arXiv subjects

Sang-Jin Sin

Publications and source records attributed to Sang-Jin Sin.

At least 19 recordsLinked to original sources

arXiv2026

Symmetric Tensor Coupling in Holographic Mean-Field Theory: Deformed Dirac Cones

We extend the holographic mean-field theory to rank-two symmetric tensor field as an external source coupled with fermion. We classify the roles of symmetric tensor coupling according to the effect on the spectral density: cone-angle change, squashing, and tilting of the spectral light cones. The over-tilted light cone is also achieved in a generalized prescription, which consistently retains the causality condition. Our results provide agreements between the holographic spectra with those observed in real materials, such as type-II Dirac cones and strained graphene.

hep-th↗

arXiv2026

Linear Resistivity from Spatially Random Interactions and the Uniqueness of Yukawa Coupling

Recent studies have shown that a spatially random Yukawa-type interaction between a Fermi surface and critical bosons can produce linear-in-temperature resistivity, the defining signature of strange metals. In this article, we systematically classify all scalar couplings of the form $(ψ^{\dagger}ψ)^nϕ^m$ in arbitrary dimensions to identify possible candidates for strange-metal behaviour within this disordered framework. We find that only spatially random Yukawa-type interaction in $(2+1)$ dimensions can yield linear resistivity. This indicates that linear resistivity is not a universal property of all spatially random scalar coupling, and strange-metal property replies on both dimensions and interaction type.

hep-th↗

arXiv2025

Lifshitz transition in a holographic finite density flavour brane Weyl semimetal

We extend a top-down holographic model of a Weyl semimetal to finite charge density and compute the fermionic spectral function by introducing two probe fermions of opposite chirality. The model is controlled by the boundary fermion mass M and the chemical potential $μ$. In the zero density, small-M limit, we recover four energy bands, two Weyl points, and linear dispersion in their vicinity, the hallmarks of a Weyl semimetal. As M increases, the bands between the Weyl points become progressively compressed and the spectral weight associated with those bands is smeared out. At finite charge density, we map the Fermi surface in momentum space and identify a Lifshitz transition: two distinct Fermi pockets, each enclosing a different Weyl point, merge into a single large Fermi surface that encloses both. This transition can be induced by either control parameter. Varying M alters the band structure and thus the band shape, which drives the Lifshitz transition, whereas changing $μ$ shifts the bands relative to the Fermi level without qualitatively changing the band structure, producing the Lifshitz transition by moving the band positions.

hep-th↗

arXiv2025

Tilted Dirac cones and their topology in Holographic Materials

We explore strongly correlated materials with tilted Dirac cone by introducing a method to realize this spectral feature within a holographic setup. Following the work by Moradpouri et al., we construct an asymptotically AdS spacetime by uplifting the vielbein of Volovik et al to tilt the flat spacetime light cone. We then couple the resulting metric to holographic fermions and compute their spectral functions, confirming the presence of a tilted Dirac cone in momentum space. We also calculate the topological number using the holographic Green's function and find that the Chern number is independent of the tilting parameter. Additionally, we show that the optical conductivity exhibits a Drude peak even at zero chemical potential, revealing nontrivial strong-coupling effects absent in field-theoretic models.

hep-th↗

arXiv2025

ER=EPR and Strange Metals from Quantum Entanglement: Disorder theory vs quantum gravity

We give an understanding how strange metals arise from the spatially random Yukawa-SYK model based on the wormhole picture and find a parallelism between the disorder theory and quantum gravity. We start from the observation that the Gaussian average over the spatial random coupling gives a wormhole, defined as a mechanism for long range interaction without causal suppression outside the lightcone. We find that the large-$N$ limit equivalence of the quenched and annealed averages provides a field theory version of the ER=EPR. Since the wormhole establishes momentum exchanges over arbitrary distance without causal suppression, it provides a mechanism of the planckian dissipation. It also tells us why SYK-like models describe strongly interacting systems even in the small coupling case. We classify the disorder samples into two classes: I) spatially random coupling with wormholes and no information loss, II) spatially uniform coupling with decoherence.

hep-th↗

arXiv2025

Topology in Holographic Mean-Field Theory at Zero and Finite Temperature

We investigate topological invariants in strongly interacting many-body systems within holographic mean-field theory (H-MFT) framework. Analytic expressions for retarded Green's functions are obtained for all possible fermionic bilinear interactions in the limit of probe background limit $\mathrm{AdS}_4$, from which we construct topological Hamiltonians. Integrating Berry curvature over the momentum domain for the gapped spectra yields well-defined and quantized Chern numbers, enabling a systematic classification of them across interaction types. These topological invariants remain robust under deformation parameters like interaction and temperature, indicating that H-MFT encodes effective single-particle-state topology near a quantum critical point in strongly correlated systems. We point out why topological number is defined in the holographic theories while it is not in the perturbative field theory.

