arXiv subjects

Shibaji Roy

Publications and source records attributed to Shibaji Roy.

At least 37 records · Page 2Linked to original sources

arXiv2013

From AdS to Schrödinger/Lifshitz dual space-times without or with hyperscaling violation

It is observed that the (intersecting) branes of M/string theory, which are known to give AdS geometry (directly or upto a conformal transformation) in the near horizon limit, do also lead to Schr\" odinger/Lifshitz dual space-times (without or with hyperscaling violation) upon using appropriate solution generating transformation and dimensional reduction. We show that the dynamical exponents of the Schr\" odinger and the Lifshitz space-times obtained in this way always add upto 2. We illustrate this by several examples, including M2-, M5-branes of M-theory and D$(p+1)$-branes ($p\neq 4$, since in this case the near horizon limit does not give AdS geometry) of string theory as well as many of their intersecting solutions. The Schr\" odinger space-time can be obtained by the standard wave generating technique along one of the brane directions (for single brane) or one of the common brane directions (for intersecting branes) and then interchanging the light-cone coordinates by double Wick rotations, whereas, the Lifshitz space-time can be obtained by dimensionally reducing (for M-theory) along the wave direction or taking T-duality (for string theory) along the same direction. We thus obtain Schr\" odinger/Lifshitz dual space-times without or with hyperscaling violation from the same M/string theory solutions and they preserve some fraction of the supersymmetry.

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arXiv2013

Zero sound in strange metals with hyperscaling violation from holography

Hyperscaling violating `strange metal' phase of heavy fermion compounds can be described holographically by probe D-branes in the background of a Lifshitz space-time (dynamical exponent $z$ and spatial dimensions $d$) with hyperscaling violation (corresponding exponent $θ$). Without the hyperscaling violation, strange metals are known to exhibit zero sound mode for $z<2$ analogous to the Fermi liquids. In this paper, we study its fate in the presence of hyperscaling violation and find that in this case the zero sound mode exists for $z < 2(1+|θ|/d)$, where the positivity of the specific heat and the null energy condition of the background dictate that $θ<0$ and $z\geq 1$. However, for $z \geq 2(1+|θ|/d)$, there is no well-defined quasiparticle for the zero sound. The systems behave like Fermi liquid for $2|θ|=dz$ and like Bose liquid for $2|θ| = qdz$ (where $q$ is the number of spatial dimensions along which D-branes are extended in the background space), but in general they behave as a new kind of quantum liquid. We also compute the AC conductivity of the systems and briefly comment on the results.

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arXiv2013

Holographic entanglement entropy of the near horizon 1/4 BPS F-D$p$ bound states

It was shown in Dey and Roy (2012) that the near horizon limit of the 1/4 BPS threshold F-D$p$ (for $0\leq p \leq 5$, $p \neq 4$) bound state solutions of type II string theories give rise to space-time metrics endowed with Lifshitz scaling along with hyperscaling violation. Here we compute the holographic entanglement entropy of this system for all $p \neq 4$ (for $p=4$ the space-time has AdS$_2$ structure). For $p=3,5$, we get the expected area law behavior of the entanglement entropy. For $p=0,1$, the entanglement entropy has new area law violations and has the behavior which is in between the linear and logarithmic behaviors. For $p=2$, we get a logarithmic violation of the area law. We also compute the entanglement entropy at finite temperature and show that as the temperature rises, the entanglement entropy makes a crossover to the thermal entropy of the system. We thus obtain the string theoretic realization of holographic EE and various of its aspects noted earlier for generic metric with hyperscaling violation.

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arXiv2013

Lifshitz metric with hyperscaling violation from NS5-Dp states in string theory

In our previous papers we have shown how Lifshitz-like space-time (space-time having a Lifshitz scaling along with a hyperscaling violation) can arise by taking near horizon limits of certain intersecting solutions (F-string with D$p$-branes and also with two D-branes) of string theory. In this paper we construct intersecting bound state solutions in the form of NS5-D$p$-branes (with $1\leq p \leq 6$) of type II string theories. These are 1/4 BPS and threshold bound states unlike the known NS5-D$p$ bound states which are 1/2 BPS and non-threshold. We show that the near horizon limits of these solutions also lead to Lifshitz-like space-time with the dynamical scaling exponent $z=0$ and the hyperscaling violation exponent $θ=9-p$. The spatial dimension of the boundary theory is $d=7-p$. The dilatons in these theories are not constant in general (except for $p=5$) and therefore produce RG flows. So, we also consider the strong coupling phases of these theories and find that these phases also have similar Lifshitz-like structures, except for $p=2$, where it has an AdS$_3$ structure.

