arXiv subjects

Xian-Hui Ge

Publications and source records attributed to Xian-Hui Ge.

At least 55 records · Page 3Linked to original sources

arXiv2023

Stationary equilibrium of test particles near charged black branes with the hyperscaling violating factor

We explore the upper bound of the Lyapunov exponent for test particles that maintain equilibrium in the radial direction near the charged black brane with the hyperscaling violating factor. The influences of black brane parameters (hyperscaling violation exponent $θ$ and dynamical exponent $z$) are investigated. We show that the equilibrium in the radial direction of test particles can violate the chaos bound. The chaos bound is more easily violated for the near-extremal charged black branes. When the null energy condition ($T_{μν}ξ^μξ^ν\geq 0$) is broken, the bound is also more likely to be violated. These results indicate that the chaos bound of particle motion is related to the temperature of the black hole and the null energy condition (NEC). By considering the zero-temperature and $T_{μν}ξ^μξ^ν=0$ cases, we obtain the critical parameters $θ_c$ and $z_c$ for the violation of chaos bound. The chaos bound is always satisfied in the range $θ> θ_c$ or $z>z_c$.

hep-th↗

arXiv2023

Page Curve and Phase Transition in deformed Jackiw-Teitelboim Gravity

We consider the entanglement island in a deformed Jackiw-Teitelboim black hole in the presence of the phase transition. This black hole has the van der Waals-Maxwell-like phase structure as it is coupled with a Maxwell field. We study the behavior of the Page curve of this black hole by using the island paradigm. In the fixed charge ensemble, we discuss different situations with different charges that influence the system's phase structure. There is only a Hawking-Page phase transition in the absence of charges, which leads to an unstable small black hole. Hence, the related Page curve does not exist. However, a van der Waals-Maxwell-like phase transition occurs in the presence of charges. This yields three black hole solutions. The Page curve of the middle size black hole does not exist. For the extremal black hole, the Page time approaches zero in the phase transition situation but becomes divergent without the phase transition. In a word, we study the Page curve and the island paradigm for different black hole phases and in different phase transition situations.

hep-th↗

arXiv2022

Non-Hermitian quantum system generated from two coupled Sachdev-Ye-Kitaev models

We show that a non-Hermitian two coupled Sachdev-Ye-Kitaev (SYK) model can provide thermodynamic structure equivalent to Hermitian two coupled SYK model. The energy spectrum, the entanglement degree of the ground states and the low energy effective action of this model are not influenced by the non-Hermiticity. The novel biorthogonal ground states demonstrates that two SYK sites, one of which can be in the ground state and the other in the Schwarzian excited state by tuning the non-Hermiticity. We find evidence that the free energy is independent of the non-Hermiticity.

hep-th↗

arXiv2022

Entanglement Islands in Generalized Two-dimensional Dilaton Black Holes

The Fabbri-Russo model is a generalized model of a two-dimensional dilaton gravity theory with various parameters "$n$" describing various specific gravities. Particularly, the Russo-Susskind-Thorlacius gravity model fits the case $n=1$. In the Fabbri-Russo model, we investigate Page curves and the entanglement island. Islands are considered in eternal and evaporating black holes. Surprisingly, in any black hole, the emergence of islands causes the rise of the entanglement entropy of the radiation to decelerate after the Page time, satisfying the principle of unitarity. For eternal black holes, the fine-grained entropy reaches a saturation value that is twice the Bekenstein-Hawking entropy. For evaporating black holes, the fine-grained entropy finally reaches zero. The parameter "$n$" significantly impacts the Page curve at extremely early times. However, at late times and large distance limit, the impact of the parameter "$n$" is a subleading term and is exponentially suppressed. As a result, the shape of Page curves is "$n$"-independent in the leading order. Furthermore, we discuss the relationship between islands and firewalls. We show that the island is a better candidate than firewalls for encountering the quantum entanglement-monogamy problem. Finally, we briefly review the gravity/ensemble duality as a potential resolution to the state conundrum resulting from the island formula.

