Publications (35)160.39 Total impact

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ABSTRACT: In recent papers, it was hypothesized that there exist dissipationless quantum liquids, i.e., liquids with zero or vanishingly small viscosity and zero entropy production, which nevertheless have nontrivial secondorder transport coefficients. A natural candidate for a dissipationless liquid is the hypothetical conformal quantum liquid, whose holographically dual description in the infrared limit is given by the fivedimensional GaussBonnet gravity. It is known that shear viscosity in that theory can be made arbitrarily small as the GaussBonnet coupling parameter approaches a critical value. We evaluate the transport coefficients of a GaussBonnet liquid (nonperturbatively in the coupling parameter; three of the six coefficients were previously unknown) and consider the zeroviscosity limit. We show that three of the five secondorder coefficients are nonzero in this limit, but they do not satisfy the criterion of zero entropy production. Hence, the holographic GaussBonnet liquid is not a dissipationless quantum liquid.Theoretical and Mathematical Physics 01/2015; 182(1):6173. DOI:10.1007/s1123201502457 · 0.70 Impact Factor 
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ABSTRACT: We compute the 't Hooft coupling correction to the infinite coupling expression for the second order transport coefficient $\lambda_2$ in ${\cal N}=4$ $SU(N_c)$ supersymmetric YangMills theory at finite temperature in the limit of infinite $N_c$, which originates from the $R^4$ terms in the low energy effective action of the dual type IIB string theory. Using this result, we show that the identity involving the three second order transport coefficients, $2 \eta \tau_\Pi  4 \lambda_1  \lambda_2 =0$, previously shown by Haack and Yarom to hold universally in relativistic conformal field theories with string dual descriptions to leading order in supergravity approximation, holds also at next to leading order in this theory. We also compute corrections to transport coefficients in a (hypothetical) strongly interacting conformal fluid arising from the generic curvature squared terms in the corresponding dual gravity action (in particular, GaussBonnet action), and show that the identity holds to linear order in the higherderivative couplings. We discuss potential implications of these results for the nearequilibrium entropy production rate at strong coupling. 
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ABSTRACT: We use gaugegravity duality to study the temperature dependence of the zero sound mode and the fundamental matter diffusion mode in the strongly coupled {\cal N}=4 SU(N_c) supersymmetric YangMills theory with N_f {\cal N}=2 hypermultiplets in the N_c>>1, N_c>>N_f limit, which is holographically realized via the D3/D7 brane system. In the high density limit \mu>>T, three regimes can be identified in the behavior of these modes, analogous to the collisionless quantum, collisionless thermal and hydrodynamic regimes of a Landau Fermiliquid. The transitions between the three regimes are characterized by the parameters T/\mu and (T/\mu)^2 respectively, and in each of these regimes the modes have a distinctively different temperature and momentum dependence. The collisionlesshydrodynamic transition occurs when the zero sound poles of the densitydensity correlator in the complex frequency plane collide on the imaginary axis to produce a hydrodynamic diffusion pole. We observe that the properties characteristic of a Landau Fermiliquid zero sound mode are present in the D3/D7 system despite the atypical T^6/\mu^3 temperature scaling of the specific heat and an apparent lack of a directly identifiable Fermi surface.Physical review D: Particles and fields 09/2011; DOI:10.1103/PhysRevD.85.026004 · 4.86 Impact Factor 
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ABSTRACT: Quasinormal modes are eigenmodes of dissipative systems. Perturbations of classical gravitational backgrounds involving black holes or branes naturally lead to quasinormal modes. The analysis and classification of the quasinormal spectra require solving nonHermitian eigenvalue problems for the associated linear differential equations. Within the recently developed gaugegravity duality, these modes serve as an important tool for determining the nearequilibrium properties of strongly coupled quantum field theories, in particular their transport coefficients, such as viscosity, conductivity and diffusion constants. In astrophysics, the detection of quasinormal modes in gravitational wave experiments would allow precise measurements of the mass and spin of black holes as well as new tests of general relativity. This review is meant as an introduction to the subject, with a focus on the recent developments in the field.Classical and Quantum Gravity 08/2009; 26(16). DOI:10.1088/02649381/26/16/163001 · 3.10 Impact Factor 
Article: Holographic Quantum Liquid
