arXiv subjects

Sang-Jin Sin

Publications and source records attributed to Sang-Jin Sin.

At least 73 records · Page 4Linked to original sources

arXiv2017

Universality of DC Electrical Conductivity from Holography

We propose a universal formula of dc electrical conductivity in rotational- and translational- symmetries breaking systems via the holographic duality. This formula states that the ratio of the determinant of the dc electrical conductivities along any spatial directions to the black hole area density in zero-charge limit has a universal value. As explicit illustrations, we give several examples elucidating the validation of this formula: We construct an anisotropic black brane solution, which yields linear in temperature for the in-plane resistivity and insulating behavior for the out-of-plane resistivity; We also construct a spatially isotropic black brane solution that both the linear-T and quadratic-T contributions to the resistivity can be realized.

hep-th↗

arXiv2016

Holography of the Dirac Fluid in Graphene with two currents

Recent experiments have uncovered evidence of the strongly coupled nature of the graphene: the Wiedemann-Franz law is violated by up to a factor of 20 near the charge neutral point. We describe this strongly-coupled plasma by a holographic model in which there are two distinct conserved U(1) currents. We find that our analytic results for the transport coefficients for two current model have a significantly improved match to the density dependence of the experimental data than the models with only one current. The additive structure in the transports coefficients plays an important role. We also suggest the origin of the two currents.

hep-th↗

arXiv2015

Character of Matter in Holography: Spin-Orbit Interaction

Gauge/Gravity duality as a theory of matter needs a systematic way to characterise a system. We suggest a `dimensional lifting' of the least irrelevant interaction to the bulk theory. As an example, we consider the spin-orbit interaction, which causes magneto-electric interaction term. We show that its lifting is an axionic coupling. We present an exact and analytic solution describing diamagnetic response. Experimental data on annealed graphite shows a remarkable similarity to our theoretical result. We also find an analytic formulas of DC transport coefficients, according to which, the anomalous Hall coefficient interpolates between the coherent metallic regime with $ρ_{xx}^{2}$ and incoherent metallic regime with $ρ_{xx}$ as we increase the disorder parameter $β$. The strength of the spin-orbit interaction also interpolates between the two scaling regimes.

hep-th↗

arXiv2015

Thermoelectric conductivities, shear viscosity, and stability in an anisotropic linear axion model

We study thermoelectric conductivities and shear viscosities in a holographically anisotropic model, which is dual to a spatially anisotropic $\mathcal{N}=4$ super-Yang-Mills theory at finite chemical potential. Momentum relaxation is realized through perturbing the linear axion field. Ac conductivity exhibits a coherent/incoherent metal transition. Deviations from the Wiedemann-Franz law are also observed in our model. The longitudinal shear viscosity for prolate anisotropy violates the bound conjectured by Kovtun-Son-Starinets. We also find that thermodynamic and dynamical instabilities are not always equivalent by examining the Gubser-Mitra conjecture.

hep-th↗

arXiv2015

Thermoelectric Conductivities at Finite Magnetic Field and the Nernst Effect

We study the thermoelectric conductivities of a strongly correlated system in the presence of a magnetic field by the gauge/gravity duality. We consider a class of Einstein-Maxwell-Dilaton theories with axion fields imposing momentum relaxation. General analytic formulas for the direct current(DC) conductivities and the Nernst signal are derived in terms of the black hole horizon data. For an explicit model study, we analyse in detail the dyonic black hole modified by momentum relaxation. In this model, for small momentum relaxation, the Nernst signal shows a bell-shaped dependence on the magnetic field, which is a feature of the normal phase of cuprates. We compute all alternating current(AC) electric, thermoelectric, and thermal conductivities by numerical analysis and confirm that their zero frequency limits precisely reproduce our analytic DC formulas, which is a non-trivial consistency check of our methods. We discuss the momentum relaxation effects on the conductivities including cyclotron resonance poles.

hep-th↗

arXiv2015

Gauge Invariance and Holographic Renormalization

We study the gauge invariance of physical observables in holographic theories under the local diffeomorphism. We find that gauge invariance is intimately related to the holographic renormalisation: the local counter terms defined in the boundary cancel most of gauge dependences of the on-shell action as well as the divergences. There is a mismatch in the degrees of freedom between the bulk theory and the boundary one. We resolve this problem by noticing that there is a residual gauge symmetry(RGS). By extending the RGS such that it satisfies infalling boundary condition at the horizon, we can understand the problem in the context of general holographic embedding of a global symmetry at the boundary into the local gauge symmetry in the bulk.