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arXiv2012

Intersecting D-branes and Lifshitz-like space-time

In a previous paper (Dey and Roy, arXiv:1203.5381) we have shown how Lifshitz-like space-times (space-times having Lifshitz scaling with hyperscaling violation) arise from 1/4 BPS, threshold F-D$p$ bound state solutions of type II string theories in the near horizon limit. In this paper we show that similar structures also arise from the near horizon limit of 1/4 BPS, threshold intersecting D-brane solutions of type II string theories. Some of these solutions are standard (D$p$-D($p+4$) for $p=0, 2$) and some are non-standard (D$p$-D($p+2)$ for $p = 1, 2, 3$) including D2-D2$'$, D3-D3$'$ and D4-D4$'$ solutions. The dilatons of these solutions in general run (except in D2-D4 and D3-D3$'$ cases) and produce RG flows. We discuss the phase structures of these solutions. D2-D4 and D3-D3$'$ in the near horizon limit do not produce Lifshitz-like space-time, but give AdS$_3$ spaces.

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arXiv2012

Lifshitz-like space-time from intersecting branes in string/M theory

We construct 1/4 BPS, threshold F-D$p$ bound states (with $0\leq p \leq 5$) of type II string theories by applying S- and T-dualities to the D1-D5 system of type IIB string theory. These are different from the known 1/2 BPS, non-threshold F-D$p$ bound states. The near horizon limits of these solutions yield Lifshitz-like space-times with varying dynamical critical exponent $z=2(5-p)/(4-p)$, for $p\neq 4$, along with the hyperscaling violation exponent $θ= p - (p-2)/(4-p)$, showing how Lifshitz-like space-time can be obtained from string theory. The dilatons are in general non-constant (except for $p=1$). We discuss the holographic RG flows and the phase structures of these solutions. For $p=4$, we do not get a Lifshitz-like space-time, but the near horizon limit in this case leads to an AdS$_2$ space.

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arXiv2012

Wilson loops in noncommutative Yang-Mills theory using gauge/gravity duality

By using the gauge/gravity duality and the Maldacena prescription we compute the expectation values of the Wilson loops in noncommutative Yang-Mills (NCYM) theory in (3+1) dimensions. We consider both the time-like and the light-like Wilson loops. The gravity dual background is given by a particular decoupling limit of (D1,D3) bound state of type IIB string theory. We obtain the velocity dependent quark-antiquark potential and numerically study how the dipole length and the potential change with velocity (for 0 < v < 1, i.e., the Wilson loop is time-like) of the dipole as well as noncommutativity. We discuss and compare the results with the known commutative results. We also obtain an analytic expression for the screening length when the rapidity is large and the noncommutativity parameter is small with the product remaining small. When v \rightarrow 1, the time-like Wilson loop becomes light-like and in that case we obtain the form of the jet quenching parameter for the strongly coupled noncommutative Yang-Mills plasma which matches with our earlier results obtained using different method.

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arXiv2012

Calculating the jet quenching parameter in the plasma of NCYM theory from gauge/gravity duality

A particular decoupling limit of non-extremal (D1, D3) brane bound state system of type IIB string theory is known to give the gravity dual of space-space non- commutative Yang-Mills (NCYM) theory at finite temperature. We use a string probe in this background to compute the jet quenching parameter in the strongly coupled plasma of hot non-commutative Yang-Mills theory in (3+1)-dimensions from gauge/gravity duality. We give the expressions for the jet quenching parameter for both small and large non-commutativity. For small non-commutativity, we find that the value of the jet quenching parameter gets reduced from its commutative value. The reduction is enhanced with temperature as T^{7} for fixed non-commutativity and the fixed 't Hooft coupling. We further generalize the results for the non-commutative Yang-Mills theories in diverse dimensions.

