arXiv subjects

Shesansu Sekhar Pal

Publications and source records attributed to Shesansu Sekhar Pal.

At least 19 recordsLinked to original sources

arXiv2021

Holographic universal relations among transport coefficients

In this paper, we show universal relations among the transport coefficients by calculating the electrical conductivity, thermal conductivity and thermo-electric conductivity in the presence of a chemical potential and magnetic fields for Einstein-Maxwell-dilaton-axion system in arbitrary but even dimensional bulk spacetime as well as for Einstein-DBI-dilaton-axion system in $3+1$ dimensional bulk spacetime. Moreover, we have also obtained a new hyperscale violating black hole solution with finite charge density and magnetic fields but with a trivial dilaton field at IR.

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arXiv2020

Transport properties of DBI action

We obtain precise temperature dependence of three transport coefficients that resembles strange metal: (a) longitudinal electrical conductivity (b) longitudinal thermoelectric conductivity (c) longitudinal thermal conductivity in the absence of magnetic field. This is achieved through one parameter, namely, when the dynamical exponent, $z=4$. Moreover, in the presence of magnetic field, we have cooked up a model where it exhibits the desired temperature dependence of the four transport quantities that resembles strange metal.

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arXiv2019

Thermal and thermoelectric conductivity of Einstein-DBI system

We calculate the thermoelectric and thermal conductivity of Einstein-DBI system without the translational symmetry. We show that strictly, the longitudinal component of the thermal conductivity in the vanishing dissipation limit, $k^2\rightarrow 0$, is determined only by the metric components evaluated at the horizon of the black hole in the same limit. The temperature dependence of the transport quantities are obtained for $AdS_4$ and $AdS_2\times R^2$ solutions. The Lorentz ratio and Hall-Lorentz ratio are found to be independent of temperature for $AdS_4$ solution in the low temperature limit, whereas the above mentioned quantities are independent of temperature for $AdS_2\times R^2$ solution.

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arXiv2018

Thermodynamics of Einstein-DBI System

We continue our study of the Einstein-Dirac-Born-Infeld system as started in \cite{Pal:2012zn}. In this paper, we studied the thermodynamical aspect of the black hole by keeping the horizon as planar in the grand canonical ensemble. In particular, we obtain the first law of thermodynamics and the Smarr type relation. Also, studied the thermodynamic stability.

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arXiv2015

Entanglement Temperature With Gauss-Bonnet Term

We compute the entanglement temperature using the first law-like of thermodynamics, $ΔE=T_{ent} ΔS_{EE}$, up to Gauss-Bonnet term in the Jacobson-Myers entropy functional in any arbitrary spacetime dimension. The computation is done when the entangling region is the geometry of a slab. We also show that such a Gauss-Bonnet term, which becomes a total derivative, when the co-dimension two hypersurface is four dimensional, does not contribute to the finite term in the entanglement entropy. We observe that the Weyl-squared term does not contribute to the entanglement entropy. It is important to note that the calculations are performed when the entangling region is very small and the energy is calculated using the normal Hamiltonian.

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arXiv2015

Weak Minimal Area In Entanglement Entropy

We re-visit the minimal area condition of Ryu-Takayanagi in the holographic calculation of the entanglement entropy. In particular, the Legendre test and the Jacobi test. The necessary condition for the weak minimality is checked via Legendre test and its sufficient nature via Jacobi test. We show for AdS black hole with a strip type entangling region that it is this minimality condition that makes the hypersurface not to cross the horizon, which is in agreement with that studied earlier by {\it Engelhardt et al.} and {\it Hubeny} using a different approach. Moreover, demanding the weak minimality condition on the entanglement entropy functional with the higher derivative term puts a constraint on the Gauss-Bonnet coupling: that is there should be an upper bound on the value of the coupling, $λ_a< \f{(d-3)}{4(d-1)}$.

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arXiv2014

Extremal Surfaces And Entanglement Entropy

We have obtained the equation of the extremal hypersurface by considering the Jacobson-Myers functional and computed the entanglement entropy. In this context, we show that the higher derivative corrected extremal surfaces can not penetrate the horizon. Also, we have studied the entanglement temperature and entanglement entropy for low excited states for such higher derivative theories when the entangling region is of the strip type.

