arXiv subjects

Shuta Ishigaki

Publications and source records attributed to Shuta Ishigaki.

13 recordsLinked to original sources

arXiv2026

Quantum Mpemba effect in holography

We investigate the quantum Mpemba effect in a holographic superfluid, in which states with stronger initial symmetry breaking relax faster toward the symmetry-restored equilibrium. We demonstrate its emergence by identifying the shifted free energy computed from the energy flux into the black hole horizon as monotonic distance measure. By decomposing the nonlinear bulk dynamics based on quasinormal modes, we reveal that the anomalous relaxation is governed by a dynamical competition in which the slowest-decaying mode is suppressed while the second mode is amplified. These findings provide a holographic perspective on the quantum Mpemba effect in nonequilibrium relaxation involving strongly coupled degrees of freedom.

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arXiv2026

Scheme dependence and instability of double-trace deformations for gauge fields in AdS$_5$

In holography, double-trace deformations provide a general framework for deforming boundary field theories. In particular, they can be utilized to introduce dynamical gauge fields in the boundary theory through double-trace deformations of bulk gauge fields. In this work, we study this construction in the case where the bulk geometry is asymptotically AdS$_5$, and find that such a system involves tachyon and ghost modes. This instability originates from the logarithmic behavior of the gauge fields in the vicinity of the AdS boundary, which leads to a scheme-dependent ambiguity in the double-trace deformation. We investigate this instability by using both analytical and numerical methods in several holographic setups, including bottom-up models and the top-down D3-D7 construction.

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arXiv2025

Dynamical Stability and Critical Exponents of the Neutral (S-type) Gubser-Rocha Model with Momentum Dissipation

The (S-type) Gubser-Rocha model is a holographic model that shows the linear dependence of the entropy density on the temperature. With an appropriate choice of the boundary action, this model exhibits a continuous phase transition in the neutral limit. In this paper, we investigate several aspects of this phase transition. Firstly, we show that the critical exponents of the phase transition match those in the mean-field percolation theory. Subsequently, we also investigate the dynamical stability, and the emergence of the Nambu-Goldstone modes by analyzing the quasinormal modes of the perturbation fields. The dynamical stability agrees with the thermodynamic stability. In addition, we find that there is an emergent Nambu-Goldstone mode in the broken phase of the S-type model.

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arXiv2025

Dependence of the critical temperature and disorder in holographic superconductors on superfluid density

Recent experiments strongly indicate deep connections between transports of strange metal and high $T_c$ superconductors. For instance, it is known that the dependence of the zero-temperature phase stiffness on the critical superconducting temperature becomes linear in underdoped materials. In this paper, we investigate relation meticulously between the phase stiffness and the critical superconducting temperature for the Gubser-Rocha holographic superconductor model in the probe limit. The superfluid density (or phase stiffness) can be extracted from the low-frequency dependence of the AC conductivity. More importantly, we find linear dependence of zero-temperature superfluid density on the critical superconducting temperature, which has a similarity to a recent experiment in a film cuprate. In addition, we also provide some approximate formulas for the critical temperatures and the AC conductivity.

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arXiv2024

Thermodynamics, magnetic properties, and global $U(1)$ symmetry breaking of the S-type Gubser-Rocha model

We study an explicit formula for the thermodynamic potential of the AdS dyonic black brane solution with an axio-dilaton hair, which was discovered in an extension of the $(3+1)$d Gubser-Rocha model enjoying S-duality. From the thermodynamic potential, we can compute the magnetization and the magnetic susceptibilities of the dyonic solution. The result of the magnetization is negative implying that the system is diamagnetic. Subsequently, we consider a specific neutral limit of the dyonic solution. In this setup, we find that the system exhibits spontaneous breaking of a global $U(1)$ symmetry. The order parameter is given by a complex operator which is dual to the axio-dilaton field in the bulk. Interestingly, the system has a finite Hall conductivity even in the absence of the external magnetic field, and it is related to the phase of the complex operator.

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arXiv2024

Patchwork Conditions for Holographic Nonlinear Responses: A Computational Method for Electric Conductivity and Friction Coefficient

We propose a new method to compute nonlinear transport coefficients in holography, such as nonlinear DC conductivity and nonlinear friction coefficient. The conventional method can be applied only to the models whose action in the gravity dual has the ``square-root structure,'' i.e., the Dirac-Born-Infeld action of the probe D-branes or the Nambu-Goto action of the probe strings. Our method is applicable to a wider range of holographic models whose action does not have such a square-root structure. We propose a condition to obtain regular physical configurations in the gravity dual in the form of two simultaneous equations, which we call the patchwork condition. Our method also enables us to estimate the effective temperature of the nonequilibrium steady states in a wider range of holographic models. We show that a general model exhibits different effective temperatures for different fluctuation modes.

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arXiv2024

First order phase transition in the D3-D7 model from the point of view of the fermionic spectral functions

We consider the D3-D7 model and use the spectral function of a probe fermion on D7 to analyze the first order phase transition from the black-hole embedding phase to another black-hole embedding phase in the presence of the finite density and temperature. From the fermionic spectral functions, we study the temperature dependence of the decay rate, and we observe various phenomena that support the first order phase transition including jump in it at the critical temperature that corresponds to the first order phase transition.

