arXiv subjects

Yunseok Seo

Publications and source records attributed to Yunseok Seo.

At least 19 recordsLinked to original sources

arXiv2026

D-instanton Effects on the Holographic Weyl Semimetals

We investigate the effects of D-instantons on a holographic Weyl semimetal in a top-down approach. We find that the presence of D-instantons induces a novel phase transition from a Weyl semimetal to a gapped phase, providing a new mechanism for gap formation in strongly coupled systems. By analyzing the free energy of probe D7-brane embeddings, we construct the phase diagram in terms of the fermion mass, instanton density, and temperature, measured in units of the Weyl parameter. We show that, in addition to the conventional mass-driven transition associated with band inversion, the instanton density gives rise to a qualitatively different transition whose mechanism cannot be understood within the standard Dirac picture. We compute nonlinear electric conductivities from the regularity condition at the black hole horizon and demonstrate that the two types of transitions exhibit distinct transport behavior. In particular, the instanton-induced transition leads to a gapped phase with properties that differ from the conventional insulating phase. We also discuss possible interpretations of this phase in the boundary theory and comment on its relation to nonperturbative effects in strongly interacting systems.

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arXiv2026

Rotating End of the World

We study the thermodynamics and interior structures of dynamical end of the world (EoW) branes in the rotating BTZ black hole. By mapping the induced metric of the branes to an effective Jackiw-Teitelboim (JT) system, we derive the first law of thermodynamics for the boundary conformal field theory (BCFT), incorporating boundary degrees of freedom. To construct this thermodynamics, we adopt two frameworks of black hole chemistry and the duality between the JT black hole and the Sachdev-Ye-Kitaev (SYK) model. In addition, we verify that the shadow entropy is equivalent to the boundary entropy via Hubeny-Ryu-Takayanagi (HRT) surfaces. Furthermore, we explore the possible interior configurations of the EoW brane inside the horizon. Two representative configurations, namely single and double-joint EoW branes, are considered. These disparate configurations share the same exterior brane configuration outside the horizon. By comparing their energies, we show that a transition could occur within the horizon.

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arXiv2025

Phase Diagram from Nonlinear Interaction between Superconducting Order and Density: Toward Data-Based Holographic Superconductor

We address an inverse problem in modeling holographic superconductors. We focus our research on the critical temperature behavior depicted by experiments. We use a physics-informed neural network method to find a mass function $M(F^2)$, which is necessary to understand phase transition behavior. This mass function describes a nonlinear interaction between superconducting order and charge carrier density. We introduce positional embedding layers to improve the learning process in our algorithm, and the Adam optimization is used to predict the critical temperature data via holographic calculation with appropriate accuracy. Consideration of the positional embedding layers is motivated by the transformer model of natural-language processing in the artificial intelligence (AI) field. We obtain holographic models that reproduce borderlines of the normal and superconducting phases provided by actual data. Our work is the first holographic attempt to match phase transition data quantitatively obtained from experiments. Also, the present work offers a new methodology for data-based holographic models.

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arXiv2023

End of the World Perspective to BCFT

In this work, we study the end-of-the-world (EOW) branes anchored to the boundaries of BCFT${}_2$ dual to the BTZ black hole. First, we explore the thermodynamics of the boundary system consisting of the conformal boundary and two EOW branes. This thermodynamics is extended by the tension appearing as the effective cosmological constant of JT black holes on the EOW branes. The tension contribution is identified with the shadow entropy equivalent to the boundary entropy of the BCFT${}_2$. The thermodynamics of the JT black holes and the bulk of BCFT${}_2$ can be combined into a novel grafted thermodynamics based on the first law. Second, we focus on the observer's view of the EOW branes by lowering the temperature. We show that the EOW branes generate a scale called ``reefs" inside the horizon. This scale also appears in the grafted thermodynamics. At high temperatures, observers on the EOW branes see their respective event horizons. The reef starts to grow relatively to the horizon size at the temperature, $T_{grow}$. As the temperature cools down the reef area fills the entire interior of the JT black holes at the temperature $T_{out}$. Then, the observers recognize their horizons disappear and see the large density of the energy flux. At this temperature, the two JT regions become causally connected. This connected spacetime has two asymptotic $AdS_2$ boundaries with a conformal matter. Also, we comment on the grafted thermodynamics to higher dimensions in Appendix B.

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arXiv2023

Construction of Superconducting Dome and Emergence of Quantum Critical Region in Holography

In this work, we investigate an extended model of holographic superconductor by a non-linear electrodynamic interaction coupled to a complex scalar field. This non-linear interaction term can make a quantum phase transition at zero temperature with finite charge carrier density. By solving full equations of motion, we can construct various shapes of the superconducting phase in the phase diagram. With a specific choice of interaction coefficients, we can construct a phase diagram with a superconducting dome. Also, we find a new geometric solution inside the superconducting dome, which turns out to be a Lifshitz-type geometry. This geometry is characterized by a dynamical critical exponent, which plays a crucial role near the quantum critical point. We refer to this region in the phase diagram as a `quantum critical region.'

