arXiv subjects

Moshe Rozali

Publications and source records attributed to Moshe Rozali.

At least 37 records · Page 2Linked to original sources

arXiv2017

Holographic Turbulence in a Large Number of Dimensions

We consider relativistic hydrodynamics in the limit where the number of spatial dimensions is very large. We show that under certain restrictions, the resulting equations of motion simplify significantly. Holographic theories in a large number of dimensions satisfy the aforementioned restrictions and their dynamics are captured by hydrodynamics with a naturally truncated derivative expansion. Using analytic and numerical techniques we analyze two and three-dimensional turbulent flow of such fluids in various regimes and its relation to geometric data.

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arXiv2017

Comments on Entanglement Propagation

We extend our work on entanglement propagation following a local quench in 2+1 dimensional holographic conformal field theories. We find that entanglement propagates along an emergent lightcone, whose speed of propagation $v_E$ seems distinct from other measures of quantum information spreading. We compare the relations we find to information and hydrodynamic velocities in strongly coupled 2+1 dimensional theories. While early-time entanglement velocities corresponding to small entangling regions are numerically close to the butterfly velocity, late-time entanglement velocities for large regions show less regularity. We also generalize and extend our previous results regarding the late-time decay of the entanglement entropy back to its equilibrium value.

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arXiv2017

Holographic Quenches in a Confined Phase

We investigate quenches of holographic theories in a confined phase, where the energy injected is insufficient to reach the deconfined phase. In such quenches, thermalization is not associated with gravitational collapse and the formation of a black hole. Nevertheless, we attempt to characterize the late-time state of this scenario. We check a number of notions of thermalization that do not require horizon formation, and find no evidence for thermalization in our chosen parameters and initial states, even in the weakest sense. We find that the post-quench behaviour of both local and nonlocal observables exhibit oscillatory behaviour rather than decaying towards equilibrium. We generally find that the response of the nonlocal observables is smoother than that of the local ones. We discuss mechanisms which generate such smoothing, as well as "beats" which appear in the time-dependence of the nonlocal operators for certain classes of quenches. When tuning the quench parameters such that the smoothing is ineffective, we are able to perform "entanglement spectroscopy", recovering the spectrum of the confined phase of the theory from the time dependence of the entanglement entropy, as well as other nonlocal observables.

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arXiv2017

Comments on the Random Thirring Model

The Thirring model with random couplings is a translationally invariant generalisation of the SYK model to 1+1 dimensions, which is tractable in the large N limit. We compute its two point function, at large distances, for any strength of the random coupling. For a given realisation, the couplings contain both irrelevant and relevant marginal operators, but statistically, in the large N limit, the random couplings are overall always marginally irrelevant, in sharp distinction to the usual Thirring model. We show the leading term to the $β$ function in conformal perturbation theory, which is quadratic in the couplings, vanishes, while its usually subleading cubic term matches our RG flow.

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arXiv2016

Higher Dimensional Generalizations of the SYK Model

We discuss a 1+1 dimensional generalization of the Sachdev-Ye-Kitaev model. The model contains $N$ Majorana fermions at each lattice site with a nearest-neighbour hopping term. The SYK random interaction is restricted to low momentum fermions of definite chirality within each lattice site. This gives rise to an ordinary 1+1 field theory above some energy scale and a low energy SYK-like behavior. We exhibit a class of low-pass filters which give rise to a rich variety of hyperscaling behaviour in the IR. We also discuss another set of generalizations which describes probing an SYK system with an external fermion, together with the new scaling behavior they exhibit in the IR.

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arXiv2016

On Brane Instabilities in the Large $D$ Limit

Using an expansion in large number of dimensions, taken to subleading orders, we discuss several issues concerning the Gregory-Laflamme instabilities. We map out the phase diagram of neutral and charged black strings, and comment on the possible transition in the nature of the final state of the instability at higher order in the 1/D expansion. We also discuss unstable black membranes, and show that in certain limits the preferred shape of the non-uniform phase is a triangular lattice.

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arXiv2016

Dynamics of Holographic Entanglement Entropy Following a Local Quench

We discuss the behaviour of holographic entanglement entropy following a local quench in 2+1 dimensional strongly coupled CFTs. The entanglement generated by the quench propagates along an emergent light-cone, reminiscent of the Lieb-Robinson light-cone propagation of correlations in non-relativistic systems. We find the speed of propagation is bounded from below by the entanglement tsunami velocity obtained earlier for global quenches in holographic systems, and from above by the speed of light. The former is realized for sufficiently broad quenches, while the latter pertains for well localized quenches. The non-universal behavior in the intermediate regime appears to stem from finite-size effects. We also note that the entanglement entropy of subsystems reverts to the equilibrium value exponentially fast, in contrast to a much slower equilibration seen in certain spin models.

