arXiv subjects

Mehdi Sadeghi

Publications and source records attributed to Mehdi Sadeghi.

At least 37 records · Page 2Linked to original sources

arXiv2024

Exponential Modification of AdS Black Hole and Thermodynamic Behavior

In this paper, we present an exponential modification for the action of an AdS black hole in the absence of a matter field. An approximated black hole solution is obtained up to the third order of perturbation coefficient. A thermodynamic investigation in canonical ensemble shows that the behavior of a Van der Waals fluid is not seen in this model. Nevertheless, the study of thermodynamic potentials and other related quantities suggests that the thermodynamic phase transitions of the first and second types can occur in this model. The forms of the phase transitions are more similar to the Hawking-Page phase transitions.

hep-th↗

arXiv2024

Non-Abelian Exponential Yang-Mills AdS Black Brane and Transport Coefficients

In this paper, AdS black brane solution of Einstein-Hilbert gravity with non-abelian exponential guage theory of Yang-Mills type is introduced. DC conductivity and the ratio of shear viscosity to entropy density as two important transport coefficients are calculated by using of Kubo formula in the context of AdS/CFT duality. Our results recover the Yang-Mills model in $q\to \infty$ limit.

hep-th↗

arXiv2024

The Phase Transition of Non-minimal Yang-Mills AdS Black Brane

In this paper, we shall study the phase transition of non-minimal coupling of Einstein-Hilbert gravity and electric field of Yang-Mills type in AdS space-time. We couple the Ricci scalar to the Yang-Mills invariant to obtain a modified theory of gravity. A black brane solution is introduced up to the first order of the term $RF^{(a)}_{μα}F^{(a)μα} $ in this model. Then, the phase transition of this solution will be investigated in canonical ensemble. Our investigation shows that only the second order phase transition behavior is seen in this model. Also, due to the coupling of the Yang-Mills field and Ricci scalar, there are differences with the phase transitions of the usual minimal models. We shall show that in the absence of non-minimal coupling there is no any phase transition.

hep-th↗

arXiv2023

The effect of three matters on KSS bound

In this paper we introduce the black brane solutions in AdS space in 4-dimensional (4D) Einstein-Gauss-Bonnet-Yang-Mills theory in the presence of string cloud and quintessence. Shear viscosity to entropy density ratio is computed via fluid-gravity duality, as a transport coefficient for this model.

hep-th↗

arXiv2023

Conductivity and Shear Viscosity of $arcsin$-Yang-Mills AdS Black Brane

In this paper, a non-abelian $arcsin$-Yang-Mills AdS black brane solution is introduced. Then, the color non-abelian direct current (DC) conductivity and shear viscosity to entropy density ratio of this model is calculated using fluid-gravity duality. Our results show that the Kovtun, Son and Starinets (KSS) bound is saturated and is exactly equal to $\frac{1}{4 π}$ but the color conductivity bound is violated for this model. Also, our outcomes recover the Yang-Mills AdS black brane when the coupling of Yang-Mills and gravity fields approaches zero.

hep-th↗

arXiv2023

Holographic Aspects of Non-minimal $R_{μανβ} F^{(a)μα} F^{(a)νβ} $ AdS Black Brane

In this paper, we study the holographic dual to an asymptotically anti-de Sitter black brane in an Einstein-Yang-Mills model with a non-minimal coupling between the Riemann and Yang-Mills fields. First, we construct a planar black hole solution of this model up to the first order of the non-minimal coupling of the Yang-Mills field with the Riemann-Christoffel tensor, denoted as $q_2$. Then, we calculate the color non-abelian direct current (DC) conductivity and the ratio of shear viscosity to entropy density for this solution. Our result for the shear viscosity $η$ to entropy density $s$ ratio saturates the Kovtun, Son, and Starinets (KSS) bound, which is proportional to $\frac{1}{4 π}$. However, our result for the conductivity is new up to the first order of $q_2$.

hep-th↗

arXiv2023

AdS black brane coupled to non-abelian logarithmic gauge theory and color DC conductivity

In this paper, we consider Einstein-Hilbert gravity in the presence of cosmological constant and non-abelian nonlinear electromagnetic field of logarithmic type which is minimally coupled to gravity. First, black brane solution of this model is introduced and then color non-abelian DC conductivity is calculated for this solution by using AdS/CFT duality. Our result recovers the Yang-Mills model in $λ\to 0$ limit.

hep-th↗

arXiv2023

The phase transition of Rastall AdS black hole with cloud of strings and quintessence

In this paper, we introduce the black hole solution in Rastall theory of gravity in the presence of quintessence and the cloud of strings. Our investigations show that this model meets only second-order phase transition in four dimensions. While both the first and second order phase transitions are seen in five dimensions. Therefore, according to the AdS/CFT duality, the confinement-deconfinement phase transition only occurs in five dimensions for this model.

hep-th↗

arXiv2023

Holographic Aspects of Non-minimal $RF^{(a)}_{μα}F^{(a)μα} $ Black Brane

In this paper, we consider Einstein-Hilbert gravity in the presence of cosmological constant and an electric field of Yang-Mills type, which is minimally coupled to gravity. We couple the Ricci scalar to the Yang-Mills invariant to obtain a modified theory of gravity. The black brane solution of this model is introduced up to the first order of the $RF^{(a)}_{μα}F^{(a)μα} $ term. Then, the color non-abelian direct current (DC) conductivity and the ratio of shear viscosity to entropy density are calculated for this solution. Our results recover the Yang-Mills Schwarzschild AdS black brane in the limit of $q_2 \to 0$.

