38 research outputs found
Universal Properties of the Langevin Diffusion Coefficients
We show that in generic isotropic holographic theories the longitudinal
Langevin diffusion coefficient along the string motion is larger compared to
that of the transverse direction. We argue that this is a universal relation
and we derive the generic conditions in order to be satisfied. A way to violate
the relation is to consider anisotropic gauge/gravity dualities. We give an
explicit example of this violation where the noise along the transverse
direction is larger than the noise occurring along the quark motion. Moreover,
we derive the effective world-sheet temperature for any generic theory and then
the conditions for negative excess noise. We argue that isotropic theories can
not have negative excess noise and we additionally remark that these conditions
are difficult to get satisfied, indicating positivity of the excess noise, even
in a large class of anisotropic holographic theories.Comment: 5 pages, double column format; v3:Published versio
Instability of BTZ black holes in parity-even massive gravity
We investigate the linearized equations of motion in three dimensional
Born-Infeld gravity theory. Motivated by this model, we calculate the
quasinormal modes of BTZ black hole solutions of parity-even gravity theories
in three dimensions by using numerical methods. The results are classified in
three families and they are so accurate such that it allows us to propose
analytical form for the quasinormal frequencies. We find new quasinormal modes
which have been missing in the literature of the analytical studies of three
dimensional massive gravitons. These new modes do not have the known tower
structure and they are purely imaginary for any value of the angular momenta.
Considering the complete set of the quasinormal modes, we show that the BTZ
black hole solutions are unstable for any value of the parameters of the
theory. We confirm our numerical results by computing the new eigenmodes
analytically at zero angular momentum.Comment: 21 pages, 2 figures, two plots and analytical analysis to support our
numerical results are added, typos correcte
On Born-Infeld Gravity in Three Dimensions
In this paper we explore different aspects of three dimensional Born-Infeld
as well as Born-Infeld-Chern-Simons gravity. We show that the models have AdS
and AdS-wave vacuum solutions. Moreover we observe that although
Born-Infeld-Chern-Simons gravity admits a logarithmic solution, Born-Infeld
gravity does not, though it has a limiting logarithmic solution as we approach
the critical point.Comment: 12 page
Non-equilibrium dynamics and phase transitions
We study the poles of the retarded Green's functions of strongly coupled
field theories exhibiting a variety of phase structures from a crossover up to
a first order phase transition. These theories are modeled by a dual
gravitational description. The poles of the holographic Green's functions
appear at the frequencies of the quasinormal modes of the dual black hole
background. We establish that near the transition, in all cases considered, the
applicability of a hydrodynamic description breaks down already at lower
momenta than in the conformal case. We establish the appearance of the spinodal
region in the case of the first order phase transition at temperatures for
which the speed of sound squared is negative. An estimate of the preferential
scale attained by the unstable modes is also given. We additionally observe a
novel diffusive regime for sound modes for a range of wavelengths.Comment: 5 pages, 4 figures. Some points are clarified. Typos corrtecte
The Imaginary Part of the Static Potential in Strongly Coupled Anisotropic Plasma
Using the gauge/gravity duality we study the imaginary part of the static
potential associated to the thermal width in finite temperature strongly
coupled anisotropic plasma. We firstly derive the potential for a generic
anisotropic background. Then we apply our formulas to a theory where the
anisotropy has been generated by a space dependent axion term. We find that
using our method there exist a peculiar turning point in the imaginary part of
the potential, similar to the one appearing in the real part. The presence of
anisotropy leads to decrease of the imaginary potential, where larger decrease
happens along the anisotropic direction when the temperature is kept fixed.
When the entropy density is fixed, increase happens along the parallel
direction while along the transverse plane we observe a decrease. To estimate
the thermal width we use an approximate extrapolation beyond the turning point
and we find a decrease in presence of the anisotropy, independently of the
comparison scheme used.Comment: 20+4 pages, 15 figures, v2: version published in JHE
Quasinormal modes and the phase structure of strongly coupled matter
We investigate the poles of the retarded Green's functions of strongly
coupled field theories exhibiting a variety of phase structures from a
crossover up to different first order phase transitions. These theories are
modeled by a dual gravitational description. The poles of the holographic
Green's functions appear at the frequencies of the quasinormal modes of the
dual black hole background. We focus on quantifying linearized level dynamical
response of the system in the critical region of phase diagram. Generically
non-hydrodynamic degrees of freedom are important for the low energy physics in
the vicinity of a phase transition. For a model with linear confinement in the
meson spectrum we find degeneracy of hydrodynamic and non-hydrodynamic modes
close to the minimal black hole temperature, and we establish a region of
temperatures with unstable non-hydrodynamic modes in a branch of black hole
solutions.Comment: 33 pages, 14 figure
Linearized nonequilibrium dynamics in nonconformal plasma
We investigate the behaviour of the lowest nonhydrodynamic modes in a class
of holographic models which exhibit an equation of state closely mimicking the
one determined from lattice QCD. We calculate the lowest quasinormal mode
frequencies for a range of scalar self-interaction potentials and find that the
damping of the quasinormal modes at the phase transition/crossover falls off by
a factor of around two from conformality after factoring out standard conformal
temperature dependence. The damping encoded in the imaginary part of the
frequencies turns out to be correlated with the speed of sound and is basically
independent of the UV details of the model. We also find that the dynamics of
the nonhydrodynamic degrees of freedom remains ultralocal, even to a higher
degree, as we deviate from conformality. These results indicate that the role
of nonhydrodynamic degrees of freedom in the vicinity of the crossover
transition may be enhanced
Nonlinear Oscillatory Shear Tests in Viscoelastic Holography
We provide the first characterization of the nonlinear and time dependent
rheologic response of viscoelastic bottom-up holographic models. More
precisely, we perform oscillatory shear tests in holographic massive gravity
theories with finite elastic response, focusing on the large amplitude
oscillatory shear (LAOS) regime. The characterization of these systems is done
using several techniques: (I) the Lissajous figures, (II) the Fourier analysis
of the stress signal, (III) the Pipkin diagram and (IV) the dependence of the
storage and loss moduli on the amplitude of the applied strain. We find
substantial evidence for a strong strain stiffening mechanism, typical of
hyper-elastic materials such as rubbers and complex polymers. This indicates
that the holographic models considered are not a good description for rigid
metals, where strain stiffening is not commonly observed. Additionally, a
crossover between a viscoelastic liquid regime at small graviton mass (compared
to the temperature scale), and a viscoelastic solid regime at large values is
observed. Finally, we discuss the relevance of our results for soft matter and
for the understanding of the widely used homogeneous holographic models with
broken translations.Comment: v2: Matching the version published in PRL