Magnetic Catalysis in Bulk and Boundary
We study various aspects of fermions and their chiral condensates, both in
the bulk of AdS4 spacetime and in the dual boundary theory. For the most part,
we focus on a geometry with an infra-red hard wall. We show that, contrary to
common lore, there exist boundary conditions in which the hard wall gives rise
to a discrete, but gapless, fermionic spectrum. In such a setting, the presence
of a magnetic field induces a bulk fermion condensate which spontaneously
breaks CP invariance. We develop the holographic dictionary between composite
operators and show that this bulk condensate has the interpretation of boundary
magnetic catalysis involving a double-trace operator. Finally, we explain how
one can replace the hard wall with bulk magnetic monopoles. In such a
framework, magnetic catalysis can be viewed as a consequence of the
Callan-Rubakov effect.Comment: 43 pages. v2: reference adde
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