hep-th↗

arXiv2025

Hall Angle of a Spatially Random Vector Model

Strange metals exhibit linear resistivity and anomalous Hall transport, yet a comprehensive theory that accounts for both phenomena is still lacking. Recent studies have shown SYK-like spatially random couplings between a Fermi surface and a bosonic field, either scalar or vector type, can yield linear-$T$ resistivity. In this paper, we continue the investigation on a vector coupling in the presence of a magnetic field. We compute the fermion and boson propagators, along with the self-energy and polarization functions, and determine their dependence on the magnetic field. Although the Hall angle does not exhibit the signature of strange-metal, the linear-in-temperature resistivity remains at low temperatures. Results indicate that random interactions can robustly support linear transport, though additional ingredients may be required to capture the full phenomenology of strange metals.

hep-th↗

arXiv2025

Topological transition as a percolation of the Berry curvature

We first study the importance of the sign of the Berry curvature in the Euler characteristic of the two-dimensional topological material with two bands. Then we report an observation of a character of the topological transition as a percolation of the sign of the Berry curvature. The Berry curvature F has peaks at the Dirac points, enabling us to divide the Brillouin zone into two regions depending on the sign of the F: one with the same sign with a peak and the other with the opposite sign. We observed that when the Chern number is non-zero, the oppositely signed regions are localized. In contrast, in the case of a trivial topology, the oppositely signed regions are delocalized dominantly. Therefore, the oppositely signed region will percolate under the topological phase transition from non-trivial to trivial. We checked this for several models including the Haldane model, the extended Haldane model, and the QWZ model. Our observation may serve as a novel feature of the topological phase transition.

cond-mat.mes-hall↗

arXiv2025

Eta-pairing state in flatband lattice: Interband coupling effect on entanglement entropy logarithm

The eta-pairing state is the eigenstate of the hypercubic Hubbard model, which exhibits anomalous logarithmic scaling of entanglement entropy. In multi-band systems, eta-pairing can be exact eigenstate when the band is flat without interband coupling. However, typical flatband systems such as Lieb and Kagome lattices often feature band touchings, where interband coupling effects are non-negligible. Using the Creutz ladder, we investigate the deformation of eta-pairing states under the interband coupling effect. Our results show corrections to entanglement entropy scaling, with modified eta-pairing states displaying broadened doublons, nonuniform energy spacing, and deviations from exact behavior for configurations with more than one eta-pair, even in the large band gap limit, except at t = 0. Through a Schrieffer-Wolff transformation, we quantify corrections to the spectrum generating algebra, offering insights into the interplay between interaction-driven phenomena and band structure effects. These findings illuminate the robustness and limitations of eta-pairing in realistic flatband systems.

cond-mat.str-el↗

arXiv2025

Linear-T Resistivity from Spatially Random Vector Coupling

Recently, Patel et al. introduced a higher dimensional version of the SYK model with random coupling in a Yukawa interaction to find the linear-$T$ resistivity. We test the universality of the mechanism by replacing the scalar field with a vector field in various dimensions. We find that it works for vector and scalar interactions, although the details are largely different. However, this mechanism for the linear-$T$ resistivity works only in $(2+1)$ dimensions and not in higher dimensions, regardless of the interaction type. Based on these results, we explore the rôle of spatial random disorder and find a simple explanation of how such random scattering converts the Fermi liquid to a strange metal by changing the self-energies of the involved bosons and fermions.

hep-th↗

arXiv2025

Holographic mean field theory and Kondo lattice

We first study a non-relativistic field theory model for the Kondo lattice by introducing the Kondo condensation, whose main effect is the hybridization of the flat band of the localized electron with dispersive one of the itinerant electron. The problem here is that the resulting Kondo condensation arises only in strong coupling where the validity of the mean field theory is questionable. Therefore, we build a holographic mean field theory of the Kondo lattice with strong coupling by identifying the effect of the lattice with the fermion's spectral shape due to the coupling with the order parameter representing the symmetry breaking. For the flat band spectrum we use the mixed quantization, and for the dispersive spectrum we intoduce the second fermion in standard quantization. The coupling of the two fermions with the scalar order representing the Kondo condensation provides the hybrization of the two spectrum, reproducing the main feature of the Kondo lattice together with the fuzzy character of the spectrum of the strongly coupled system.