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arXiv2024

A domain wall and chiral edge currents in holographic chiral phase transitions

We investigate spatially inhomogeneous solutions in a top-down holographic model: the D3/D7 model which provides a holographic description of the chiral phase transition for a finite external magnetic field, chemical potential, and temperature. We numerically find a domain wall (or kink) solution in the three dimensional space, which incorporates between the chiral symmetry broken phase at the spatial infinity, under the homogeneous sources. Along with the inhomogeneity of the chiral condensate, the charge density is also spatially modulated. The modulated charge density and finite magnetic field lead to the chiral edge current close to the domain wall. We explore the dependences of those profiles on the chemical potential and temperature near the first and second order phase transition points. Our results indicate that the inhomogeneous solutions we found are in good agreement with those obtained by the Ginzburg--Landau theory in the vicinity of the transition points.

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arXiv2024

Doped Holographic Superconductors in Gubser-Rocha model

We construct a doped holographic superconductor in the Gubser-Rocha model, and realize a superconducting dome in the middle of the temperature-doping phase diagram. It is worth noting that unlike the previous researches, the profile of our dome shrinks inward near zero temperature. From the numerical observation for the coupling dependence of the phase diagram, we find that the coupling between the two gauge fields plays a crucial role in the formation of dome. We also analytically calculate the DC conductivity of the normal phase of the system in the momentum dissipation and obtain the resistivity which is proportional to temperature. The AC conductivity is calculated numerically.

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arXiv2023

Stark effect and dissociation of mesons in holographic conductor

We study the meson spectrum of the ${\cal{N}}=4$ supersymmetric Yang-Mills theory with ${\cal{N}}=2$ fundamental hypermultiplets for a finite electric field by using the D3/D7 model. The spectrum for scalar and vector mesons is computed by analyzing the (quasi-)normal modes for the fluctuations of the D7-brane embedding and gauge fields. In the presence of an electric field, two different phases in the background are realized: the meson and dissociation phases. In this paper, we analyze the meson spectrum of scalar and vector mesons for all ranges of the electric field and explore the effect of the electric field on the meson spectrum, that is, the Stark effect. In the meson spectrum, we observe the avoided crossing between different levels due to the coupling of fluctuations via the electric field.

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arXiv2022

Dynamical stability and filamentary instability in holographic conductors

In this study, we analyze the dynamical stability of the D3-D7 model dual to a holographic conductor with a constant current under an external electric field. We particularly focus on the stability around the parameter region where the multivalued relation between the external electric field and the current is shown due to nonlinear conductivity. The dynamical stability of the states can be analyzed by considering linear perturbations in the background states and computing the quasinormal modes. In the multivalued region, we find that the states in one branch with a low electric current can be dynamically unstable. The turning point in the $J$-$E$ characteristic coincides with the stability switching. Further, we also find that the perturbations around the unstable states can become stable with finite wavenumber. In other words, the perturbations in the background states become static at a critical wavenumber, implying the existence of inhomogeneous steady states with current filaments.

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arXiv2021

Nambu-Goldstone modes in non-equilibrium systems from AdS/CFT correspondence

We investigate dispersion relation of Nambu-Goldstone modes in a dissipative system realized by the AdS/CFT correspondence. We employ the D3/D7 model which represents ${\cal N} = 4$ supersymmetric Yang-Mills theory coupled to ${\cal N = 2}$ flavor fields. If we consider massless quarks in the presence of an external magnetic field, the system exhibits the phase transition associated with the spontaneous symmetry breaking of the chiral symmetry. We find that the Nambu-Goldstone modes shows a diffusive behavior in the dispersion relation, which agrees with that found with the effective field theory approach. We also study a non-equilibrium steady state which has a constant current flow in the presence of an external electric field. In a non-equilibrium steady state, we find that the Nambu-Goldstone modes shows a linear dispersion in the real part of the frequency in addition to the diffusive behavior. Moreover, we analyze the linear dispersion of the Nambu-Goldstone modes in the hydrodynamic approximation. As a result, we find that the linear dispersion can be written as the analytic functions of quantities in the dual field theory. Our results imply that such a linear dispersion is a characteristic behavior of Nambu-Goldstone modes in a non-equilibrium steady state.

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arXiv2020

Mechanism for Negative Differential Conductivity in Holographic Conductors

We clarify the mechanism for negative differential conductivity in holographic conductors. Negative differential conductivity is a phenomenon in which the electric field decreses with the increase of the current. This phenomenon is widely observed in strongly correlated insulators, and it has been known that some models of AdS/CFT correspondence (holographic conductors) reproduces this behaviour. We study the mechanism for negative differential conductivity in holographic conductors by analyzing the lifetime of the bound states of the charge carriers. We find that when the system exhibits negative differential conductivity, the lifetime of the bound states grows as the electric field increases. This suggests that the negative differential conductivity in this system is realized by the supression of the ionization of the bound states that supplies the free carriers.

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