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arXiv2015

Spatial Modulation and Conductivities in Effective Holographic Theories

We analyze a class of bottom-up holographic models for low energy thermo-electric transport. The models we focus on belong to a family of Einstein-Maxwell-dilaton theories parameterized by two scalar functions, characterizing the dilaton self-interaction and the gauge coupling function. We impose spatially inhomogeneous lattice boundary conditions for the dilaton on the AdS boundary and study the resulting phase structure attained at low energies. We find that as we dial the scalar functions at our disposal (changing thus the theory under consideration), we obtain either (i) coherent metallic, or (ii) insulating, or (iii) incoherent metallic phases. We chart out the domain where the incoherent metals appear in a restricted parameter space of theories. We also analyze the optical conductivity, noting that non-trivial scaling behaviour at intermediate frequencies appears to only be possible for very narrow regions of parameter space.

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arXiv2015

Fermi Liquids from D-Branes

We discuss finite density configurations on probe D-branes, in the presence of worldvolume fermions. To this end we consider a phenomenological model whose bosonic sector is governed by the DBI action, and whose charged sector is purely fermionic. In this model, we demonstrate the existence of a compact worldvolume embedding, stabilized by a Fermi surface on the D- brane. The finite density state in the boundary QFT is a Fermi-like liquid. We comment on the possibility of realizing non-Fermi liquids in this setup.

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arXiv2015

Holographic Higgs Phases

We discuss phases of gauge theories in the holographic context, and formulate a criterion for the existence of a Higgs phase, where the gauge redundancy is "spontaneously broken", in purely bulk language. This condition, the existence of a finite tension solitonic string representing a narrow magnetic flux tube, is necessary for a bulk theory to be interpreted as a Higgs phase of a boundary gauge theory. We demonstrate the existence of such solitons in both top-down and bottom-up examples of holographic theories. In particular, we numerically construct new solitonic solutions in AdS black hole background, for various values of the boundary gauge coupling, which are used to demonstrate that the bulk theory models a superconductor, rather than a superfluid. The criterion we find is expected to be useful in finding holographic duals of color superconducting phases of gauge theories at finite density.

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arXiv2015

Cosmological Particle Production at Strong Coupling

We study the dynamics of a strongly-coupled quantum field theory in a cosmological spacetime using the holographic AdS/CFT correspondence. Specifically we consider a confining gauge theory in an expanding FRW universe and track the evolution of the stress-energy tensor during a period of expansion, varying the initial temperature as well as the rate and amplitude of the expansion. At strong coupling, particle production is inseparable from entropy production. As a result, we find significant qualitative differences from the weak coupling results: at strong coupling the system rapidly loses memory of its initial state as the amplitude is increased. Furthermore, in the regime where the Hubble parameter is parametrically smaller than the initial temperature, the dynamics is well modelled as a plasma evolving hydrodynamically towards equilibrium.

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arXiv2015

Driven Holographic CFTs

We study the dynamical evolution of strongly coupled field theories, initially in thermal equilibrium, under the influence of an external driving force. We model the field theory holographically using classical gravitational dynamics in an asymptotically AdS spacetime. The system is driven by a source for a (composite) scalar operator. We focus on a scenario where the external source is periodic in time and chart out the response of several observables. We find an interesting phase structure in the response as a function of the amplitude of the source and driving frequency. Specifically the system transitions from a dissipation dominated phase, via a dynamical crossover to a highly resonant amplification phase. The diagnostics of these phases include the response of the operator in question, entropy production, energy fluctuations, and the temporal change of entanglement entropy for small subsystems. We comment on evidence for a potential phase transition in the energy fluctuations of the system.