hep-th↗

arXiv2022

Scrambling time for analogue black holes embedded in AdS space

We propose a gedanken experiment on realizing thermofield double state (TFD) by using analog black holes and provide an approach to test the scrambling time. Through this approach, we demonstrate clearly how shock wave changes the TFD state as time evolves. As the whole system evolves forward in time, the perturbation of space-time geometry will increase exponentially. Finally, it will destroy the entanglement between the two states of the thermal field, and the mutual information between them is reduced to zero in the time scale of scrambling. The results show that for perturbations of analogue black holes embedded in AdS space, the scale of the scrambling time is closely related to the logarithm of entropy of the black hole. The results provide further theoretical argument for the scrambling time, which can be further falsified in experiments.

hep-th↗

arXiv2022

Toward the nonequilibrium thermodynamic analog of complexity and the Jarzynski identity

The Jarzynski identity can describe small-scale nonequilibrium systems through stochastic thermodynamics. The identity considers fluctuating trajectories in a phase space. The complexity geometry frames the discussions on quantum computational complexity using the method of Riemannian geometry, which builds a bridge between optimal quantum circuits and classical geodesics in the space of unitary operators. Complexity geometry enables the application of the methods of classical physics to deal with pure quantum problems. By combining the two frameworks, i.e., the Jarzynski identity and complexity geometry, we derived a complexity analog of the Jarzynski identity using the complexity geometry. We considered a set of geodesics in the space of unitary operators instead of the trajectories in a phase space. The obtained complexity version of the Jarzynski identity strengthened the evidence for the existence of a well-defined resource theory of uncomplexity and presented an extensive discussion on the second law of complexity. Furthermore, analogous to the thermodynamic fluctuation-dissipation theorem, we proposed a version of the fluctuation-dissipation theorem for the complexity. Although this study does not focus on holographic fluctuations, we found that the results are surprisingly suitable for capturing their information. The results obtained using nonequilibrium methods may contribute to understand the nature of the complexity and study the features of the holographic fluctuations.

hep-th↗

arXiv2022

Holographic heat engine efficiency of hyperbolic charged black holes

We consider a four-dimensional charged hyperbolic black hole as working matter to establish a black hole holographic heat engine, and use the rectangular cycle to obtain the heat engine efficiency. We find that when the increasing of entropy is zero, the heat engine efficiency of the hyperbolic black hole becomes the well-known Carnot efficiency. We also find that less charge corresponds to higher efficiency in the case of q > 0. Furthermore, we study the efficiency of the flat case and spherical case and compare the efficiency with that of the hyperbolic charged black holes. Finally, we use numerical simulation to study the efficiency in benchmark scheme.

gr-qc↗

arXiv2022

Circular Motion of Charged Particles near Charged Black Hole

We study the circular motion of charged particles near a charged black hole. The general form of the Lyapunov exponent of charged particles is obtained by using the Jacobian matrix. The results show that the chaos bound can be saturated by the circular motion of charged particles on the horizon. By further expanding the Lyapunov exponent near the horizon and investigating Reissner-Nordström(RN) black holes with different $M/Q$, we find that in contrast to the static equilibrium, the circular motion of charged particles can have a larger Lyapunov exponent due to the existence of angular momentum. For the RN black holes which have the mass-charge ratio $1<M/Q <1.1547$, the chaos bound is locally violated by the null and the time-like circular motion with large angular momentum. As an illustration of the universality of our results, we study the charged particles' circular motion near the Reissner-Nordström Anti-de Sitter (RN-AdS) black hole and find that the null and the time-like circular motion with large angular momentum can exceed the chaos bound under the background of RN-AdS black holes with the mass-charge ratio in the range $1.23132<M/Q<1.75225$.

hep-th↗

arXiv2022

Island, Page Curve and Superradiance of Rotating BTZ Black Holes

We study the Page curve and the information paradox for the BTZ black hole coupled to two thermal baths by applying the island paradigm. We prove that as the island locates outside the event horizon, the entanglement entropy of Hawking radiation for finite temperature black holes obeys the Page curve. However, for extremal rotating black holes, the Page time and the scrambling time become divergent. To avoid the ill-defined of the Page time, we consider the contribution of superradiance, which is a process that can extract rotational energy from black holes. The superradiance continues in a period much shorter than the Page time when the central charge $c$ is not too large. In this process, the Page time, the scrambling time and the black hole thermal entropy all decrease. Whenever the superradiance finishes, the black hole turn out to be a neutral black hole and the Hawking radiation dominates. The Page curve can then be well reproduced.

