arXiv subjects

Ahmad Sheykhi

Publications and source records attributed to Ahmad Sheykhi.

At least 73 records · Page 4Linked to original sources

arXiv2020

Mimetic Black Strings

We present two new classes of black string solutions in the context of mimetic gravity. The horizon topology of these solutions can be either a flat $T^2$ torus with topology $S^1 \times S^1$, or a standard cylindrical model with topology $R\times S^1$. The first class describes uncharged rotating black string which its asymptotic behavior is a quotient of anti-de Sitter (AdS) space, while the second class represents asymptotically AdS charged rotating black string. We study the casual structure and physical properties of these spacetimes and calculate, the entropy, electric charge, mass and angular momentum per unit length of rotating black strings.

gr-qc↗

arXiv2020

Critical behavior of charged dilaton black holes in AdS space

We revisit critical behaviour and phase structure of charged anti-deSitter (AdS) dilaton black holes for arbitrary values of dilaton coupling $α$, and realize several novel phase behaviour for this system. We adopt the viewpoint that cosmological constant (pressure) is fixed and treat the charge of the black hole as a thermodynamical variable. We study critical behaviour and phase structure by analyzing the phase diagrams in $T-S$ and $ q-T$ planes. We numerically derive the critical point in terms of $α$ and observe that for $α=1$ and $α\geq \sqrt{3}$, the system does not admit any critical point, while for $0<α<1$, the critical quantities are not significantly affected by $α$. We find that unstable behavior of the Gibbs free energy for $q q_{c}$, however, a novel first order phase transition occurs between small and large black hole, which has not been observed in the previous studies on phase transition of charged AdS black holes.

gr-qc↗

arXiv2020

Meissner-like effect and conductivity of power-Maxwell holographic superconductors

The full description of a superconductor requires that it has an infinite DC conductivity (or zero electrical resistivity) as well as expels the external magnetic fields. Thus, for any holographic superconductor which is dual to a real superconductor, it is necessary to examine, simultaneously, these two features based on the gauge/gravity duality. In this paper, we explore numerically these two aspects of the higher dimensional holographic superconductors, in the presence of a Power-Maxwell electrodynamics as the gauge field. At first, we calculate the critical temperature, condensation, conductivity, and superconducting gap, in the absence of magnetic field and disclose the effects of both power parameter, $s$, as well as the spacetime dimensions, $d$, on this quantities. Then, we immerse the superconductor into an external magnetic field, $B$, and observe that with increasing the magnetic field, the starting point of condensation occurs at temperature less than the critical temperature, $T_{c}$, in the absence of magnetic field. This implies that at a fixed temperature, we can define a critical magnetic field, above which the critical temperature goes to zero which is similar to the Meissner effect in superconductor. In these indications, we also try to show the distinction of the conformal invariance of the Power-Maxwell Lagrangian that occurs for $s=d/4$.

hep-th↗

arXiv2020

New explanation for accelerated expansion and flat galactic rotation curves

Employing the non-additive Tsallis entropy, $S\sim A^β$, for the large-scale gravitational systems, we disclose that in the cosmological scales both Friedmann equation and the equation of motion for the Newtonian cosmology get modified, accordingly. We then derive the modified Newton's law of gravitation which is valid on the large scales. We show that, in the relativistic regime, the modified Friedmann equation admits an accelerated expansion, for a universe filled with ordinary matter, without invoking any kind of dark energy, provided the non-extensive parameter is chosen $β<1/2$. In the non-relativistic regime, however, the modified Newton's law of gravitation can explain the flat galactic rotation curves without invoking particle dark matter provided $β\lesssim1/2$. Our study may be regarded as an alternative explanation for the "dark side of the universe", through modification of the gravitational field equations.

physics.gen-ph↗

arXiv2019

Conductivity of the one-dimensional holographic p-wave superconductors in the presence of nonlinear electrodynamics

We investigate analytically as well as numerically effects of nonlinear Born-Infeld (BI) electrodynamics on the properties of (1+1)-dimensional holographic $p$-wave superconductor in the context of gauge/gravity duality. We consider the case in which the gauge and vector fields backreact on the background geometry. We apply the Sturm-Liouville eigenvalue problem for the analytical approach as well as the shooting method for the numerical calculations. In both methods, we find out the relation between critical temperature $T_{c}$ and chemical potential $μ$ and show that both approaches are in good agreement with each other. We find that if one strengthen the effect of backreaction as well as nonlinearity, the critical temperature decreases which means that the condensation is harder to form. We also explore the conductivity of the one-dimensional holographic $p$-wave superconductor for different values of b and $T/T_{c}$. We find out that the real and imaginary parts of the conductivity have different behaviors in higher dimensions. The effects of different values of temperature is more apparent for larger values of nonlinearity parameter. In addition, for the fixed value of $T/T_{c}$ by increasing the effect of nonlinearity we observe larger values for Drude-like peak in real part of conductivity and deeper minimum for imaginary part.