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arXiv2013

Fermi-like Liquid From Einstein-DBI-Dilaton System

We have obtained an expression of the entropy density depending on the scale transformation of the spatial directions in the field theory. It takes the following form in $d+1$ dimensional bulk spacetime: $s\sim T^{\f{δ(d-1)-θ}{z}}_H$, where $z$ and $T_H$ are the dynamical exponent and temperature in the field theory, respectively. $θ$ is related to the scaling violation exponent, whereas $δ$ gives us the information about the scaling behavior of the spatial field theoretic direction. This we demonstrate by finding solutions to the Einstein-DBI-dilaton system in generic spacetime dimensions. Upon restricting to $d=3$, we show the linear temperature dependence of the specific heat and inverse quadratic temperature dependence of the resistivity for $z=2,θ=0$ and $δ=1$, which resembles that of the Fermi-like liquid. Whereas for $z=2, θ=-2$ and $δ=0$ gives us a solution that is conformal to $AdS_2\otimes R^2$, which resembles with the non-Fermi-like liquid. Moreover, it shows the logarithmic violation of the entanglement entropy when the entangling region is of the strip type.

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arXiv2012

Approximate strange metallic behavior in AdS

We show for unit dynamical exponent, $z=1$, the appearance of the Fermi liquid and non-Fermi liquid behavior as we tune the charge density and the magnetic field in 3+1 dimensional field theory using the gauge-gravity duality. There exists an upturn behavior of the resistivity only along the direction perpendicular to the magnetic field. Also, there exists a universal behavior of the resistivity, independent of the dimensionality of the spacetime, in a specific corner of the parameter space, namely, in the large charge density and small magnetic field limit: the longitudinal conductivity goes as $T^{-2/z}$, whereas the Hall conductivity goes as $T^{-4/z}$. It means the Hall coefficient goes as $T^{4/z}$. We compute the diffusion constant from the flow equation of the conductivity.

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arXiv2011

Model building in AdS/CMT: DC conductivity and Hall angle

Using the bottom-up approach in a holographic setting, we attempt to study both the transport and thermodynamic properties of a generic system in 3+1 dimensional bulk spacetime. We show the exact 1/T and $T^2$ dependence of the longitudinal conductivity and Hall angle, as seen experimentally in most copper-oxide systems, which are believed to be close to quantum critical point. This particular temperature dependence to conductivities are possible in two different cases: (1) Background solutions with scale invariant and broken rotational symmetry, (2) solutions with pseudo-scaling and unbroken rotational symmetry but only at low density limit. Generically, the study of the transport properties in a scale invariant background solution, using the probe brane approach, at high density and at low temperature limit suggests us to consider only metrics with two exponents. More precisely, the spatial part of the metric components should not be same i.e., $g_{xx}\neq g_{yy}$. In doing so, we have generated the above mentioned behavior to conductivity with a very special behavior to specific heat which at low temperature goes as: $C_V\sim T^3$. However, if we break the scaling symmetry of the background solution by including a nontrivial dilaton, axion or both and keep the rotational symmetry then also we can generate such a behavior to conductivity but only in the low density regime. As far as we are aware, this particular temperature dependence to both the conductivity and Hall angle is being shown for the first time using holography.

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arXiv2011

RG flow of transport quantities

The RG flow equation of various transport quantities are studied in arbitrary spacetime dimensions, in the fixed as well as fluctuating background geometry both for the Maxwellian and DBI type of actions. The regularity condition on the flow equation of the conductivity at the horizon for the DBI action reproduces naturally the leading order result of {\it Hartnoll et al.}, [{\it JHEP}, {\bf 04}, 120 (2010)]. Motivated by the result of {\it van der Marel et al.}, [{\it science}, {\bf 425}, 271 (2003], we studied, analytically, the conductivity versus frequency plane by dividing it into three distinct parts: $ω T$ and $ω>> T$. In order to compare, we choose 3+1 dimensional bulk spacetime for the computation of the conductivity. In the $ω T$ range and staying away from the horizon, for the DBI action with unit dynamical exponent, non-zero magnetic field and charge density, the conductivity goes as $ω^{-2/3}$, whereas the phase of the conductivity, goes as, $ArcTan(Imσ^{xx}/Reσ^{xx})=π/6$ and $ArcTan(Imσ^{xy}/Reσ^{xy})=-π/3$. There exists a universal quantity at the horizon that is the phase angle of conductivity, which either vanishes or an integral multiple of $π$. Furthermore, we calculate the temperature dependence to the thermoelectric and the thermal conductivity at the horizon. The charge diffusion constant for the DBI action is studied.