hep-th↗

arXiv2021

Analogue of the pole-skipping phenomenon in acoustic black holes

The pole-skipping phenomenon is a special property of the retarded Green's function of black hole perturbations. We turn to its analog in acoustic black holes, which may relate to experiments. The frequencies of these special points are located at negative integer (imaginary) Matsubara frequencies $ω=-i2πTn$, which are consistent with the imaginary frequencies of quasinormal modes (QNMs). This implies that the lower-half plane pole-skipping phenomena have the same physical meaning as the imaginary part of QNMs, which represents the dissipation of perturbation of acoustic black holes and is related to the instability time scale of perturbation.

hep-th↗

arXiv2021

Chaos of Particle Motion near the Black Hole with Quasi-topological Electromagnetism

We explore the chaotic behavior of particle motion in a black hole with quasi-topological electromagnetism. The chaos bound is found to be violated in the higher order expansion of the metric function and the electric potential near the horizon. We draw the Poincare sections of particle motion corresponding to the chaos bound violated and non-violated cases, respectively. Then we study the relationship between the maximal Lyapunov exponent λ_s defined by the static equilibrium and the Lyapunov exponent of the particle geodesic motion near the Reissner-Nordstrom(RN) black hole and the black hole with quasi-topological electromagnetism. We find an interesting relationship between the Lyapunov exponent λ_{ph} of photon's radial falling into the black hole and the maximal Lyapunov exponent λ_s. For the black holes whose metric function increases monotonically with radius outside horizon, this leads to λ_{ph} \geq 2λ_s.

hep-th↗

arXiv2021

Page Curves and Islands in Charged Dilaton Black Holes

We study the Page curve in the four dimensional asymptotically flat eternal Garfinkle-Horowitz-Strominger dilaton black holes by applying the "quantum extremal surface" prescription. The results demonstrate that without island, the entanglement entropy of Hawking radiation is proportional to time and divergent at late times. While taking account of the emergence of the island extending slightly outside the event horizon, the entanglement entropy of Hawking radiation stops growing and eventually reaches a saturation value, which is twice of the Bekenstein-Hawking entropy and consistent with the finiteness of the von Neumann entropy of eternal black holes. Moreover, we discuss the impact of the parameter $n$ and charge $Q$ on the Page time. The influence of $n$ on it can neglected when the ratio of the charge $Q$ to the black hole mass $M$ is sufficiently small, but the charge $Q$ has a significant impact on Page time. Importantly, at the extremal case, the Page time becomes divergent or vanishing, in which the semiclassical theory needs further investigation.

hep-th↗

arXiv2021

Pole-skipping and hydrodynamic analysis in Lifshitz, AdS$_2$ and Rindler geometries

The "pole-skipping" phenomenon reflects that the retarded Green's function is not unique at a pole-skipping point in momentum space $(ω,k)$. We explore the universality of the pole-skipping in different geometries. In holography, near horizon analysis of the bulk equation of motion is a simpler way to derive a pole-skipping point and we use this method in Lifshitz, AdS$_2$ and Rindler geometries. We also study the complex hydrodynamic analyses and find that the dispersion relations in terms of dimensionless variables $\fracω{2πT}$ and $\frac{\vert k\vert}{2πT}$ pass through pole-skipping points $(\frac{ω_n}{2πT}, \frac{\vert k_n\vert}{2πT}$) at small $ω$ and $k$ in Lifshitz background. We verify that the position of the pole-skipping points does not depend on the standard quantization or alternative quantization in the boundary theory in AdS$_2\times\mathbb{R}^{d-1}$ geometry. In Rindler geometry, we cannot find the corresponding Green's function to calculate pole-skipping points because it is difficult to impose the boundary condition. However we can obtain "special points" near horizon where bulk equations of motion have two incoming solutions. These "special points" correspond to nonunique of the Green's function in physical meaning from the perspective of holography.