hep-th↗

arXiv2019

Alternative approach to the critical behavior and microscopic structure of the power-Maxwell black holes

Employing a new approach toward thermodynamic phase space, we investigate the phase transition, critical behavior and microscopic structure of higher dimensional black holes in an Anti-de Sitter (AdS) background and in the presence of Power-Maxwell field. In contrast to the usual extended $P-V$ phase space where the cosmological constant (pressure) is treated as a thermodynamic variable, we fix the cosmological constant and treat the charge of the black hole (or more precisely $Q^s$) as a thermodynamic variable. Based on this new standpoint, we develop the resemblance between higher dimensional nonlinear black hole and Van der Waals liquid-gas system. We write down the equation of state as $% Q^s=Q^s(T,ψ)$, where $ψ$ is the conjugate of $Q^s$, and construct a Smarr relation based on this new phase space as $M=M(S,P,Q^s)$, while $% s=2p/(2p-1)$ and $p$ is the power parameter of the Power-Maxwell Lagrangian. We obtain the Gibbs free energy of the system and find a swallowtail behaviour in Gibbs diagrams, which is a characteristic of first-order phase transition and express the analogy between our system and van der Waals fluid-gas system. Moreover, we calculate the critical exponents and show that they are independent of the model parameters and are the same as those of Van der Waals system which is predicted by the mean field theory. Finally , we successfully explain the microscopic behavior of the black hole by using thermodynamic geometry. We observe a gap in the scalar curvature $R$ occurs between small and large black hole. The maximum amount of the gap increases as the number of dimensions increases. We finally find that character of the interaction among the internal constituents of the black hole thermodynamic system is intrinsically a strong repulsive interaction.

physics.gen-ph↗

arXiv2019

Holographic paramagnetic-ferromagnetic phase transition with Power-Maxwell electrodynamics

We explore the effects of Power-Maxwell nonlinear electrodynamics on the properties of holographic s-wave paramagnetic-ferromagnetic phase transition in the background of Schwarzchild Anti-de Sitter (AdS) black hole. For this purpose, we introduce a massive 2-form coupled to the Power-Maxwell field. We perform the numerical shooting method in the probe limit by assuming the Power-Maxwell and the 2-form fields do not back react on the background geometry. We observe that increasing the strength of the power parameter causes the formation of magnetic moment in the black hole background harder and critical temperature lower. In the absence of external magnetic field and at the low temperatures, the spontaneous magnetization and the ferromagnetic phase transition happen. In this case, the critical exponent for magnetic moment is always 1/2 which is in agreement with the result from the mean field theory. In the presence of external magnetic field, the magnetic susceptibility satisfies the Cure-Weiss law.

hep-th↗

arXiv2019

Conductivity of the holographic p-wave superconductors with higher order corrections

We investigate the holographic $p$-wave superconductors in the presence of the higher order corrections on the gravity as well as on the gauge field side. On the gravity side, we add the Gauss-Bonnet curvature correction terms, while on the gauge field side we take the nonlinear Lagrangian in the form $F + b F^2$, where F is the Maxwell Lagrangian and b indicates the strength of nonlinearity. We employ the shooting method for the numerical calculations in order to obtain the ratio of the critical temperature $T_c$ over $ρ^ {1/(d-2)}$. We observe that by increasing the values of the mass and the nonlinear parameters the critical temperature decreases and thus the condensation becomes harder to form. In addition, the stronger Gauss-Bonnet parameter $α$ hinders the superconducting phase in Gauss-Bonnet gravity. Furthermore, we calculate the electrical conductivity based on the holographic setup. The real and imaginary parts are related to each other by Kramers-Kronig relation which indicates a delta function and pole in low frequency regime, respectively. However, at enough large frequencies the trend of real part can be interpreted by $Re[σ] = ω^{(d-4)}$. Moreover, in holographic model the ratio $ω_g/T_c$ is always much larger than the BCS value $3.5$ due to the strong coupling of holographic superconductors. In both gravity kinds, decreasing the temperature or increasing the effect of nonlinearity shifts the gap frequency toward larger values. Besides, the gap frequency is occurred at larger values by enlarging the Gauss-Bonnet parameter. In general, the behavior of conductivity depends on the choice of the mass, the nonlinear and the Gauss-Bonnet parameters.