hep-th↗

arXiv2024

Holographic fermions in the Dyonic Gubser-Rocha black hole

We investigate the fermionic properties of a dyonic Gubser-Rocha model in the context of gauge/gravity duality. This model incorporates both a magnetic field and momentum relaxation. We have derived this model's scaling exponent, revealing the influence of the magnetic field and momentum relaxation on low-energy physics. As the magnetic field strength and momentum relaxation increase, the spectral function of the dual field changes significantly. Specifically, we observe variations in the scaling exponent, Fermi momentum, and dispersion relations as the magnetic field increases, highlighting the system's transition from a Fermi liquid to a non-Fermi liquid, and eventually to an insulating state. Our analysis of the magneto-scattering rate reveals that it is nearly zero in the Fermi liquid region, increases significantly in the non-Fermi liquid region, and ultimately arrives at a maximum value in the insulating state.

hep-th↗

arXiv2024

Classes of Holographic Mott Gaps

The fermion gaps are classified into order gap or Mott gap depending on the presence/absence of the order parameter. We construct the holographic model of the Mott gap using the field that is supported by the density only without introducing any order parameter. We then classify the Mott gap, depending on the shape of the gap in the density of states and whether the Fermi surface is touching the valence bond or not, into three classes: i) Symmetric gap, ii) Asymmetric gap with isolated Fermi sea. iii) Asymmetric gap with Fermi sea touching the valence band. Finally, we identify possible non-minimal gauge interactions that produce a flatband without symmetry breaking.

hep-th↗

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.

hep-th↗

arXiv2024

Quantum Scaling Dimension from the Equivalence principle

We propose a method to constrain the scaling dimension of the operators of the strongly interacting systems (SIS) using the holographic setup. %where the (d+1)-dimensional black hole is used to describe the d-dimensional SIS. We demonstrate our method using the holographic superconductor theory. The idea is to consider the inside as well as the outside of the AdS black hole in which the gap equations has higher order singularities. Then the equivalence principle requests the solution be smoothly connected at the horizon, which request the vanishing of log divergent term as well as an indefinite conditionally convergent terms that can lead to any real number according to Riemann. As a result, one gets quantized values of the scaling dimension of the condensing operator. This is a pleasant surprise because so far one gets the constraints on the scaling dimension only by a hard analysis with bootstrap ansatz.

hep-th↗

arXiv2024

Encoding the lattice in the Holography

One of the most wanted features of holography in its condensed matter physics application is to encode the structure of lattice, which is the most direct data of the material. In this paper, we propose a method to encode the lattice structure by embedding the tight binding data into the Dirac equation in the AdS bulk. We explicitly worked out the idea for the Graphene and Haldane model, and the result shows that some degrees of freedom escape the free-electron on-shell curve, and Green's function loses the pole structure completely. It implies that the electronic structure is not described by the band structure only, which is consistent with what many ARPES data tell us, and it also implies that the system is in non-fermi liquid even for the graphene, which is consistent with recent experiments for the clean graphene.

hep-th↗

arXiv2024

Mean field theory for strongly coupled systems: Holographic approach

In this paper, we develop the holographic mean field theory for strongly interacting fermion systems. We investigate various types of the symmetry-breakings and their effect on the spectral function. We found analytic expressions of fermion Green's functions in the probe-limit for all types of tensor order parameter fields. We classified the spectral shapes and singularity types from the analytic Green's function. We calculated the fermions spectral function in the full backreacted background and then compared it with the analytic results to show the reliability of analytic results in the probe limit.

hep-th↗

arXiv2024

Order parameter and spectral function in $d$-wave holographic superconductors

We consider the $d$-wave holographic superconductor model with full backreaction on the metric, addressing a missing part in the literature. We have identified the corrected order parameter by comparing the fermionic spectral function with the momentum-dependent order parameter. By numerical investigations of the fermionic spectral function in the presence of a tensor condensate, we find the Fermi arc and the gapped behavior, which closely resemble ARPES data. Moreover, we have examined the influence of the coupling constant, chemical potential, and temperature on the spectral function. We find that $d$-wave fermionic spectral function can be obtained through $p_x$ and $p_y$ condensates combined with two fermion flavors. Similarly, combining $d_{x^2-y^2}$ and $d_{xy}$ orbitals symmetry with two fermion flavors leads to a $g$-wave spectral function.

hep-th↗