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arXiv2010

Quantum phase transition in a Dp-Dq system

Using the top-down approach, we study intersecting Dp-Dq brane configuration in string theory and find examples, where there can be a quantum phase transition at zero temperature induced by the violation of the Breitenlohner-Freedman (BF) bound at IR, which is done essentially by a combination of charge density and magnetic fields. In particular, there exists a Berezinskii-Kosterlitz-Thouless (BKT) type of transition for D3-D5 and D5-D5 systems. The study of the BKT type of transition is initiated by Jensen et al. [Phys. Rev. Lett. {\bf 105}, 041601 (2010) ] for a D3-D5 system with nonzero charge density and a magnetic field. Here, we show that one can have the BKT transition for a D3-D5 system even in the absence of charge density but requires multiple magnetic fields. In this case the field theory lives in 2+1 dimensions, whereas for the D5-D5 type, the transition requires the presence of both the charge density and magnetic fields and the dual field theory lives on a 3+1 dimensional spacetime. We also study the D3-D7 system but it does not show the BKT type of transition.

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arXiv2010

Weak Gravity Conjecture, Central Charges and $η/s$

We correlate the weak gravity conjecture (WGC), the KSS conjecture with chemical potential at extremality and the central charges by going through a particular example in five dimensional AdS spacetime with two unknown coefficients $c_1, c_2$, assuming WGC exists in AdS spacetime. The result that follows from this example suggests that WGC makes the KSS conjecture to hold in the extremal limit but only when one of the coefficient vanishes ($c_1=0, c_2\neq 0$ or $c_2=0, c_1\neq 0$) and when both the coefficients are non zero it can respect and/or violate the KSS conjecture depending on the choice to $c_1$ at extremality, even though $η/s$ do not depend on $c_1$ at extremality. Moreover, WGC is not fully compatible with the calculation of central charges even though the bounds on coefficient $c_1$ that follows from demanding WGC stays within the bounds that central charges predict. As usual, the KSS conjecture is violated, of course, in the non-extremal limit.

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arXiv2010

$η/s$ at finite coupling

We compute the ratio of the coefficient of shear viscosity to entropy density at finite coupling and at zero chemical potential using holographic duality up to ten derivative terms in the low energy effective 5-dimensional action, of a specific kind, which may or may not be connected to the supersymmetric completion of Type IIB theory. The result suggests that this ratio can be positive only for the 8th derivative term even with the form of that term in the action as ${{\cal C}^{ij}}_{kl}{{\cal C}^{kl}}_{mn}{{\cal C}^{mn}}_{rs}{{\cal C}^{rs}}_{ij}$, where ${\cal C}$ is the Weyl tensor.

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arXiv2009

Non-relativistic supersymmetric Dp branes

The supergravity solutions to various $p$-branes are presented and typically it breaks one quarter of supersymmetry except for the D1 brane case, where it breaks one half of supersymmetry and are manifestly non-relativistic in nature. The symmetries that the solutions enjoys are that of space and time translations, rotations, boosts but without any scaling and special conformal transformations except for $p\neq 3$. We have also constructed supersymmetric non-relativistic cascading solutions to intersecting $D3$ and $D5$ branes on both the singular as well as on deformed conifold, where the $D5$'s are wrapped on the $S^2$ of the Calabi-Yau i.e. the analogs of Klebanov-Tseytlin and Klebanov-Strassler solutions and the supersymmetric non-relativistic M2 brane solution.

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arXiv2004

Probing Maldacena-Nunez in IR with ${\bar D3}$ branes

We probed Maldacena-Nunez solution in IR with p coincident anti D3 branes and found that these probe branes become a fuzzy NS5 brane. Doing the dual analysis i.e. from the NS5 brane point of view with the charge of p anti D3 brane on the world-volume of NS5 brane, we showed that to leading order this potential matches with that of p anti D3 branes and the potential on the NS5 brane has a stable minima and have also calculated the potential, from the NS5 brane point of view, for a small fluctuation along the radial direction.

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arXiv2003

Nonsingular Cosmologies from Branes

We analyse possible cosmological scenarios on a brane where the brane acts as a dynamical boundary of various black holes with anti-de Sitter or de Sitter asymptotics. In many cases, the brane is found to describe completely non-singular universe. In some cases, quantum gravity era of the brane-universe can also be avoided by properly tuning bulk parameters. We further discuss the creation of a brane-universe by studying its wave function. This is done by employing Wheeler-De Witt equation in the mini superspace formalism.

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arXiv2002

Coincident Dp-branes in codimension two

We derive the mass shell condition for N coincident D0 branes in codimension two i.e. in space-time dimension three. Using this we present the action for this system in first order formalism. Our analysis is restricted to flat space-time.

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