hep-th↗

arXiv2019

One-dimensional backreacting holographic p-wave superconductors

We analytically as well as numerically study the properties of one-dimensional holographic p- wave superconductors in the presence of backreaction. We employ the Sturm-Liouville eigenvalue problem for the analytical calculation and the shooting method for the numerical investigations. We apply the AdS3/CFT2 correspondence and determine the relation between the critical temperature Tc and the chemical potential μfor different values of mass m of charged spin-1 field and backreacting parameters. We observe that the data of both analytical and numerical studies are in good agreement. We find out that increasing the backreaction as well as the mass parameter cause the greater values for Tc/ μ. Therefore, it makes the condensation harder to form. In addition, the analytical and numerical approaches show that the value of the critical exponent βis 1/2 which is the same as in the mean field theory. Moreover, both methods confirm the exhibition of a second order phase transition.

hep-th↗

arXiv2018

Universality class of alternative phase space and Van der Waals criticality

A new perspective toward thermodynamic phase space of Reisser-Nordstrom (RN) black holes in an anti-de-Sitter (AdS) spaces was recently proposed [Phys. Lett. B 768 (2017) 235], where the square of the electric charge $(Q^2)$ of black hole was regarded as a thermodynamic variable and the cosmological constant (pressure) as a fixed quantity. In this paper, we address the universality class and critical properties of any AdS black hole in this alternative phase space. We disclose the critical behavior of AdS black hole in the alternative phase space in which a continuous phase transition happens and in a very general framework, independent of the spacetime metric. Based on the expansion of the equation of state and Landau thermodynamic potential in the neighborhood of a critical point in the alternative phase space, we confirm that the set of values for critical exponents for generic black hole is analogous to the Van der Waals fluid system. Finally, we reveal that the scalar curvature in geometry thermodynamic diverges at the critical point of black hole. Our study shows that the approach here is powerful enough to investigate the critical behavior of any black holes and further supports the viability of the alternative viewpoint toward phase space of black holes suggested in [Phys. Lett. B 768 (2017) 235].

hep-th↗

arXiv2018

Modified Friedmann Equations from Tsallis Entropy

It was shown by Tsallis and Cirto that thermodynamical entropy of a gravitational system such as black hole must be generalized to the non-additive entropy, which is given by $S_h=γA^β$, where $A$ is the horizon area and $β$ is the nonextensive parameter \cite{Tsa}. In this paper, by taking the entropy associated with the apparent horizon of the Friedmann-Robertson-Walker (FRW) Universe in the form of Tsallis entropy, and assuming the first law of thermodynamics, $dE=T_hdS_h+WdV$, holds on the apparent horizon, we are able to derive the corresponding Friedmann equations describing the dynamics of the universe with any spatial curvature. We also examine the time evolution of the total entropy and show that the generalized second law of thermodynamics is fulfilled in a region enclosed by the apparent horizon. Then, modifying the emergence proposal of gravity proposed by Padmanabhan and calculating the difference between the surface degrees of freedom and the bulk degrees of freedom in a region of space, we again arrive at the modified Friedmann equation of the FRW Universe with any spatial curvature which is the same as one obtained from the first law of thermodynamics. We also study the cosmological consequences of Tsallis cosmology. Interestingly enough, we find that this model can explain simultaneously the late time acceleration in the universe filled with pressureless matter without invoking dark energy, as well as the early deceleration. Besides, the age problem can be circumvented automatically for an accelerated universe and is estimated larger than $3/2$ age of the universe in standard cosmology. For $β=2/5$, we find $13.12$ Gyr $< t_0 < 16.32 $ Gyr, which is consistent with recent observations. We also comment on the density perturbation in the context of Tsallis cosmology.

