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George Siopsis

Publications and source records attributed to George Siopsis.

At least 109 records · Page 6Linked to original sources
arXiv2012

Enhancement of Critical Temperature of a Striped Holographic Superconductor

We study the interplay between the stripe order and the superconducting order in a strongly coupled striped superconductor using gauge/gravity duality. In particular, we study the effects of inhomogeneity introduced by the stripe order on the superconducting transition temperature beyond the mean field level by including the effects of backreaction onto the spacetime geometry in the dual gravitational picture. We find that inhomogeneity \emph{enhances} the critical temperature relative to its value for the uniform system.

hep-th↗
arXiv2012

Strongly Coupled Striped Superconductor with Large Modulation

We analytically calculate properties of a strongly coupled striped superconductor, with the charge density wave sourced by a modulated chemical potential, in the large modulation wavenumber Q limit. In the absence of a homogeneous term in the chemical potential, we show that the critical temperature scales as a negative power of Q for scaling dimensions Δ< 3/2, whereas for Δ> 3/2, there is no phase transition above a certain critical value of Q. The condensate is found to scale as a positive power of Q such that the gap is proportional to Q. We discuss how these results change if a homogeneous term is added to the chemical potential. We compare our analytic results with numerical calculations whenever the latter are available and find good agreement.

cond-mat.str-el↗
arXiv2011

Holographic superconductors near the Breitenlohner-Freedman bound

We discuss holographic superconductors in an arbitrary dimension whose dual black holes have scalar hair of mass near the Breitenlohner-Freedman bound. We concentrate on low temperatures in the probe limit. We show analytically that when the bound is saturated, the condensate diverges at low temperatures as $|\ln T|^δ$, where $δ$ depends on the dimension. This mild divergence was missed in earlier numerical studies. We calculate the conductivity analytically and show that at low temperatures, all poles move toward the real axis. We obtain an increasingly large number of poles which approach the zeroes of the Airy function in 2+1 dimensions and of the Gamma function in 3+1 dimensions. Our analytic results are in good agreement with numerical results whenever the latter are available.

hep-th↗
arXiv2011

Holographic Charge Density Waves

We discuss a gravity dual of a charge density wave consisting of a U(1) gauge field and two scalar fields in the background of an AdS$_4$ Schwarzschild black hole together with an antisymmetric field (probe limit). Interactions drive the system to a phase transition below a critical temperature. We numerically compute the ground states characterized by modulated solutions for the gauge potential corresponding to a dynamically generated unidirectional charge density wave in the conformal field theory. Signatures of the holographic density waves are retrieved by studying the dynamical response to an external electric field. We find that this novel holographic state shares many common features with the standard condensed matter version of charge density wave systems.

hep-th↗
arXiv2010

A New Class of Exact Hairy Black Hole Solutions

We present a new class of black hole solutions with minimally coupled scalar field in the presence of a negative cosmological constant. We consider a one-parameter family of self-interaction potentials parametrized by a dimensionless parameter $g$. When $g=0$, we recover the conformally invariant solution of the Martinez-Troncoso-Zanelli (MTZ) black hole. A non-vanishing $g$ signals the departure from conformal invariance. All solutions are perturbatively stable for negative black hole mass and they may develop instabilities for positive mass. Thermodynamically, there is a critical temperature at vanishing black hole mass, where a higher-order phase transition occurs, as in the case of the MTZ black hole. Additionally, we obtain a branch of hairy solutions which undergo a first-order phase transition at a second critical temperature which depends on $g$ and it is higher than the MTZ critical temperature. As $g\to 0$, this second critical temperature diverges.

hep-th↗
arXiv2010

Time-dependent flow from an AdS Schwarzschild black hole

I discuss two examples of time-dependent flow which can be described in terms of an AdS Schwarzschild black hole via holography. The first example involves Bjorken hydrodynamics which should be applicable to the formation of the quark gluon plasma in heavy ion collisions. The second example is the cosmological evolution of our Universe.

hep-th↗
arXiv2010

Perturbations of anti-de Sitter black holes

I review perturbations of black holes in asymptotically anti-de Sitter space. I show how the quasi-normal modes governing these perturbations can be calculated analytically and discuss the implications on the hydrodynamics of gauge theory fluids per the AdS/CFT correspondence. I also discuss phase transitions of hairy black holes with hyperbolic horizons and the dual superconductors emphasizing the analytical calculation of their properties.