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arXiv2023

Impurity-Driven Metal-Insulator Transitions in Holography

In this work, we study Metal-Insulator transition in a holographic model containing an interaction between the order parameter and charge-carrier density. It turns out that the impurity density of this model can drive the phase transition whose ordered phase corresponds to the insulating phase. The temperature behavior of DC conductivity distinguishes the insulating phase from the metal phase. We confirm this behavior by a numerical method and an analytic calculation. As a byproduct, we show the existence of a `quantum phase transition' supported by the Breitenlohner-Freedman bound argument.

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arXiv2021

Impurity Effect on Hysteric Magnetoconductance: Holographic Approach

In this paper we study a hysteric phase transition from weak localization phase to hysteric magnetoconductance phase using gauge/gravity duality. This hysteric phase is triggered by a spontaneous magnetization related to Z2 symmetry and time reversal symmetry in a 2+1 dimensional system with momentum relaxation. We derive thermoelectric conductivity formulas describing non-hysteric and hysteric phases. At low temperatures, this magneto-conductance shows similar phase transitions of topological insulator surface states. We also obtain hysteresis curves of Seebeck coefficient and Nernst signal. It turns out that our impurity parameter changes magnetic properties of the dual system. This is justified by showing increasing susceptibility and the spontaneous magnetization with increasing impurity parameter.

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arXiv2021

Phase Transition in JT Gravity and $T\bar{T}$ Deformation

In this paper we study a black hole phase transition in a generalized JT gravity noticed in arXiv:2006.03494. We investigate the effect of the phase transition on the Euclidean geodesic and holographic two-point function in models with dilaton potential which interpolates two ordinary JT gravities with different cosmological constants. It is noted that there exists a closed geodesic with a new scale at low temperature phase when the potential has a locally negative region. This scale causes several peaks in the two-point function. We also comment on the phase transition of charged black holes. We then consider coupling generalized JT gravity to a matter and study its relation to a $T\bar{T}$ deformation of CFT at the classical level. We find the deformation parameter as a function of the dilaton and provide examples showing Janus-type couplings.

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arXiv2020

Determination of Dynamical exponents of Graphene at quantum critical point by holography

We calculate the transport of a theory with two conserved currents by holographic method and compare it with graphene data to determine its dynamical exponents $(z,θ)$ which characterizes a QCP. As a result, we find that the electric and the thermal conductivity data can be fit much more naturally if we assume $(z,θ)=(3/2,1)$ rather than $(1,0)$. Furthermore, we find that thermoelectric power data at high temperature can be fit if we use $(3/2,1)$ but not at all by $(1,0)$. The $θ=1$ result can be interpreted as taking into account the fermionic nature of the electrons and $z=3/2$ can be interpreted as the flattened band by the strong interaction.

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arXiv2020

Ginzberg-Landau-Wilson theory for Flat band, Fermi-arc and surface states of strongly correlated systems

We consider a holographic theory as a Ginzberg-Landau theory working for strongly interacting system near the quantum critical point: we take the bulk matter field $Φ^I(r,x)$, the dual of the fermion bilinear, as the order parameter. We calculate and classify the fermion spectral functions in the presence of such orders. Depending on the symmetry, we found spectral features like the gap, pseudo-gap, flat disk bands and the Fermi-arc connecting the two Dirac cones, which are familiar in Dirac material and Kondo lattice. Many of above features are associated with the zero modes whose presence is tied with a discrete symmetry of the interaction. The interaction induced zero modes either makes the strongly correlated system fermi-liquid like, or creates a disk-like flat band. Some of the order parameters in the bulk theory do not have an interpretation of symmetry breaking in terms of the boundary space, which opens the possibility of 'an order without symmetry breaking'.

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arXiv2020

New black holes with hyperscaling violation and transports of quantum critical points with magnetic impurity

We consider the magneto-transports of quantum matters doped with magnetic impurities near the quantum critical points(QCP). For this, we first find new black hole solution with hyper-scaling violation which is dual to such system. By considering the fluctuation near this exact solution, we calculated all transport coefficients using the holographic method. We applied our result to the surface state of the topological insulator with magnetic doping and found two QCP's, one bosonic and the other fermionic. It turns out that doped Bi$_2$Se$_3$ and Bi$_2$Te$_3$ correspond to different QCP's. We also investigated transports of QCP's as functions of physical parameters and found that there are phase transitions as well as crossovers from weak localization to weak anti-localization.