gr-qc↗

arXiv2018

Reentrant phase transition of Born-Infeld-AdS black holes

We investigate thermodynamic phase structure and critical behaviour of Born-Infeld (BI) black holes in an anti-de Sitter (AdS) space, where the charge of the system can vary and the cosmological constant (pressure) is fixed. We find that the BI parameter crucially affects the temperature of the black hole when the horizon radius, $r_{+}$, is small. We observe that depending on the value of the nonlinear parameter, $β$, BI-AdS black hole may be identified as RN black hole for $Q\geq Q_{m}$, and Schwarzschild-like black hole for $Q Q_{c}$, the Gibbs free energy is single valued and the system is locally stable ($C_{Q}>0$), while for $% Q<Q_{c}$, it becomes multivalued and $C_{Q}<0$. In the range of $% Q_{z}<Q<Q_{c}$, a first order phase transition occurs between small black hole (SBH) and large black hole (LBH). Interestingly enough, in the range of $Q_{t}\leq Q\leq Q_{z}$, a reentrant phase transition occurs between intermediate (large) black hole, SBH and LBH in Schwarzschild-type region. This means that in addition to the first order phase transition which separates SBH and LBH, a finite jump in Gibbs free energy leads to a \textit{% zeroth order} phase transition between SBH and intermediate black hole (LBH) where initiates from $Q=Q_{z}$ and terminates at $Q=Q_{t}$.

gr-qc↗

arXiv2018

Critical behaviour of Lifshitz dilaton black holes

Till now, critical behaviour of Lifshitz black holes, in an extended $P-v$ space, has not been studied, because it is impossible to find an analytical equation of state, $P=P(v,T)$, for an arbitrary Lifshitz exponent $z$. In this paper, we adopt a new approach toward thermodynamic phase space and successfully explore the critical behaviour of $(n+1)$-dimensional Lifshitz dilaton black holes. For this purpose, we write down the equation of state as $Q^s=Q^s(T,Ψ)$ with $Ψ=\left({\partial M}/{\partial Q^{s} }\right)_{S,P}$ is the conjugate of $Q^s$ and construct Smarr relation based on this new phase space as $ M=M(S,Q^{s},P)$, where $s=2p/(2p-1)$ with $p$ is the power of the power-law Maxwell Lagrangian. We justify such a choice mathematically and show that with this new phase space, the system admits the critical behaviour and resembles the Van der Waals fluid system when the cosmological constant (pressure) is treated as a fixed parameter, while the charge of the system varies. We obtain Gibbs free energy of the system and find swallow tail shape in Gibbs diagrams which represents the first order phase transition. Finally, we calculate the critical exponents and show that although thermodynamic quantities depend on the metric parameters such as $z$ , $p$ and $n$, the critical exponents are the same as Van der Walls fluid-gas system. This alternative viewpoint toward phase space of lifshitz dilaton black hole can be understood easily since one can imagine such a change for a given single black hole i.e., acquiring charge which induces the phase transition. Our results further support the viewpoint suggested in \cite{Dehy}.

hep-th↗

arXiv2018

Analytical and numerical study of backreacting one-dimensional holographic superconductors in the presence of Born-Infeld electrodynamics

We analytically as well as numerically study the effects of Born-Infeld nonlinear electrodynamics on the properties of $(1+1)$-dimensional s-wave holographic superconductors. We relax the probe limit and further assume the scalar and gauge fields affect on the background spacetime. We thus explore the effects of backreaction on the condensation of the scalar hair. For the analytical method, we employ the Sturm-Liouville eigen value problem and for the numerical method, we employ the shooting method. We show that these methods are powerful enough to analyze the critical temperature and phase transition of the one dimensional holographic superconductor. We find out that increasing the backreaction as well as nonlinearity makes the condensation harder to form. In addition, this one-dimensional holographic superconductor faces with second order phase transition and their critical exponent has the mean field value $β={1}/{2}$.

hep-th↗

arXiv2018

Conductivity of higher dimensional holographic superconductors with nonlinear electrodynamics

We investigate analytically as well as numerically the properties of s-wave holographic superconductors in $d$-dimensional spacetime and in the presence of Logarithmic nonlinear electrodynamics. We study three aspects of these kind of superconductors. First, we obtain, by employing analytical Sturm-Liouville method as well as numerical shooting method, the relation between critical temperature and charge density, $ρ$, and disclose the effects of both nonlinear parameter $b$ and the dimensions of spacetime, $d$, on the critical temperature $T_c$. We find that in each dimension, $T_c/ρ^{1/(d-2)}$ decreases with increasing the nonlinear parameter $b$ while it increases with increasing the dimension of spacetime for a fixed value of $b$. Then, we calculate the condensation value and critical exponent of the system analytically and numerically and observe that in each dimension, the condensation get larger with increasing the nonlinear parameter $b$. Besides, for a fixed value of $b$, it increases with increasing the spacetime dimension. We confirm that the results obtained from our analytical method are in agreement with the results obtained from numerical shooting method. This fact may support the correctness of our analytical method. Finally, we explore the holographic conductivity of this system and find out that the superconducting gap increases with increasing either the nonlinear parameter or the spacetime dimension.