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arXiv2014

Holographic Baryons from Oblate Instantons

We investigate properties of baryons in a family of holographic field theories related to the Sakai-Sugimoto model of holographic QCD. Starting with the $N_f=2$ Sakai-Sugimoto model, we truncate to a 5D Yang-Mills action for the gauge fields associated with the noncompact directions of the flavour D8-branes. We define a free parameter $γ$ that controls the strength of this Yang-Mills term relative to the Chern-Simons term that couples the abelian gauge field to the SU(2) instanton density. Moving away from $γ= 0$ should incorporate some of the effects of taking the Sakai-Sugimoto model away from large 't Hooft coupling $λ$. In this case, the baryon ground state corresponds to an oblate SU(2) instanton on the bulk flavour branes: the usual SO(4) symmetric instanton is deformed to spread more along the field theory directions than the radial direction. We numerically construct these anisotropic instanton solutions for various values of $γ$ and calculate the mass and baryon charge profile of the corresponding baryons. Using the value $γ= 2.55$ that has been found to best fit the mesonic spectrum of QCD, we find a value for the baryon mass of 1.19 GeV, significantly more realistic than the value 1.60 GeV computed previously using an SO(4) symmetric ansatz for the instanton.

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arXiv2013

Entanglement and Correlations near Extremality: CFTs dual to Reissner-Nordström AdS${}_5$

We use the AdS/CFT correspondence to study models of entanglement and correlations between two $d=4$ CFTs in thermofield double states at finite chemical potential. Our bulk spacetimes are planar Reissner-Nordström AdS black holes. We compute both thermo-mutual information and the two-point correlators of large-dimension scalar operators, focussing on the small-temperature behavior -- an infrared limit with behavior similar to that seen at large times. The interesting feature of this model is of course that the entropy density remains finite as $T \rightarrow 0$ while the bulk geometry develops an infinite throat. This leads to a logarithmic divergence in the scale required for non-zero mutual information between equal-sized strips in the two CFTs, though the mutual information between one entire CFT and a finite-sized strip in the other can remain non-zero even at $T=0$. Furthermore, despite the infinite throat, there can be extremally charged operators for which the two-point correlations remain finite as $T \rightarrow 0$. This suggests an interestingly mixed picture in which some aspects of the entanglement remain localized on scales set by the chemical potential, while others shift to larger and larger scales. We also comment on implications for the localized-quasiparticle picture of entanglement.

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arXiv2013

Chiral Edge Currents in a Holographic Josephson Junction

We discuss the Josephson effect and the appearance of dissipationless edge currents in a holographic Josephson junction configuration involving a chiral, time-reversal breaking, superconductor in 2+1 dimensions. Such a superconductor is expected to be topological, thereby supporting topologically protected gapless Majorana-Weyl edge modes. Such modes manifest themselves in chiral dissipationless edge currents, which we exhibit and investigate in the context of our construction. The physics of the Josephson current itself, though expected to be unconventional in some non-equilibrium settings, is shown to be conventional in our setup which takes place in thermal equilibrium. We comment on various ways in which the expected Majorana nature of the edge excitations, and relatedly the unconventional nature of topological Josephson junctions, can be verified in the holographic context.

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arXiv2013

Striped Order in AdS/CFT

We study the formation of inhomogeneous order in the Einstein-Maxwell-axion system, dual to a 2+1 dimensional field theory that exhibits a spontaneously generated current density, momentum density and modulated scalar operator. Below the critical temperature, the Reissner-Nordstrom-AdS black hole becomes unstable and stripes form in the bulk and on the boundary. The bulk geometry possesses striking geometrical features, including a modulated horizon that tends to pinch off as T -> 0. On a domain of fixed length, we find a second order phase transition to the striped solution in each of the grand canonical, canonical and microcanonical ensembles, with modulated charges that grow and saturate as we lower the temperature and descend into the inhomogeneous phase. For the black hole on an infinite domain, a similar second order transition occurs, and the width of the dominant stripe increases in the zero temperature limit.

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arXiv2013

Holographic Stripes

We construct inhomogeneous asymptotically AdS black hole solutions corresponding to the spontaneous breaking of translational invariance and the formation of striped order in the boundary field theory. We find that the system undergoes a second order phase transition in both the fixed density and fixed chemical potential ensembles, for sufficiently large values of the axion coupling. We investigate the phase structure as function of the temperature, axion coupling and the stripe width. The bulk solutions have striking geometrical features related to a magnetoelectric effect associated with the existence of a near horizon topological insulator. At low temperatures the horizon becomes highly inhomogeneous and tends to pinch off.}

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arXiv2012

Compressible Matter at a Holographic Interface

We study the interface between a fractional topological insulator and an ordinary insulator, both described using holography. By turning on a chemical potential we induce a finite density of matter localized at the interface. These are gapless surface excitations which are expected to have a fermionic character. We study the thermodynamics of the system, finding a symmetry preserving compressible state at low temperatures, whose excitations exhibit hyperscaling violation. These results are consistent with the expectation of gapless fermionic excitations forming a Fermi surface at finite density.

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