hep-th↗
arXiv2010

Analytic calculation of properties of holographic superconductors

We calculate analytically properties of holographic superconductors in the probe limit. We analyze the range $1/2 < Δ< 3$, where $Δ$ is the dimension of the operator that condenses. We obtain the critical temperature in terms of a solution to a certain eigenvalue problem. Near the critical temperature, we apply perturbation theory to determine the temperature dependence of the condensate. In the low temperature limit we show that the condensate diverges as $T^{-Δ/3}$ for $Δ< 3/2$ whereas it asymptotes to a constant value for which we provide analytic estimates for $Δ> 3/2$. We also obtain the frequency dependence of the conductivity by solving analytically the wave equation of electromagnetic perturbations. We show that the real part of the DC conductivity behaves as $e^{-Δ_g /T}$ and estimate the gap $Δ_g$ analytically. Our results are in good agreement with numerical results.

hep-th↗
arXiv2009

Exact Gravity Dual of a Gapless Superconductor

A model of an exact gravity dual of a gapless superconductor is presented in which the condensate is provided by a charged scalar field coupled to a bulk black hole of hyperbolic horizon in asymptotically AdS spacetime. Below a critical temperature, the black hole acquires its hair through a phase transition while an electromagnetic perturbation of the background Maxwell field determines the conductivity of the boundary theory.

hep-th↗
arXiv2009

Shear Viscosity and Chern-Simons Diffusion Rate from Hyperbolic Horizons

We calculate the shear viscosity and anomalous baryon number violation rate in quantum field theories at finite temperature having gravity duals with hyperbolic horizons. We find the explicit dependence of these quantities on the temperature. We show that the ratio of shear viscosity to entropy density is below $1/(4π)$ at all temperatures and it can be made arbitrarily small in the low temperature limit.

hep-th↗
arXiv2009

Discontinuities in Scalar Perturbations of Topological Black Holes

We study the perturbative behaviour of topological black holes. We calculate both analytically and numerically the quasi-normal modes of scalar perturbations. In the case of small black holes we find discontinuities of the quasi-normal modes spectrum at the critical temperature and we argue that this is evidence of a second-order phase transition.

hep-th↗
arXiv2009

Dissipative Bjorken hydrodynamics from an AdS Schwarzschild black hole

We discuss the derivation of dissipative Bjorken hydrodynamics from a Schwarzschild black hole in asymptotically AdS spacetime of arbitrary dimension in the limit of large longitudinal proper time $τ$. Using an appropriate slicing near the boundary, we calculate the Schwarzschild metric to next-to-next-to-leading order in the large $τ$ expansion as well as the dual stress-energy tensor on the boundary via holographic renormalization. At next-to-next-to-leading order, it is necessary to perturb the Schwarzschild metric in order to maintain boost invariance. The perturbation has a power law time dependence and leads to the same value of the ratio of viscosity to entropy density, $1/(4π)$, as in the case of sinusoidal perturbations. Our results are in agreement with known time-dependent asymptotic solutions of the Einstein equations in five dimensions.

hep-th↗
arXiv2009

Particle emission from a black hole on a tense codimension-2 brane

We calculate analytically grey-body factors of Schwarzschild black-holes localized on a 3-brane of finite tension and codimension 2. We obtain explicit expressions for various types of particles emitted in the bulk as well as on the brane in both the low and high frequency regimes. In the latter case, we obtain expressions which are valid for arbitrary number of extra dimensions if the brane tension vanishes.

hep-th↗
arXiv2009

Black holes and the quark-gluon plasma

I discuss the possibility that the quark-gluon plasma at strong coupling admits a description in terms of a black hole in asymptotically anti-de Sitter space.

hep-th↗
arXiv2008

Bjorken flow from an AdS Schwarzschild black hole

We consider a large black hole in asymptotically AdS spacetime of arbitrary dimension with a Minkowski boundary. By performing an appropriate slicing as we approach the boundary, we obtain via holographic renormalization a gauge theory fluid obeying Bjorken hydrodynamics in the limit of large longitudinal proper time. The metric we obtain reproduces to leading order the metric recently found as a direct solution of the Einstein equations in five dimensions. Our results are also in agreement with recent exact results in three dimensions.

hep-th↗
arXiv2008

Phase Transitions in Charged Topological-AdS Black Holes

We study the perturbative behaviour of charged topological-AdS black holes. We calculate both analytically and numerically the quasi-normal modes of the electromagnetic and gravitational perturbations. Keeping the charge-to-mass ratio constant, we show that there is a second-order phase transition at a critical temperature at which the mass of the black hole vanishes. We pay special attention to the purely dissipative modes appearing in the spectrum as they behave singularly at the critical point.

hep-th↗
arXiv2008

Low-lying quasinormal modes of topological AdS black holes and hydrodynamics

We analytically calculate the low-lying gravitational quasinormal modes of a topological AdS black hole of arbitrary dimension. We show that they are in agreement with corresponding results from the hydrodynamics of the gauge theory plasma on the boundary, as required by the AdS/CFT correspondence. For some of these modes, we obtain a lifetime which is comparable to or longer than the longest lifetime of perturbations of spherical black holes. Thus, these modes are expected to play an important role in the late time behavior of the gauge theory plasma.

hep-th↗