hep-th↗

arXiv2023

AdS Black Hole with Cylindrical Symmetry

In this paper, we consider Einstein-Hilbert gravity in the presence of cosmological constant with cylindrical symmetry to introduce the black hole solution of this model. Here, we solve the Einstein's vacuum field equation, and then we calculate the appropriate metric for this problem.

gr-qc↗

arXiv2022

Non-abelian Einstein-Born-Infeld AdS black brane and color DC conductivity

In this paper, we consider Einstein-Hilbert gravity in the presence of cosmological constant and non-abelian nonlinear electromagnetic field of Born-Infeld type which is minimally coupled to gravity. First, black brane solution of this model is introduced and then color non-abelian DC conductivity is calculated for this solution by using AdS/CFT duality.

hep-th↗

arXiv2020

The Effect of Bulk Dimension in the presence of String Cloud on Viscosity Bound

In this paper, the Einstein AdS black brane solution in the presence of a string cloud in the context of d-dimensional massive gravity is introduced. The ratio of shear viscosity to entropy density for this solution violates the KSS bound by applying the Dirichlet boundary and regularity on the horizon conditions. Our result shows that this value is independent of string cloud in any arbitrary dimensions.

hep-th↗

arXiv2020

AdS Black Brane Solution Surrounded by Quintessence in Massive Gravity and KSS Bound

In this paper, the Einstein AdS black brane solution in the presence of quintessence in context of massive gravity is introduced. The ratio of shear viscosity to entropy density for this solution violates the KSS bound by applying the Dirichlet boundary and regularity on the horizon conditions. Our result shows that this value is independent of quintessence in arbitrary dimensions.

hep-th↗

arXiv2020

Effects of the external string cloud on the Van der Waals like behavior and efficiency of AdS-Schwarzschild black holes in massive gravity

In this paper we study AdS-Schwarzschild black holes in four and five dimensions in dRGT minimally coupled to a cloud of strings and investigate the effects of this string cloud on the thermodynamics of the black holes. It is observed that the entropy of the string cloud and massive terms does not affect the black hole entropy. The observations about four dimensions indicate that the massive term in the presence of external string cloud can not exhibit Van der Waals like behavior for AdS-Schwarzschild black holes and therefore there is only the Hawking-Page phase transition. In contrast, in five dimensions, the graviton mass modifies this behavior through the third massive term, so that a critical behavior and second order phase transition is deduced. Also, the Joule-Thomson effect is not observed. The black hole stability conditions are also studied in four and five dimensions and a critical value for the string cloud parameter is presented. In five dimensions a degeneracy between states for extremal black holes is investigated. After studying black holes as thermodynamic systems, we consider such systems as heat engines, and finally the efficiency of them is calculated.

hep-th↗

arXiv2019

Critical behavior of AdS Gauss-Bonnet massive black holes in the presence of external string cloud

Following previous study about AdS-Schwarzschild black holes minimally coupled to a cloud of strings in the context of massive gravity {Ghanaatian:2019} and inspired by strong connection between Gauss-Bonnet Gravity and heterotic string theory, in this paper, we first take into account the Gauss-Bonnet term and we study thermodynamics and critical behavior of these black holes in the extended phase space. The effects of Gauss-Bonnet, massive, and string cloud parameters on the criticality of these black holes has been investigated. It can be seen that the Gauss-Bonnet and massive parameters have opposite effects on the criticality and phase transition of the solutions. We also observe that the increase in the value of the string cloud parameter above a critical value, eliminates the van der Waals like behavior of these solutions. Also, the Joule-Thomson effect is not observed. Then we examine thermal stability of these black holes in canonical ensemble by calculating the heat capacity. In addition, we explore critical behavior in extended phase space by employing heat capacity and consequently, we observe that the results are in agreement with the previous results from the usual method in section 3.

hep-th↗

arXiv2019

Does a Cloud of Strings Affect Shear Viscosity Bound?

The Einstein AdS black brane with a cloud of strings background in context of massive gravity is introduced. There is a momentum dissipation on the boundary because of graviton mass on the bulk. The ratio of shear viscosity to entropy density is calculated for this solution. This value violates the KSS bound if we apply the Dirichlet boundary and regularity on the horizon conditions. Our result shows that this value is independent of the cloud of strings.

hep-th↗

arXiv2019

Transverse Shear Viscosity to Entropy Density for the General Anisotropic Black Brane in Horava-Lifshitz Gravity

In this paper we calculate the ratio of transverse shear viscosity to entropy density for the general anisotropic black brane in Horava-Lifshitz gravity. There is a well-known conjecture that states this ratio should be larger than $\frac{1}{4π}$. The ratio of shear viscosity to entropy density is proportional to the inverse square coupling of quantum thermal field theory,$\frac{η}{s} \sim \frac{1}{λ^2 }$. Especially in QFT with gravity dual the stronger coupling means the shear viscosity per entropy density gets closer to the lower bound of $\frac{1}{4π}$. The KSS bound preserves in the anisotropic scaling model.

hep-th↗

arXiv2019

Holographic Aspects of a Higher Curvature Massive Gravity

We study the holographic dual of a massive gravity with Gauss-Bonnet and cubic quasi-topological higher curvature terms. Firstly, we find the energy-momentum two-point function of the 4-dimensional boundary theory where the massive term breaks the conformal symmetry as expected. An $a$-theorem is introduced based on the null energy condition. Then we focus on a black brane solution in this background and derive the ratio of shear viscosity to entropy density for the dual theory. It is worth mentioning that the concept of viscosity as a transport coefficient is obscure in a nontranslational invariant theory as in our case. So although we use the Green-Kubo's formula to derive it, we rather call it the rate of entropy production per the Planckian time due to a strain. Results smoothly cover the massless limit.

hep-th↗