hep-th↗

arXiv2018

Holographic conductivity of holographic superconductors with higher order corrections

We analytically as well as numerically disclose the effects of the higher order correction terms in the gravity and in the gauge field on the properties of $s$-wave holographic superconductors. On the gravity side, we consider the higher curvature Gauss-Bonnet corrections and on the gauge field side, we add a quadratic correction term to the Maxwell Lagrangian. We show that for this system, one can still obtain an analytical relation between the critical temperature and the charge density. We also calculate the critical exponent and the condensation value both analytically and numerically. We use a variational method, based on the Sturm-Liouville eigenvalue problem for our analytical study, as well as a numerical shooting method in order to compare with our analytical results. For a fixed value of the Gauss-Bonnet parameter, we observe that the critical temperature decreases with increasing the nonlinearity of the gauge field. This implies that the nonlinear correction term to the Maxwell electrodynamics make the condensation harder. We also study the holographic conductivity of the system and disclose the effects of Gauss-Bonnet and nonlinear parameters $α$ and $b$ on the superconducting gap. We observe that for various values of $α$ and $b$, the real part of conductivity is proportional to the frequency per temperature, $ω/T$, as frequency is enough large. Besides, the conductivity has a minimum in the imaginary part which is shifted toward greater frequency with decreasing the temperature.

hep-th↗

arXiv2017

Novel phase transition in charged dilaton black holes

We disclose a novel phase transition in black hole physics by investigating thermodynamics of charged dilaton black holes in an extended phase space where the charge of the black hole is regarded as a fixed quantity. Along with the usual critical (second-order) as well as the first-order phase transitions in charged black holes, we find that a finite jump in Gibbs free energy is generated by dilaton-electromagnetic coupling constant, $α$, for a certain range of pressure. This novel behavior indicates a small/large black hole \emph{zeroth-order} phase transition in which the response functions of black holes thermodynamics diverge e.g. isothermal compressibility. Such zeroth-order transition separates the usual critical point and the standard first-order transition curve. We show that increasing the dilaton parameter($α$) increases the zeroth-order portion of the transition curve. Additionally, we find that the second-order (critical) phase transition exponents are unaffected by the dilaton parameter, however, the condition of positive critical temperature puts an upper bound on the dilaton parameter ($α<1$).

hep-th↗

arXiv2016

Critical behaviour and microscopic structure of charged AdS black holes via an alternative phase space

It has been argued that charged Ads black holes have similar thermodynamic behavior as the Van der Waals fluid system, provided one treats the cosmological constant as a thermodynamic variable in an extended phase space. In this paper, we disclose the deep connection between charged AdS black holes and Van der Waals fluid system without extending the phase space. We keep the cosmological constant as a fixed parameter and instead, treat the square of the charge of black hole as a thermodynamic variable. Therefore, we write the equation of state as $Q^{2}=Q^{2}(T,Ψ)$ where $Ψ$ (conjugate of $Q^{2} $) is the inverse of the specific volume, $Ψ=1/v$. This allows us to complete the analogy of charged AdS black holes with Van der Waals fluid system and derive the phase transition as well as critical exponents of the system. We identify a thermodynamic instability in this new picture with real analogy to Van der Waals fluid with physically relevant Maxwell construction. We therefore study the critical behavior of isotherms in $% Q^2-Ψ$ diagram and deduce all the critical exponents of the system and determine that the system exhibits a small-large black hole phase transition at the critical point $(T_c,Q^2_c, Ψ_c)$. This alternative view is important as one can imagine such a change for a given single black hole i. e. acquiring charge which induces the phase transition. Finally, we disclose the microscopic properties of charged AdS black holes by using thermodynamic geometry. Interestingly, we find that scalar curvature has a gap between small and large black holes, and this gap becomes exceedingly large as one moves away from the critical point along the transition line. Therefore, we are able to attribute the sudden enlargement of the black hole to the strong repulsive nature of the internal constituents at the phase transition.

gr-qc↗