365 research outputs found
Origin of spin-driven ferroelectricity and effect of external pressure on the complex magnetism of the perovskite
Shear Modes, Criticality and Extremal Black Holes
We consider a (2+1)-dimensional field theory, assumed to be holographically
dual to the extremal Reissner-Nordstrom AdS(4) black hole background, and
calculate the retarded correlators of charge (vector) current and
energy-momentum (tensor) operators at finite momentum and frequency. We show
that, similar to what was observed previously for the correlators of scalar and
spinor operators, these correlators exhibit emergent scaling behavior at low
frequency. We numerically compute the electromagnetic and gravitational
quasinormal frequencies (in the shear channel) of the extremal
Reissner-Nordstrom AdS(4) black hole corresponding to the spectrum of poles in
the retarded correlators. The picture that emerges is quite simple: there is a
branch cut along the negative imaginary frequency axis, and a series of
isolated poles corresponding to damped excitations. All of these poles are
always in the lower half complex frequency plane, indicating stability. We show
that this analytic structure can be understood as the proper limit of finite
temperature results as T is taken to zero holding the chemical potential fixed.Comment: 28 pages, 7 figures, added reference
Holographic RG flow of the shear viscosity to entropy density ratio in strongly coupled anisotropic plasma
We study holographic RG flow of the shear viscosity tensor of anisotropic,
strongly coupled N=4 super-Yang-Mills plasma by using its type IIB supergravity
dual in anisotropic bulk spacetime. We find that the shear viscosity tensor has
three independent components in the anisotropic bulk spacetime away from the
boundary, and one of the components has a non-trivial RG flow while the other
two have a trivial one. For the component of the shear viscosity tensor with
non-trivial RG flow, we derive its RG flow equation, and solve the equation
analytically to second order in the anisotropy parameter 'a'. We derive the RG
equation using the equation of motion, holographic Wilsonian RG method, and
Kubo's formula. All methods give the same result. Solving the equation, we find
that the ratio of the component of the shear viscosity tensor to entropy
density 'eta/s' flows from above '1/4pi' at the horizon (IR) to below '1/4pi'
at the boundary (UV) where it violates the holographic shear viscosity
(Kovtun-Son-Starinets) bound and where it agrees with the other longitudinal
component.Comment: 17 pages, 2 figures, slight change on the title, more background
material added, references added, accepted for publication 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
Pointlike probes of superstring-theoretic superfluids
In analogy with an experimental setup used in liquid helium, we use a
pointlike probe to study superfluids which have a gravity dual. In the gravity
description, the probe is represented by a hanging string. We demonstrate that
there is a critical velocity below which the probe particle feels neither drag
nor stochastic forces. Above this critical velocity, there is power-law scaling
for the drag force, and the stochastic forces are characterized by a finite,
velocity-dependent temperature. This temperature participates in two simple and
general relations between the drag force and stochastic forces. The formula we
derive for the critical velocity indicates that the low-energy excitations are
massless, and they demonstrate the power of stringy methods in describing
strongly coupled superfluids.Comment: 17 pages, 2 figures, added a figure, a reference, and moved material
to an appendi
Transport in holographic superfluids
We construct a slowly varying space-time dependent holographic superfluid and
compute its transport coefficients. Our solution is presented as a series
expansion in inverse powers of the charge of the order parameter. We find that
the shear viscosity associated with the motion of the condensate vanishes. The
diffusion coefficient of the superfluid is continuous across the phase
transition while its third bulk viscosity is found to diverge at the critical
temperature. As was previously shown, the ratio of the shear viscosity of the
normal component to the entropy density is 1/(4 pi). As a consequence of our
analysis we obtain an analytic expression for the backreacted metric near the
phase transition for a particular type of holographic superfluid.Comment: 45 pages + appendice
Dissipative superfluid dynamics from gravity
Charged asymptotically AdS black branes in five dimensions are sometimes
unstable to the condensation of charged scalar fields. For fields of infinite
charge and squared mass -4 Herzog was able to analytically determine the phase
transition temperature and compute the endpoint of this instability in the
neighborhood of the phase transition. We generalize Herzog's construction by
perturbing away from infinite charge in an expansion in inverse charge and use
the solutions so obtained as input for the fluid gravity map. Our tube wise
construction of patched up locally hairy black brane solutions yields a one to
one map from the space of solutions of superfluid dynamics to the long
wavelength solutions of the Einstein Maxwell system. We obtain explicit
expressions for the metric, gauge field and scalar field dual to an arbitrary
superfluid flow at first order in the derivative expansion. Our construction
allows us to read off the the leading dissipative corrections to the perfect
superfluid stress tensor, current and Josephson equations. A general framework
for dissipative superfluid dynamics was worked out by Landau and Lifshitz for
zero superfluid velocity and generalized to nonzero fluid velocity by Clark and
Putterman. Our gravitational results do not fit into the 13 parameter
Clark-Putterman framework. Purely within fluid dynamics we present a consistent
new generalization of Clark and Putterman's equations to a set of superfluid
equations parameterized by 14 dissipative parameters. The results of our
gravitational calculation fit perfectly into this enlarged framework. In
particular we compute all the dissipative constants for the gravitational
superfluid.Comment: v1: 58 + 1 pages; v2: 83 + 1 page
QCD with Chemical Potential in a Small Hyperspherical Box
To leading order in perturbation theory, we solve QCD, defined on a small
three sphere in the large N and Nf limit, at finite chemical potential and map
out the phase diagram in the (mu,T) plane. The action of QCD is complex in the
presence of a non-zero quark chemical potential which results in the sign
problem for lattice simulations. In the large N theory, which at low
temperatures becomes a conventional unitary matrix model with a complex action,
we find that the dominant contribution to the functional integral comes from
complexified gauge field configurations. For this reason the eigenvalues of the
Polyakov line lie off the unit circle on a contour in the complex plane. We
find at low temperatures that as mu passes one of the quark energy levels there
is a third-order Gross-Witten transition from a confined to a deconfined phase
and back again giving rise to a rich phase structure. We compare a range of
physical observables in the large N theory to those calculated numerically in
the theory with N=3. In the latter case there are no genuine phase transitions
in a finite volume but nevertheless the observables are remarkably similar to
the large N theory.Comment: 44 pages, 18 figures, jhep3 format. Small corrections and
clarifications added in v3. Conclusions cleaned up. Published versio
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
Transport in Anisotropic Superfluids: A Holographic Description
We study transport phenomena in p-wave superfluids in the context of
gauge/gravity duality. Due to the spacetime anisotropy of this system, the
tensorial structure of the transport coefficients is non-trivial in contrast to
the isotropic case. In particular, there is an additional shear mode which
leads to a non-universal value of the shear viscosity even in an Einstein
gravity setup. In this paper, we present a complete study of the helicity two
and helicity one fluctuation modes. In addition to the non-universal shear
viscosity, we also investigate the thermoelectric effect, i.e. the mixing of
electric and heat current. Moreover, we also find an additional effect due to
the anisotropy, the so-called flexoelectric effect.Comment: 38 pages, 11 figures; v2: typos corrected. arXiv admin note:
substantial text overlap with arXiv:1011.591
- …