arXiv subjects

Mohammad Ali-Akbari

Publications and source records attributed to Mohammad Ali-Akbari.

At least 19 recordsLinked to original sources

arXiv2026

Probe Dependence of the Imaginary Part of HTEE

We investigate the holographic timelike entanglement entropy (HTEE) in a five-dimensional anisotropic background, dual to a strongly coupled anisotropic plasma. Using the complex extremal surface method, we compute the HTEE analytically in the high-temperature, small-anisotropy limit $aT \ll 1$. We consider two different orientations of the boundary timelike interval: one perpendicular to the anisotropy direction and one parallel to it. We find that the imaginary part of the HTEE is not a universal property of the geometry but depends sensitively on the orientation of the extremal surface relative to the anisotropy. This demonstrates that the imaginary part arising from the UV logarithmic divergence is a probe-dependent quantity. Our results suggest that the imaginary part of HTEE can serve as a diagnostic of the coupling between the extremal surface and the anisotropic degrees of freedom of the dual field theory.

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arXiv2026

Holographic Entanglement and Emergent Gravity

We investigate the emergence of bulk geometry from boundary entanglement entropy in the context of holographic duality. Considering a thermal $1+1$-dimensional conformal field theory dual to the BTZ black hole, we compute four distinct holographic entanglement measures: the standard spacelike HEE, its complementary spacelike counterpart HEE$^s$, the timelike HTEE and its complementary timelike counterpart HEE$^t$. These measures probe both the exterior and interior regions of the black hole horizon. Remarkably, by taking appropriate derivatives of these entanglement entropies with respect to the boundary intervals and the turning point of the extremal surface, we reconstruct the full BTZ metric, including the radial component $g_{zz}$, without imposing the Einstein equations. The reconstruction yields the correct metric components in all regions of the geometry. We further show that in the zero-temperature limit, our results consistently reduce to the pure AdS$_3$ metric and the familiar vacuum CFT$_2$ entanglement entropies. Our findings demonstrate that entanglement entropy data alone contains sufficient information to uniquely determine the dual gravitational background, offering a concrete realization of holographic emergence where gravity arises from the quantum information structure of the boundary theory.

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arXiv2026

Analytic HTEE in Moving Plasmas and Its Transition

We investigate the holographic timelike entanglement entropy of a boosted $(1+1)$-dimensional plasma within the AdS$_3$/CFT$_2$ correspondence, considering a Lorentz-boosted BTZ black hole background. We solve the coupled extremal-surface equations analytically and obtain a closed-form expression for the HTEE. We show that below a critical boost velocity, the turning points of the extremal surface are purely imaginary, leading to the universal imaginary contribution to the entropy, while the real part acquires a nontrivial dependence on the boost velocity. Moreover, we identify a critical boost velocity at which the complex extremal-surface branch terminates. Beyond this critical velocity, the physically admissible extremal surfaces become purely real, signalling a transition from the holographic timelike entanglement entropy to a spacelike-like real saddle that is geometrically similar to the standard holographic entanglement entropy. The known static BTZ result is recovered in the zero-velocity limit, as expected.

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arXiv2024

Holographic subregion complexity in a moving strongly coupled plasma

We study holographic subregion complexity in a moving strongly coupled plasma in dimensions d = 2, 3, 4, which is holographically dual to a boosted black brane metric in a higher dimensional geometry. The proposal we employ is the one that identifies the complexity of a mixed state by the volume of codimensional-one hypersurface enclosed by Hubeny-Rangamani-Takayanagi surface. Using the finite difference method, the numerical calculations reveal that temperature, velocity, and subregion length all have an increasing effect on holographic subregion complexity. For arbitrary values of temperature and subregion length, as velocity approaches its relativistic upper limit, holographic subregion complexity exhibits a divergence. This divergence behavior observed in d = 2, 3, 4 seems to demonstrate a universal behavior and is characterized by the Lorentz factor squared, γ2.

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arXiv2023

Two Concepts of Holographic Complexity under Thermal and Electromagnetic Quenches

We study the evolution of holographic subregion complexity (HSC) in a thermally and magnetically quenched strongly coupled quantum field theory in 2+1 dimension. We illustrate two concepts of complexity in this theory, (1): how much information it takes to specify a state by studying the behavior of the final value of HSC in terms of the final temperature and magnetic field and (2): how long it takes to reach the state, by considering the time it takes for HSC to relax as a function of the final temperature and magnetic field. In the first concept, we observe that the effect of temperature and magnetic field on HSC is decreasing until the energy of the probe is comparable to the final temperature and magnetic field. We present an argue based on an ensemble of microstates corresponding to a given mixed macrostate. In the second concept, we show that the time of relaxation of HSC decreases with the increase of temperature and magnetic field for fixed value of the energy of the probe. We also compare the time evolution of HSC for two quenches, in the first concept. We observe that the absolute value of the ratio of the final value of HSC for two kinds of quenches depends on the energy of the probe.