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arXiv2019

Holographic Spectral Functions with Momentum Relaxation

We study (fermionic) spectral functions in two holographic models, the Gubser-Rocha-linear axion model and the linear axion model, where translational symmetry is broken by axion fields linear to the boundary coordinates ($ψ_{I}=βδ_{Ii} x^{i}$). Here, $β$ corresponds to the strength of momentum relaxation. The spectral function is computed by the fermionic Green's function of the bulk Dirac equation, where a fermion mass, $m$, and a dipole coupling, $p$, are introduced as input parameters. By classifying the shape of spectral functions, we construct complete phase diagrams in ($m,p,β$) space for both models. We find that two phase diagrams are similar even though their background geometries are different. We also find that the effect of momentum relaxation on the (spectral function) phases of two models are similar even though the effect of momentum relaxation on the DC conductivities of two models are very different. We suspect that this is because holographic fermion does not back-react to geometry in our framework.

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arXiv2019

Building Magnetic Hysteresis in Holography

We study the spontaneous magnetization and the magnetic hysteresis using the gauge/gravity duality. We first propose a novel and general formula to compute the magnetization in a large class of holographic models. By using this formula, we compute the spontaneous magnetization in a model like a holographic superconductor. Furthermore, we turn on the external magnetic field and build the hysteresis curve of magnetization and charge density. To our knowledge, this is the first holographic model realizing the hysteresis accompanied with spontaneous symmetry breaking. By considering the Landau-Ginzburg type effective potential in the symmetry broken phase, we obtain the mass of the magnon from the bulk geometry data.

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arXiv2019

Interaction induced quasi-particle spectrum and the origin of the pinning peak in holography

It is often said that interactions destroy the particle nature of excitations. We report that, in holographic theory adding interaction term can create a new quasi particle spectrum, on the contrary. We show this by calculating the optical conductivity in a model with exact background solution and finding a new quasi-particle spectrum. We argue that such new poles are generic consequence of any non-minimal interaction like Chern-Simon term. The interaction driven metal-insulator transition and the pinning effect in holography are examples of this phenomena. We also point out that the origin of the pinning peak is the vortex formation by the anomalous magnetic moment induced by the interaction term.

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arXiv2019

Unitarity bound violation in holography and the Instability toward the Charge Density Wave

We study the spectral function of fermions in a holographic set up with bulk Dirac mass in the regime beyond the conformal unitarity bound, and find that spectral function has the dispersion relation with tachyonic behavior, indicating an instability. Based on linearity between the density and the position of the tip of the k-gap, we suggest that this instability is toward the charge density wave(CDW) and the position of the tip can be identified as the wave vector of CDW. For the physical origin, we point out the similarity of unitarity violation in our non-Fermi Liquid theory and nesting phenomena in the Fermi liquid theory as the mechanism of CDW instability.

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arXiv2018

Mott transition with Holographic Spectral function

We show that the Mott transition can be realized in a holographic model of a fermion with bulk mass, $m$, and a dipole interaction of coupling strength $p$. The phase diagram contains gapless, pseudo-gap and gapped phases and the first one can be further divided into four sub-classes. We compare the spectral densities of our holographic model with the Dynamical Mean Field Theory (DMFT) results for Hubbard model as well as the experimental data of Vanadium Oxide materials. Interestingly, single-site and cluster DMFT results of Hubbard model share some similarities with the holographic model of different parameters, although the spectral functions are quite different due to the asymmetry in the holography part. The theory can fit the X-ray absorption spectrum (XAS) data quite well, but once the theory parameters are fixed with the former it can fit the photoelectric emission spectrum (PES) data only if we symmetrize the spectral function.

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arXiv2018

Schwarzian correction to quantum correlation in SYK model

We study a class of SYK-type models in large N limit from the gravity dual side in terms of Schwarzian action analytically. The quantum correction to two point correlation function due to the Schwarzian action produces transfer of degree of freedom from the quasiparticle peak to Hubbard band in density of states (DOS), a signature strong correlation. In Schwinger-Keldysh (SK) formalism, we calculate higher point thermal out-of-time order correlation (OTOC) functions, which indicate quantum chaos by having Lyapunov exponent. Higher order local spin-spin correlations are also calculated, which can be related to the dynamical local susceptibility of quantum liquids such as spin glasses, disordered metals.

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arXiv2017

Small Fermi Surfaces and Strong Correlation Effects in Dirac Materials with Holography

Recent discovery of transport anomaly in graphene demonstrated that a system known to be weakly interacting may become strongly correlated if system parameter(s) can be tuned such that fermi surface is sufficiently small. We study the strong correlation effects in the transport coefficients of Dirac materials doped with magnetic impurity under the magnetic field using holographic method. The experimental data of magneto-conductivity are well fit by our theory, however, not much data are available for other transports of Dirac material in such regime. Therefore, our results on heat transport,thermo-electric power and Nernst coefficients are left as predictions of holographic theory for generic Dirac materials in the vicinity of charge neutral point with possible surface gap. We give detailed look over each magneto-transport observable and 3Dplots to guide future experiments.

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