hep-th↗

arXiv2021

Black holes in 4D Einstein-Maxwell-Gauss-Bonnet gravity coupled with scalar fields

Einstein-Maxwell-Gauss-Bonnet-axion theory in $4$-dimensional spacetime is investigated in this paper through a "Kaluza-Klein-like" process. Dual to systems at finite temperature with background magnetic field on three dimensions, the four-dimensional dyonic black hole solution coupled with higher derivative terms is obtained. After the tensor-type perturbation is added, the shear viscosity to entropy density ratio is calculated at high temperature and low temperature separately. The behaviour of shear viscosity to entropy density ratio of uncharged black holes is found to be similar with that in $5$-dimensional spacetime, violating the Kovtun-Starinets-Son bound as well when temperature becomes lower. In addition, the main feature of this ratio remains almost unchanged in $4$ dimensions, which is characterised by $(T/Δ)^2$ at low temperature $T$, with $Δ$ proportional to the coefficient $β$ from scalar fields. The difficulty in causal analysis is also discussed, which is mainly caused by the vanishing momentum term in equations of motion.

hep-th↗

arXiv2021

Superconducting gap ratio from strange metal phase in the absence of quasiparticles

A lattice model for strongly interacting electrons motivated by a rank-3 tensor model provides a tool for understanding the pairing mechanism of high-temperature superconductivity. This SYK-like model describes the strange metal phase in the cuprate high temperature superconductors. Our calculation indicates that the superconducting gap ratio in this model is higher than the ratio in the BCS theory due to the coupling term and the spin operator. Under certain conditions, the ratio also agrees with the BCS theory. Our results relate to the case of strong coupling, so it may pave the way to gaining insight into the cuprate high temperature superconductors.

cond-mat.str-el↗

arXiv2020

Deep learning black hole metrics from shear viscosity

Based on AdS/CFT correspondence, we build a deep neural network to learn black hole metrics from the complex frequency-dependent shear viscosity. The network architecture provides a discretized representation of the holographic renormalization group flow of the shear viscosity and can be applied to a large class of strongly coupled field theories. Given the existence of the horizon and guided by the smoothness of spacetime, we show that Schwarzschild and Reissner-Nordström metrics can be learned accurately. Moreover, we illustrate that the generalization ability of the deep neural network can be excellent, which indicates that by using the black hole spacetime as a hidden data structure, a wide spectrum of the shear viscosity can be generated from a narrow frequency range. These results are further generalized to an Einstein-Maxwell-dilaton black hole. Our work might not only suggest a data-driven way to study holographic transports but also shed some light on holographic duality and deep learning.

hep-th↗

arXiv2020

On the geometry outside of acoustic black holes in $2+1$-dimensional spacetime

Analogue black holes, which can mimic the kinetic aspects of real black holes, have been proposed for many years. The growth of the radial momentum of test particles toward the acoustic horizon is calculated for acoustic black holes in flat and curved spacetimes. Surprisingly, for a freely infalling vortex approaching the acoustic black hole, the Lyapunov exponent of the growth of the momentum at the horizon saturates the chaos bound $Λ_{\rm Lyapunov}\leq 2 πT$. We investigate the orbits of test vortices and sound wave rays in the $2+1$-dimensional "curved" spacetime of an acoustic black hole. We show that the vortices orbit, the sound wave orbit, and the time delay of sound are similar to those famous effects of general relativity. These effects can be verified experimentally in future experiments.

hep-th↗

arXiv2020

Causality of black holes in 4-dimensional Einstein-Gauss-Bonnet-Maxwell theory

We study charged black hole solutions in 4-dimensional (4D) Einstein-Gauss-Bonnet-Maxwell theory to the linearized perturbation level. We first compute the shear viscosity to entropy density ratio. We then demonstrate how bulk causal structure analysis imposes a upper bound on the Gauss-Bonnet coupling constant in the AdS space. Causality constrains the value of Gauss-Bonnet coupling constant $α_{GB}$ to be bounded by $α_{GB}\leq 0$ as $D\rightarrow 4$.

hep-th↗