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arXiv2022

Resource and stability near a critical point from the quantum information perspective

In the presence of chemical potential and temperature, we holographically study subregion complexity in a non-conformal quantum field theory with a critical point. We propose a new interpretation according to which the states, needing (more) less information to be specified, characterize the (un) stable thermodynamical solutions. We observe the increasing and decreasing effects of chemical potential and temperature on holographic subregion complexity, respectively. These two opposite behaviors lead to a point where subregion complexity of the mixed state is the same as this value for a zero temperature conformal field theory. We also present a new description of the difference between the minimum and the maximum value (the value near the critical point) of holographic subregion complexity as a resource for doing computational work to prepare the state near the critical point from the state far from it. We also calculate the critical exponent.

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arXiv2021

Note on stability and holographic subregion complexity

We study holographic subregion complexity in a spatially anisotropic field theory, which expresses a confinement-deconfinement phase transition. Its holographic dual is a five-dimensional anisotropic holographic model characterized by a Van der Waals-like phase transition between small and large black holes. We propose a new interpretation from the informational perspective to determine the stable and unstable thermodynamically solutions. According to this proposal, the states which need (more) less information to be specified characterize the (un) stable solutions. We similarly offer an interpretation to determine the stable and unstable solutions based on the resource of a computational machine, such that the solutions are (un) stable if computational resource (decreases) increases with the increase of temperature. We observe that the effect of anisotropy on holographic subregion complexity is decreasing. This decreasing effect can be interpreted by considering a whole closed system consisting of the state and its environment in which the complexity of the mixed state decreases and complexity of the environment increases.

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arXiv2021

Complexity and uncomplexity during energy injection

We consider a strongly coupled field theory with a critical point and nonzero chemical potential at finite temperature, which is dual to an asymptotically AdS charged black hole. We study the evolution of the rescaled holographic subregion complexity near and far from the critical point. We explain two distinct concepts of complexity in this theory and discuss that the state under study is complex based on how much information is needed to specify the state and is simple according to how many operations have to be done to reach the state. It has been reported before that time evolution of holographic subregion complexity contradicts the second law of complexity in these AdS-Vaidya-like geometries, but we try to provide a compatible interpretation. We justify decreasing of complexity using an increasing number of microstates of the mixed state and speculate about the description of the relative complexity of the initial state and the final state as a resource. We propose that in this process complexity of the mixed state decreases and complexity of the environment increases. We also observe that in this model, the rescaled holographic subregion complexity is a good observable for probing the dynamical critical exponent.

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arXiv2019

A note on holographic subregion complexity and QCD phase transition

Using holographic subregion complexity, we study the confinement-deconfinement phase transition of quantum chromodynamics. In the model we consider here, we observe a connection between the potential energy of probe meson and the behavior of its complexity. Moreover, near the critical point, at which the phase transition takes place, our numerical calculations indicate that we need less information to specify a meson in the non-conformal vacuum than in the conformal one, despite the fact that the non-conformal vacuum has larger energy!

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arXiv2018

Dynamically probing strongly-coupled field theories with critical point

The dependence of the rescaled equilibration time on different parameters of the field theories with a holographic dual has been investigated in this paper. We consider field theories with nonzero chemical potential at finite temperature which are dual to asymptotically AdS charged black holes. We examine a dynamical probe scalar operator where its dynamics is due to a time-dependent source, quantum quench, or out-of-equilibrium initial condition in field theory with fixed or varying temperature and chemical potential. We observe that the behavior of the scalar operator equilibration time with respect to temperature or chemical potential can not be predicted merely by field theory parameters and depends on how fast the energy is injected into the system. It is shown that in field theories with critical point the rescaled equilibration time is shorter for thermodynamically stable systems. We also observe that the rescaled equilibration time as one approaches the critical point enhances and acquires an infinite slope though its value remains finite. We show that for fast quenches, even though the system is far from equilibrium, the dynamical critical exponent is the same as the one reported for quasi-normal modes in the same background. However for slow quenches the dynamical critical exponent picks up a different value.

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arXiv2018

Meson Excitation at Finite Chemical Potential

We consider a probe stable meson in the holographic quark-gluon plasma at zero temperature and chemical potential. Due to the energy injection into the plasma, the temperature and chemical potential are increased to arbitrary finite values and the meson is also excited. Excitation time tex is the time at which the meson falls into the final excited state. We study the effect of various parameters of theory on the excitation time and observe that for larger values of final temperature and chemical potential the excitation time increases. Furthermore, our outcomes show that the more stable mesons are excited sooner.

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arXiv2015

Chiral Magnetic Effect in the Anisotropic Quark-Gluon Plasma

An anisotropic thermal plasma phase of a strongly coupled gauge theory can be holographically modelled by an anisotropic AdS black hole. The temperature and anisotropy parameter of the AdS black hole background of interest [1] is specified by the location of the horizon and the value of the Dilaton field at the horizon. Interestingly, for the first time, we obtain two functions for the values of the horizon and Dilaton field in terms of the temperature and anisotropy parameter. Then by introducing a number of spinning probe D7-branes in the anisotropic background, we compute the value of the chiral magnetic effect (CME). We observe that in the isotropic and anisotropic plasma the value of the CME is equal for the massless quarks. However, at fixed temperature, raising the anisotropy in the system will increase the value of the CME for the massive quarks.