hep-th↗

arXiv2015

Variation of Entanglement Entropy in Scattering Process

In a scattering process, the final state is determined by an initial state and an S-matrix. We focus on two-particle scattering processes and consider the entanglement between these particles. For two types initial states; i.e., an unentangled state and an entangled one, we calculate perturbatively the change of entanglement entropy from the initial state to the final one. Then we show a few examples in a field theory and in quantum mechanics.

hep-th↗

arXiv2014

Coherent/incoherent metal transition in a holographic model

We study AC electric($σ$), thermoelectric($α$), and thermal($\barκ$) conductivities in a holographic model, which is based on 3+1 dimensional Einstein-Maxwell-scalar action. There is momentum relaxation due to massless scalar fields linear to spatial coordinate. The model has three field theory parameters: temperature($T$), chemical potential($μ$), and effective impurity($β$). At low frequencies, if $β< μ$, all three AC conductivities($σ, α, \barκ$) exhibit a Drude peak modified by pair creation contribution(coherent metal). The parameters of this modified Drude peak are obtained analytically. In particular, if $β\ll μ$ the relaxation time of electric conductivity approaches to $2\sqrt{3} μ/β^2$ and the modified Drude peak becomes a standard Drude peak. If $β> μ$ the shape of peak deviates from the Drude form(incoherent metal). At intermediate frequencies($T<ω<μ$), we have analysed numerical data of three conductivities($σ, α, \barκ$) for a wide variety of parameters, searching for scaling laws, which are expected from either experimental results on cuprates superconductors or some holographic models. In the model we study, we find no clear signs of scaling behaviour.

hep-th↗

arXiv2014

Anisotropic plasma at finite $U(1)$ chemical potential

We present a type IIB supergravity solution dual to a spatially anisotropic $\mathcal{N}=4$ super Yang-Mills plasma at finite $U(1)$ chemical potential and finite temperature. The effective five-dimensional gravitational theory is a consistent Einstein-Maxwell-dilaton-Axion truncation of the gauged supergravity. We obtain the solutions both numerically and analytically. We study the phase structure and thermodynamic instabilities of the solution, and find new instabilities independent of the renormalization scheme.

hep-th↗

arXiv2014

EPR = ER and Scattering Amplitude as Entanglement Entropy Change

We study the causal structure of the minimal surface of the four-gluon scattering, and find a world-sheet wormhole parametrized by Mandelstam variables, thereby demonstrate the EPR = ER relation for gluon scattering. We also propose that scattering amplitude is the change of the entanglement entropy by generalizing the holographic entanglement entropy of Ryu-Takayanagi to the case where two regions are divided in space-time.

hep-th↗

arXiv2014

Anisotropic plasma with a chemical potential and scheme-independent instabilities

Generically, the black brane solution with planar horizons is thermodynamically stable. We find a counter-example to this statement by demonstrating that an anisotropic black brane is unstable. We present a charged black brane solution dual to a spatially anisotropic finite temperature $\mathcal{N}=4$ super Yang-Mills plasma at finite $U(1)$ chemical potential. This static and regular solution is obtained both numerically and analytically. We uncover rich thermodynamic phase structures for this system by considering the cases when the anisotropy constant "a" takes real and imaginary values, respectively. In the case $a^2>0$, the phase structure of this anisotropic black brane is similar to that of Schwarzschild-AdS black hole with $S^3$ horizon topology, yielding a thermodynamical instability at smaller horizon radii. For the condition $a^2\leq 0$, the thermodynamics is dominated by the black brane phase for all temperatures.

hep-th↗

arXiv2013

A New Model of Holographic QCD and Chiral Condensate in Dense Matter

We consider the model of holographic QCD with asymptotic freedom and gluon condensation in its vacuum. It consists of the color D4-branes and D0-branes as a background and the flavor D8-branes as a probe. By taking a specific field theory limit, the effective coupling decreases to vanish in UV region. We then introduce the uniformly distributed baryons in terms of the baryon vertices and study the density dependence of chiral condensate, which is evaluated using the worldsheet instanton method. In the confined phase, the chiral condensate as a function of density monotonically decreases in high baryon density. Such behavior is in agreement with the expectation, while in extremely low density it increases. We attribute this anomaly to the incorrect approximation of uniformity in very low density. In the deconfined phase the chiral condensate monotonically decreases in the whole region of density.