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arXiv2011

The enriched phase structure of black branes in canonical ensemble

It is found that a necessary completion of phase structure of D-dimensional charged black p-brane ($p > 0$) in a cavity requires two additional thermodynamical phases, the so-called "bubble of nothing" and/or the extremal brane, in canonical ensemble. This finding resolves the puzzle about the missing phases which are needed for the underlying phase diagram when $\tilde d = D - p - 3 \leq 2$ and gives a new (bubble) phase which can become globally stable when $\tilde d > 2$. An analog of Hawking-Page transition is also found among other new phase transitions, giving a complete phase structure in this setup.

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arXiv2011

Accelerating cosmologies from M/String theory compactifications

We point out that the solution of $(4+n)$-dimensional gravity coupled to the dilaton and an $n$-form field strength can give rise to a flat 4-dimensional universe (with a scale factor) of the type proposed recently under time dependent compactifications. The compact internal spaces could be hyperbolic, flat or spherical and the solution is identical to the space-like two brane or S2-brane. As has been shown previously for SM2 solution with a fixed field strength we show that for $n=7$ (where the dilaton is vanishing and with a general field strength), 6 the corresponding SM2 and SD2 solutions can give accelerating cosmologies in Einstein frame for both hyperbolic and flat internal spaces, thereby meeting the challenge of obtaining such a solution from M/String theory compactifications.

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arXiv2011

Phase structure of black branes in grand canonical ensemble

This is a companion paper of our previous work [1] where we studied the thermodynamics and phase structure of asymptotically flat black $p$-branes in a cavity in arbitrary dimensions $D$ in a canonical ensemble. In this work we study the thermodynamics and phase structure of the same in a grand canonical ensemble. Since the boundary data in two cases are different (for the grand canonical ensemble boundary potential is fixed instead of the charge as in canonical ensemble) the stability analysis and the phase structure in the two cases are quite different. In particular, we find that there exists an analog of one-variable analysis as in canonical ensemble, which gives the same stability condition as the rather complicated known (but generalized from black holes to the present case) two-variable analysis. When certain condition for the fixed potential is satisfied, the phase structure of charged black $p$-branes is in some sense similar to that of the zero charge black $p$-branes in canonical ensemble up to a certain temperature. The new feature in the present case is that above this temperature, unlike the zero-charge case, the stable brane phase no longer exists and `hot flat space' is the stable phase here. In the grand canonical ensemble there is an analog of Hawking-Page transition, even for the charged black $p$-brane, as opposed to the canonical ensemble. Our study applies to non-dilatonic as well as dilatonic black $p$-branes in $D$ space-time dimensions.

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arXiv2011

Wilson loops in (p+1)-dimensional Yang-Mills theories using gravity/gauge theory correspondence

We compute the expectation values of both the time-like and the light-like Wilson loops in a strongly coupled plasma of (p+1)-dimensional Yang-Mills theories using gravity/gauge theory correspondence. From the time-like Wilson loop we obtain the velocity dependent quark-antiquark potential where the dipole is moving through the plasma with an arbitrary velocity 0 1, the Wilson loop becomes light-like and we obtain the form of the jet quenching parameter in those strongly coupled plasma.

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arXiv2011

Phase transitions and critical behavior of black branes in canonical ensemble

We study the thermodynamics and phase structure of asymptotically flat non-dilatonic as well as dilatonic black branes in a cavity in arbitrary dimensions ($D$). We consider the canonical ensemble and so the charge inside the cavity and the temperature at the wall are fixed. We analyze the stability of the black brane equilibrium states and derive the phase structures. For the zero charge case we find an analog of Hawking-Page phase transition for these black branes in arbitrary dimensions. When the charge is non-zero, we find that below a critical value of the charge, the phase diagram has a line of first-order phase transition in a certain range of temperatures which ends up at a second order phase transition point (critical point) as the charge attains the critical value. We calculate the critical exponents at that critical point. Although our discussion is mainly concerned with the non-dilatonic branes, we show how it easily carries over to the dilatonic branes as well.

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arXiv2010

Drag force in a hot non-relativistic, non-commutative Yang-Mills plasma

We apply the standard technique of Null Melvin Twist to the non-extremal (D1, D3) bound state configuration of type IIB string theory. Under a particular decoupling limit, such configuration represents the gravity dual of the non-relativistic, non-commutative Yang-Mills theory at a finite temperature. We then use the AdS/CFT and the string probe approach to compute the drag force on an external quark moving through such a hot non-relativistic, non-commutative YM plasma. We discuss various limiting cases to show the interplay between the non-relativistic as well as the non-commutative effect of the general drag force expression.