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ABSTRACT: Quantum liquids are characterized by the distinctive properties such as the lowtemperature behavior of heat capacity and the spectrum of lowenergy quasiparticle excitations. In particular, at low temperature, Fermi liquids exhibit the zero sound, predicted by Landau in 1957 and subsequently observed in liquid He3. In this Letter, we ask whether such characteristic behavior is present in theories with a holographically dual description. We consider a class of gauge theories with fundamental matter fields whose holographic dual in the appropriate limit is given in terms of the DiracBornInfeld action in antide Sitter space. We find that these systems also exhibit a sound mode at zero temperature despite having a nonFermiliquid behavior of the specific heat. These properties suggest that holography identifies a new type of quantum liquid which potentially could be experimentally realized in strongly correlated systems.Physical Review Letters 03/2009; 102(5):051602. DOI:10.1103/PhysRevLett.102.051602 · 7.73 Impact Factor 
Article: Zero Sound from Holography
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ABSTRACT: Quantum liquids are characterized by the distinctive properties such as the low temperature behavior of heat capacity and the spectrum of lowenergy quasiparticle excitations. In particular, at low temperature, Fermi liquids exhibit the zero sound, predicted by L. D. Landau in 1957 and subsequently observed in liquid He3. In this paper, we ask a question whether such a characteristic behavior is present in theories with holographically dual description. We consider a class of gauge theories with fundamental matter fields whose holographic dual in the appropriate limit is given in terms of the DiracBornInfeld action in AdS_{p+1} space. An example of such a system is the N=4 SU(N_c) supersymmetric YangMills theory with N_f massless N=2 hypermultiplets at strong coupling, finite baryon number density, and low temperature. We find that these systems exhibit a zero sound mode despite having a nonFermi liquid type behavior of the specific heat. These properties suggest that holography identifies a new type of quantum liquids. 
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ABSTRACT: The membrane paradigm approach to black hole physics introduces the notion of a stretched horizon as a fictitious timelike surface endowed with physical characteristics such as entropy, viscosity and electrical conductivity. We show that certain properties of the stretched horizons are encoded in the quasinormal spectrum of black holes. We compute analytically the lowest quasinormal frequency of a vectortype perturbation for a generic black hole with a translationally invariant horizon (black brane) in terms of the background metric components. The resulting dispersion relation is identical to the one obtained in the membrane paradigm treatment of the diffusion on stretched horizons. Combined with the Buchel–Liu universality theorem for the membrane's diffusion coefficient, our result means that in the long wavelength limit the black brane spectrum of gravitational perturbations exhibits a universal, purely imaginary quasinormal frequency. In the context of gauge–gravity duality, this provides yet another (third) proof of the universality of shear viscosity to entropy density ratio in theories with gravity duals.Physics Letters B 07/2008; 670(45670):442445. DOI:10.1016/j.physletb.2008.11.028 · 6.02 Impact Factor 
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ABSTRACT: We consider secondorder viscous hydrodynamics in conformal field theories at finite temperature. We show that conformal invariance imposes powerful constraints on the form of the secondorder corrections. By matching to the AdS/CFT calculations of correlators, and to recent results for Bjorken flow obtained by Heller and Janik, we find three (out of five) secondorder transport coefficients in the strongly coupled = 4 supersymmetric YangMills theory. We also discuss how these new coefficents can arise within the kinetic theory of weakly coupled conformal plasmas. We point out that the MüllerIsraelStewart theory, often used in numerical simulations, does not contain all allowed secondorder terms and, frequently, terms required by conformal invariance.Journal of High Energy Physics 04/2008; 2008(04):100. DOI:10.1088/11266708/2008/04/100 · 5.62 Impact Factor 
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ABSTRACT: The holographic dual of largeNc superYangMills coupled to a small number of flavours of fundamental matter, Nf << Nc, is described by Nf probe D7branes in the gravitational background of Nc black D3branes. This system undergoes a first order phase transition characterised by the `melting' of the mesons. We study the high temperature phase in which the D7branes extend through the black hole horizon. In this phase, we compute the spectral function for vector, scalar and pseudoscalar modes on the D7brane probe. We also compute the diffusion constant for the flavour currents.Journal of High Energy Physics 07/2007; DOI:10.1088/11266708/2007/11/091 · 6.22 Impact Factor 