hep-th↗

arXiv2013

Symmetry Energy from Holographic QCD

We review the symmetry energy in the context of AdS/CFT correspondence. After constructing $D$ brane configurations corresponding to dense system in boundary theory, we calculate the symmetry energy by solving DBI action of $D$ branes in confining and deconfining phase. We conclude the density dependence of the symmetry energy has scaling law whose power depends only on dimensionality of the branes and space-time.

hep-th↗

arXiv2013

The physical mechanism of AdS instability and Holographic Thermalization

Gravitational falling in AdS has two characteristic properties \cite{ohsin}: i) A thick shell becomes a thin shell. ii) Any shape become spherical. Such focusing character of AdS, for the collapse of dusts, leads to the rapid thermalization mechanism in strongly interacting system. For the collapse of a wave, it explains the cascade of energy to UV through repeated bounces, which has been extensively discussed in recent numerical works. Therefore the focusing is the physical mechanism of instability of AdS. Such sharp contrast between the dust and wave in collapse, together with the experimental observation of rapid thermalization, suggest that the initial condition of created particles in RHIC is in a state with random character rather than a coherent one. Two time scales, one for thermalization and the other for hydro-nization are defined and calculated in terms of the total mass density and energy distribution of the initial particles. We find $t_{th}\sim (1-c_1/E^2)^{1/2}/T $ so that softer modes thermalize earlier. However, for hydro-nization, $t_{hyd} \sim 1/E^{2/3}T^{1/3}$ therefore harder modes come earlier. We also show that near horizon limit of Dp brane solutions have similar focusing effect which is enough to guarantee the early thermalization.

hep-th↗

arXiv2013

Non-spherical collapse in AdS and Early Thermalization in RHIC

In the flat space, non-spherical shells collapse to give globular cluster after many oscillations. We show that in anti de sitter space, they form black holes in one dynamical time. We propose that this is the mechanism of early thermalization in strong quark-gluon plasma in gravity dual. This is traced back to the a remarkable property of AdS : the period in radial motion is amplitude independent in spite of the NON-linearity of the equation of motion. We investigate the interaction effect numerically and observe the same qualitative behavior for the attractive forces. For repulsive interactions, particles halt at a small but finite radius for long time due to the specific structure of the bulk AdS propagator. It helps hair creation in the AdS black hole.

hep-th↗

arXiv2013

Stable and flexible system for glucose homeostasis

Pancreatic islets, controlling glucose homeostasis, consist of α, β, and δ cells. It has been observed that α and β cells generate out-of-phase synchronization in the release of glucagon and insulin, counter-regulatory hormones for increasing and decreasing glucose levels, while β and δ cells produce in-phase synchronization in the release of the insulin and somatostatin. Pieces of interactions between the islet cells have been observed for a long time, although their physiological role as a whole has not been explored yet. We model the synchronized hormone pulses of islets with coupled phase oscillators that incorporate the observed cellular interactions. The integrated model shows that the interaction from β to δ cells, of which sign has controversial reports, should be positive to reproduce the in-phase synchronization between β and δ cells. The model also suggests that δ cells help the islet system flexibly respond to changes of glucose environment.

q-bio.CB↗

arXiv2013

Thermal Mass and Plasmino for Strongly Interacting Fermions

We investigate fermion self energy problem in the strongly coupled dense medium in holographic approach. By working out bottom up models as well as top down ones we showed that vanishing thermal mass and non-existence of temperature generated plasmino mode is the universal feature of the strongly interacting fermion system. We identified that the dual of the bulk Rashiba effect, which was recently found by the Herzog et.al, is the presence of the plasmino mode generated by the density.

hep-th↗

arXiv2013

Self-energy of Strongly Interacting Fermions in Medium: a Holographic Approach

We consider the self-energy of strongly interacting fermions in the medium using gauge/gravity duality of $D4/D8$ system. We study the mass generation of the thermal and/or dense medium and the collective excitation called plasmino, by considering the spectral function of fermion and its dispersion relation. Our results are very different from those of the hard thermal loop method: for zero density, there is no thermal mass or plasmino in any phase. Plasmino in deconfined phase is not allowed in $D4/D8$ set up. In the confined phase, there is plasmino modes only for a window of density.

hep-th↗