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arXiv2014

Meson Life Time in the Anisotropic Quark-Gluon Plasma

In the hot (an)isotropic plasma the meson life time $τ$ is defined as a time scale after which the meson dissociates. According to the gauge/gravity duality, this time can be identified with the inverse of the imaginary part of the frequency of the quasinormal modes, $ω_I$, in the (an)isotropic black hole background. In the high temperature limit, we numerically show that at fixed temperature(entropy density) the life time of the mesons decreases(increases) as the anisotropy parameter raises. For general case, at fixed temperature we introduce a polynomial function for $ω_I$ and observe that the meson life time decreases. Moreover, we realize that $(s/T^3)^6$, where $s$ and $T$ are entropy density and temperature of the plasma respectively, can be expressed as a function of anisotropy parameter over temperature. Interestingly, this function is a Padé approximant.

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arXiv2014

Chiral Symmetry Breaking: To Probe Anisotropy and Magnetic Field in QGP

We discuss the (spontaneous) chiral symmetry breaking in a strongly coupled anisotropic quark-gluon plasma (QGP) in the presence of the magnetic field, using holography. The physical quantities related to the chiral symmetry breaking (m;B_c) distinguish between the effects of the anisotropy and magnetic field on the plasma. Anisotropy affects the system similar to the temperature and for its larger values heavier quarks can live in the QGP without getting condensed. Raising the anisotropy in the system will also increase the value of the critical magnetic field, B_c, at which the spontaneous chiral symmetry breaking happens. Both of these growths are even more when the magnetic field is applied perpendicular to the anisotropy direction. Such behaviour persists in the high temperature limit where the temperature is kept fixed. However, when the entropy density is held fixed, as one increases the anisotropy, lighter mesons melt when the magnetic field is applied along the anisotropy direction, in contrast to when the magnetic field is perpendicular to the anisotropy direction.

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arXiv2013

Non-equilibrium Phase Transition from AdS/CFT

Using AdS/CFT correspondence we study non-equilibrium phase transition in the presence of a constant external magnetic field. The transition occurs when the sign of differential conductivity reverses. Utilizing numerical method we show that the type of transition depends on the value of magnetic field as well as the temperature of gauge theory. Moreover we show that this transition does not depend on the supersymmetry and the subspace on which the fundamental matter fields live.

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arXiv2013

Thermalization in External Magnetic Field

In the AdS/CFT framework meson thermalization in the presence of a constant external magnetic field in a strongly coupled gauge theory has been studied. In the gravitational description the thermalization of mesons corresponds to the horizon formation on the flavour D7-brane which is embedded in the AdS_5 x S^5 background in the probe limit. The apparent horizon forms due to the time-dependent change in the baryon number chemical potential, the injection of baryons in the gauge theory. We will numerically show that the thermalization happens even faster in the presence of the magnetic field on the probe brane. We observe that this reduction in the thermalization time sustains up to a specific value of the magnetic field.

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arXiv2012

Meson Thermalization in Various Dimensions

In gauge/gravity duality framework the thermalization of mesons in strongly coupled (p+1)-dimensional gauge theories is studied for a general Dp-Dq system, q>=p, using the flavour Dq-brane as a probe. Thermalization corresponds to the horizon formation on the flavour Dq-brane. We calculate the thermalization time-scale due to a time-dependent change in the baryon number chemical potential, baryon injection in the field theory. We observe that for such a general system it has a universal behaviour depending only on the t'Hooft coupling constant and the two parameters which describe how we inject baryons into the system. We show that this universal behaviour is independent of the details of the theory whether it is conformal and/or supersymmetric.

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arXiv2011

Rotating strings and energy loss in non-conformal holography

We study the energy lost by an accelerating quark probe in the quark-gluon plasma produced in the heavy ion collisions in an approximate setting where the acceleration of the probe is due to uniform circular motion. The energy loss rate of the rotating probe is calculated at strong coupling in the confining SU(N) gauge theory based on N D4 branes on a circle, using the rotating string solutions in the dual gravitational background. The system is known to exhibit a confinement-deconfinement transition at a finite temperature T_c. We investigate energy loss both in the low and the high T phases. The high T phase is similar to the previously studied case of the conformal plasma, yet we find qualitative differences due to non-conformality of the underlying theory. The low T phase, on the other hand exhibits novel interesting behavior: We find a dual gravitational mechanism that yields a lower bound on the emitted energy of the rotating quark, proportional to the mass gap in the glueball spectrum. The low T energy loss is argued to be completely due to glueball brehmstrahlung, hence the energy loss rate calculated here determines the Lienard potential for syncrotron radiation in this confining gauge theory at strong coupling.

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