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arXiv2010

Remarks on the instability of black Dp-branes

We show that for black D$p$-branes having charge $Q$ and Hawking temperature $T$, the product $QT^{7-p}$ is bounded from above for $p\leq 5$ and is unbounded for $p=6$. While the maximum occurs at some finite value of a parameter for $p \leq 4$, it occurs at infinity of the parameter for $p=5$. As a consequence, for fixed charge, there are two black D$p$-branes (for $p\leq 4$) at any given temperature less than its maximum value, and when the temperature is maximum there is one black D$p$-brane. For $p=5$, there is only one black D5-brane at a given temperature less than its maximum value, whereas, for $p=6$, since there is no bound for the temperature, there is always a black D6-brane solution at a given temperature. Of the two black D$p$-branes (for $p\leq 4$), one is large which is shown to be thermodynamically unstable and the other is small which is stable. But for $p=5,6$, the black D$p$-branes are always thermodynamically unstable. The stable, small black D$p$-brane, however, under certain conditions, can become unstable quantum mechanically and decay either to a BPS D$p$-brane or to a Kaluza-Klein "bubble of nothing" through closed string tachyon condensation. The small D5, D6 branes, although classically unstable, have the same fate under similar conditions.

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arXiv2009

Holography and drag force in thermal plasma of non-commutative Yang-Mills theories in diverse dimensions

We use holography and a string probe approach to compute the drag force on a quark moving in a thermal plasma of non-commutative Yang-Mills (NCYM) theories in various dimensions. The gravity background in these cases are described by a particular decoupling limit of non-extremal (D$(p-2)$, D$p$) brane bound state system. We show how the drag force on an external quark moving in the dual NCYM theories gets corrected due to non-commutativity and as a result the effective viscosity of the plasma gets reduced. We have obtained the drag force for both small and large non-commutativity. This was known earlier for (3+1)-dimensional NCYM theory, however, we find that the corrections for the general case typically depend on the dimensionality of the NCYM theories, indicating that the structure of the drag force is non-universal.

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arXiv2009

Non-supersymmetric D1/D5, F/NS5 and closed string tachyon condensation

We construct the intersecting non-supersymmetric (non-susy) D1/D5 solution of type IIB string theory. While, as usual, the solution is charged under an electric two-form and an electric six-form gauge field, it also contains a non-susy chargeless (non-BPS) D0-brane. The S-dual of this solution is the non-susy F/NS5 solution. We show how these solutions nicely interpolate between the corresponding black (or non-extremal) solutions and the Kaluza-Klein (KK) "bubble of nothing" (BON) by continuously changing some parameters characterizing the solutions from one set of values to another. We show, by a time symmetric general bubble initial data analysis, that the final bubbles in these cases are static and stable and the interpolations can be physically interpreted as closed string tachyon condensation. As special cases, we recover the transition of two charge black F-string to BON, considered by Horowitz, and also the transition from AdS$_3$ black hole to global AdS$_3$.

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arXiv2009

Intersecting non-SUSY branes and closed string tachyon condensation

Following \cite{Bai:2006vv} we here consider the supergravity solutions representing the charged non-supersymmetric $p$-brane (for $1\leq p \leq 6$) intersecting with chargeless non-supersymmetric 1-brane and 0-brane of type II string theories. We show how these solutions nicely interpolate between black $p$-branes and the Kaluza-Klein "bubble of nothing" (BON) by continuously varying some parameters characterizing the solutions from one set of values to another. By performing a time symmetric general bubble initial data analysis, we show that the interpolation implies a possible transition from black $p$-branes to KK BON only for $p\leq 4$, as in these cases there exist locally stable static bubbles under certain conditions. However, for $p>4$, black branes always decay into dynamical bubbles. Contrary to what is known in the literature, we argue that under certain conditions, the mechanism causing this transition to stable static bubbles can be the closed string tachyon condensation for all $p\leq 4$. We also show that the configurations indeed contain (chargeless) F-strings required for the closed string tachyons to appear causing the transitions to occur.

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