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ABSTRACT: We review recent progress in applying the AdS/CFT correspondence to finitetemperature field theory. In particular, we show how the hydrodynamic behavior of field theory is reflected in the lowmomentum limit of correlation functions computed through a realtime AdS/CFT prescription, which we formulate. We also show how the hydrodynamic modes in field theory correspond to the lowlying quasinormal modes of the AdS black pbrane metric. We provide a proof of the universality of the viscosity/entropy ratio within a class of theories with gravity duals and formulate a viscosity bound conjecture. Possible implications for real systems are mentioned.Annual Review of Nuclear and Particle Science 05/2007; DOI:10.1146/annurev.nucl.57.090506.123120 · 6.49 Impact Factor 
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ABSTRACT: By weakly gauging one of the U(1) subgroups of the Rsymmetry group, N=4 superYangMills theory can be coupled to electromagnetism, thus allowing a computation of photon production and related phenomena in a QCDlike nonAbelian plasma at both weak and strong coupling. We compute photon and dilepton emission rates from finite temperature N=4 supersymmetric YangMills plasma both perturbatively at weak coupling to leading order, and nonperturbatively at strong coupling using the AdS/CFT duality conjecture. Comparison of the photoemission spectra for N=4 plasma at weak coupling, N=4 plasma at strong coupling, and QCD at weak coupling reveals several systematic trends which we discuss. We also evaluate the electric conductivity of N=4 plasma in the strong coupling limit, and to leadinglog order at weak coupling. Currentcurrent spectral functions in the strongly coupled theory exhibit hydrodynamic peaks at small frequency, but otherwise show no structure which could be interpreted as welldefined thermal resonances in the hightemperature phase. Comment: 32 pages v2: added improved discussion of choice and normalization of EM currentJournal of High Energy Physics 07/2006; DOI:10.1088/11266708/2006/12/015 · 6.22 Impact Factor 
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ABSTRACT: The transport properties of certain strongly coupled thermal gauge theories can be determined from their effective description in terms of gravity or superstring theory duals. Here we provide a short summary of the results for the shear and bulk viscosity, charge diffusion constant, and the speed of sound in supersymmetric strongly interacting plasmas. We also outline a general algorithm for computing transport coefficients in any gravity dual. The algorithm relates the transport coefficients to the coefficients in the quasinormal spectrum of fivedimensional black holes in asymptotically anti de Sitter space.European Physical Journal A 06/2006; 29(1):7781. DOI:10.1140/epja/i2005103012 · 2.42 Impact Factor 
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ABSTRACT: We use the gaugegravity duality conjecture to compute spectral functions of the stressenergy tensor in finitetemperature N = 4 supersymmetric YangMills theory in the limit of large N(c) and large 't Hooft coupling. The spectral functions exhibit peaks characteristic of hydrodynamic modes at small frequency, and oscillations at intermediate frequency. The nonperturbative spectral functions differ qualitatively from those obtained in perturbation theory. The results may prove useful for lattice studies of transport processes in thermal gauge theories.Physical Review Letters 05/2006; 96(13):131601. DOI:10.1103/PhysRevLett.96.131601 · 7.73 Impact Factor 
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ABSTRACT: We consider hydrodynamics of N=4 supersymmetric SU(N_c) YangMills plasma at a nonzero density of Rcharge. In the regime of large N_c and large 't Hooft coupling the gravity dual description involves an asymptotically Anti de Sitter fivedimensional charged black hole solution of Behrnd, Cvetic and Sabra. We compute the shear viscosity as a function of chemical potentials conjugated to the three U(1) \subset SO(6)_R charges. The ratio of the shear viscosity to entropy density is independent of the chemical potentials and is equal to 1/4\pi. For a single charge black hole we also compute the thermal conductivity, and investigate the critical behavior of the transport coefficients near the boundary of thermodynamic stability. Comment: 21 pages, 1 figure; v2: typos fixed, references added; v3: typo in eq. 2.18 fixedJournal of High Energy Physics 01/2006; DOI:10.1088/11266708/2006/03/052 · 6.22 Impact Factor 
Article: Quasinormal modes and holography
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ABSTRACT: Quasinormal frequencies of electromagnetic and gravitational perturbations in asymptotically AdS spacetime can be identified with poles of the corresponding realtime Green's functions in a holographically dual finite temperature field theory. The quasinormal modes are defined for gaugeinvariant quantities which obey incomingwave boundary condition at the horizon and Dirichlet condition at the boundary. As an application, we explicitly find poles of retarded correlation functions of Rsymmetry currents and the energymomentum tensor in strongly coupled finite temperature N=4 supersymmetric SU(Nc) YangMills theory in the limit of large Nc. Comment: 28 pages; v2: typos fixed, ref. addedPhysical review D: Particles and fields 06/2005; 72(8). DOI:10.1103/PhysRevD.72.086009 · 4.86 Impact Factor 
