arXiv subjects

Manuela Kulaxizi

Publications and source records attributed to Manuela Kulaxizi.

30 records · Page 2Linked to original sources

arXiv2011

Holographic Entanglement Entropy in Lovelock Gravities

We study entanglement entropies of simply connected surfaces in field theories dual to Lovelock gravities. We consider Gauss-Bonnet and cubic Lovelock gravities in detail. In the conformal case the logarithmic terms in the entanglement entropy are governed by the conformal anomalies of the CFT; we verify that the holographic calculations are consistent with this property. We also compute the holographic entanglement entropy of a slab in the Gauss-Bonnet examples dual to relativistic and non-relativistic CFTs and discuss its properties. Finally, we discuss features of the entanglement entropy in the backgrounds dual to renormalization group flows between fixed points and comment on the implications for a possible c-theorem in four spacetime dimensions.

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arXiv2010

Energy Flux Positivity and Unitarity in CFTs

We show that in most conformal field theories the condition of the energy flux positivity, proposed by Hofman and Maldacena, is equivalent to the absence of ghosts. At finite temperature and large energy and momenta, the two-point functions of the stress energy tensor develop lightlike poles. The residues of the poles can be computed, as long as the only spin two conserved current, which appears in the stress energy tensor OPE and acquires nonvanishing expectation value at finite temperature, is the stress energy tensor. The condition for the residues to stay positive and the theory to remain ghost free is equivalent to the condition of positivity of energy flux.

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arXiv2010

Supersymmetry Constraints in Holographic Gravities

Supersymmetric higher derivative gravities define superconformal field theories via the AdS/CFT correspondence. From the boundary theory viewpoint, supersymmetry implies a relation between the coefficients which determine the three point function of the stress energy tensor which can be tested in the dual gravitational theory. We use this relation to formulate a necessary condition for the supersymmetrization of higher derivative gravitational terms. We then show that terms quadratic in the Riemann tensor do not present obstruction to supersymmetrization, while generic higher order terms do. For technical reasons, we restrict the discussion to seven dimensions where the obstruction to supersymmetrization can be formulated in terms of the coefficients of Weyl anomaly, which can be computed holographically.

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arXiv2010

Holographic Lovelock Gravities and Black Holes

We study holographic implications of Lovelock gravities in AdS spacetimes. For a generic Lovelock gravity in arbitrary spacetime dimensions we formulate the existence condition for asymptotically AdS black holes. We consider small fluctuations around these black holes and determine the constraint on Lovelock parameters by demanding causality of the boundary theory. For the case of cubic Lovelock gravity in seven spacetime dimensions we compute the holographic Weyl anomaly and determine the three point functions of the stress energy tensor in the boundary CFT. Remarkably, these correlators happen to satisfy the same relation as the one imposed by supersymmetry. We then compute the energy flux; requiring it to be positive is shown to be completely equivalent to requiring causality of the finite temperature CFT dual to the black hole. These constraints are not stringent enough to place any positive lower bound on the value of viscosity. Finally, we conjecture an expression for the energy flux valid for any Lovelock theory in arbitrary dimensions.

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arXiv2010

AdS_7/CFT_6, Gauss-Bonnet Gravity, and Viscosity Bound

We study the relation between the causality and the positivity of energy bounds in Gauss-Bonnet gravity in AdS_7 background and find a precise agreement. Requiring the group velocity of metastable states to be bounded by the speed of light places a bound on the value of Gauss-Bonnet coupling. To find the positivity of energy constraints we compute the parameters which determine the angular distribution of the energy flux in terms of three independent coefficients specifying the three-point function of the stress-energy tensor. We then relate the latter to the Weyl anomaly of the six-dimensional CFT and compute the anomaly holographically. The resulting upper bound on the Gauss-Bonnet coupling coincides with that from causality and results in a new bound on viscosity/entropy ratio.

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arXiv2009

Notes on Properties of Holographic Matter

Probe branes with finite worldvolume electric flux in the background created by a stack of Dp branes describe holographically strongly interacting fundamental matter at finite density. We identify two quantities whose leading low temperature behavior is independent of the dimensionality of the probe branes: specific heat and DC conductivity. This behavior can be inferred from the dynamics of the fundamental strings which provide a good description of the probe branes in the regime of low temperatures and finite densities. We also comment on the speed of sound on the branes and the temperature dependence of DC conductivity at vanishing charge density.