Article: The Silence of the Little Strings
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ABSTRACT: We study the hydrodynamics of the highenergy phase of Little String Theory. The poles of the retarded twopoint function of the stress energy tensor contain information about the speed of sound and the kinetic coefficients, such as shear and bulk viscosity. We compute this twopoint function in the dual string theory and analytically continue it to Lorentzian signature. We perform an independent check of our results by the Lorentzian supergravity calculation in the background of nonextremal NS5branes. The speed of sound vanishes at the Hagedorn temperature. The ratio of shear viscosity to entropy density is equal to the universal value 1/4\pi and does not receive \alpha' corrections. The ratio of bulk viscosity to entropy density equals 1/10\pi. We also compute the Rcharge diffusion constant. In addition to the hydrodynamic singularities, the correlators have an infinite series of finitegap poles, and a massless pole with zero attenuation. Comment: 30 pages, 3 figures; harvmacJournal of High Energy Physics 06/2005; DOI:10.1088/11266708/2005/10/027 · 6.22 Impact Factor 
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ABSTRACT: The ratio of shear viscosity to volume density of entropy can be used to characterize how close a given fluid is to being perfect. Using string theory methods, we show that this ratio is equal to a universal value of variant Planck's over 2pi/4pik(B) for a large class of strongly interacting quantum field theories whose dual description involves black holes in antide Sitter space. We provide evidence that this value may serve as a lower bound for a wide class of systems, thus suggesting that black hole horizons are dual to the most ideal fluids.Physical Review Letters 04/2005; 94(11):111601. DOI:10.1103/PhysRevLett.94.111601 · 7.73 Impact Factor 
Article: Coupling constant dependence of the shear viscosity in N=4 supersymmetric Yang–Mills theory
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ABSTRACT: Gauge theory–gravity duality predicts that the shear viscosity of N=4 supersymmetric SU(Nc) Yang–Mills plasma at temperature T in the limit of large Nc and large 't Hooft coupling is independent of the coupling and equals to . In this paper, we compute the leading correction to the shear viscosity in inverse powers of 't Hooft coupling using the α′corrected lowenergy effective action of type IIB string theory. We also find the correction to the ratio of shear viscosity to the volume entropy density (equal to 1/4π in the limit of infinite coupling). The correction to 1/4π scales as with a positive coefficient.Nuclear Physics B 02/2005; DOI:10.1016/j.nuclphysb.2004.11.055 · 3.95 Impact Factor 
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ABSTRACT: Using gauge theory/gravity duality we study sound wave propagation in strongly coupled nonconformal gauge theory plasma. We compute the speed of sound and the bulk viscosity of N=2∗ supersymmetric SU(Nc) Yang–Mills plasma at a temperature much larger than the mass scale of the theory in the limit of large Nc and large 't Hooft coupling. The speed of sound is computed both from the equation of state and the hydrodynamic pole in the stress–energy tensor twopoint correlation function. Both computations lead to the same result. Bulk viscosity is determined by computing the attenuation constant of the sound wave mode.Nuclear Physics B 01/2005; DOI:10.1016/j.nuclphysb.2005.11.005 · 3.95 Impact Factor 
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ABSTRACT: We extend the correspondence between black hole physics and thermodynamics to nonequilibrium processes. The shear viscosity is shown to correspond to the absorption cross section of lowenergy graviton by the black hole. We show that the ratio of shear viscosity to volume density of entropy in theories with gravity duals is equal to a universal value of h/(4pi). We conjecture that this value is a lower limit on the ratio of shear viscosity to entropy density for a very wide class of systems. Possible applications of this viscosity bound conjecture are mentioned.
Publication Stats
6k  Citations  
160.39  Total Impact Points  
Top Journals
Institutions

2009–2011

University of Oxford
 • Rudolf Peierls Centre for Theoretical Physics
 • Department of Physics
Oxford, ENG, United Kingdom


2008

University of Southampton
 Department of Physics and Astronomy
Southampton, ENG, United Kingdom


2005–2007

Perimeter Institute for Theoretical Physics
Waterloo, Ontario, Canada


2005–2006

University of California, Santa Barbara
 Kavli Institute for Theoretical Physics
Santa Barbara, California, United States


2003–2005

University of Washington Seattle
 Institute for Nuclear Theory
Seattle, Washington, United States


2002

Columbia University
 Department of Physics
New York, New York, United States


1998–2001

CUNY Graduate Center
New York City, New York, United States


2000

NYU Langone Medical Center
New York, New York, United States