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arXiv2008

Holographic Responses of Fermion Matter

We consider the $D4-D8-\bar{D8}$ brane system which serves as ultraviolet completion of the Nambu-Jona-Lasinio model, where the only degrees of freedom carrying baryon charge are fermions. By turning on chemical potential for this charge one may expect the formation of the Fermi liquid ground state. At strong coupling we use the dual holographic description to investigate the responses of the system to small perturbations. In the chirally symmetric phase we find that the density dependent part of the heat capacity vanishes linearly with temperature. We also observe a zero sound excitation in the collisionless regime, whose speed is equal to that of normal sound in the hydrodynamic regime. Both the linear dependence of the heat capacity and the existence of zero sound are properties of the Fermi liquid ground state. We also compute the two-point function of the currents at vanishing frequency but do not find any singularities at finite values of the momentum.

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arXiv2008

Comments on Fermi Liquid from Holography

We investigate the signatures of Fermi liquid formation in the N=4 super Yang-Mills theory coupled to fundamental hypermultiplet at nonvanishing chemical potential for the global U(1) vector symmetry. At strong 't Hooft coupling the system can be analyzed in terms of the D7 brane dynamics in AdS_5 x S^5 background. The phases with vanishing and finite charge density are separated at zero temperature by a quantum phase transition. In case of vanishing hypermultiplet mass, Karch, Son and Starinets discovered a gapless excitation whose speed equals the speed of sound. We find that this zero sound mode persists to all values of the hypermultiplet mass, and its speed vanishes at the point of phase transition. The value of critical exponent and the ratio of the velocities of zero and first sounds are consistent with the predictions of Landau Fermi liquid theory at strong coupling.

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arXiv2007

Half-BPS Geometries and Thermodynamics of Free Fermions

Solutions of type IIB supergravity which preserve half of the supersymmetries have a dual description in terms of free fermions, as elucidated by the "bubbling AdS" construction of Lin, Lunin and Maldacena. In this paper we study the half-BPS geometry associated with a gas of free fermions in thermodynamic equilibrium obeying the Fermi-Dirac distribution. We consider both regimes of low and high temperature. In the former case, we present a detailed computation of the ADM mass of the supergravity solution and find agreement with the thermal energy of the fermions. The solution has a naked null singularity and, by general arguments, is expected to develop a finite area horizon once stringy corrections are included. By introducing a stretched horizon, we propose a way to match the entropy of the fermions with the entropy of the geometry in the low temperature regime. In the opposite limit of high temperature, the solution resembles a dilute gas of D3 branes. Also in this case the ADM mass of the geometry agrees with the thermal energy of the fermions.

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arXiv2006

On beta-deformations and Noncommutativity

We elucidate the connection between the N=1 beta-deformed SYM theory and noncommutativity. Our starting point is the T-duality generating transformation involved in constructing the gravity duals of both beta-deformed and noncommutative gauge theories. We show that the two methods can be identified provided that a particular submatrix of the O(3,3,R) group element employed in the former case, is interpreted as the noncommutativity parameter associated with the deformation of the transverse space. It is then explained how to construct the matrix in question, relying solely on information extracted from the gauge theory Lagrangian and basic notions of AdS/CFT. This result may provide an additional tool in exploring deformations of the N=4 SYM theory. Finally we use the uncovered relationship between beta-deformations and noncommutativity to find the gravity background dual to a noncommutative gauge theory with beta-type noncommutativity parameter.

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arXiv2005

Marginal Deformations of Tree-Level N=4 SYM from Twistor String Theory

The topological B-model with target the supertwistor space CP(3|4) is known to describe perturbative amplitudes of N=4 Super Yang-Mills theory. We review the extension of this correspondence to the superconformal gauge theories that arise as marginal deformations of N=4 by considering the effects of turning on a certain closed string background, which results in non-anticommutativity in the fermionic directions of CP(3|4). We generalise the twistor string prescription for amplitudes to this case and illustrate it with some simple examples.

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arXiv2005

Marginal Deformations of N=4 SYM from Open/Closed Twistor Strings

We investigate how the marginal deformations of N=4 supersymmetric Yang-Mills theory (analysed in particular by Leigh and Strassler) arise within B-model topological string theory on supertwistor space CP(3|4). This is achieved by turning on a certain closed string background in the fermionic directions. Through a specific open/closed correlation function, this mode induces a correction to holomorphic Chern-Simons theory, corresponding to the self-dual part of the chiral operator added on the gauge theory side. The effect of the deformation is interpreted as non-anticommutativity between some of the odd coordinates of CP(3|4). Motivated by this, we extend the twistor formalism for calculating MHV amplitudes in N=4 SYM to these N=1 theories by introducing a suitable star product between the wavefunctions. We check that our prescription yields the expected results to linear order in the deformation parameter.

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