arXiv subjects

A. Shirzad

Publications and source records attributed to A. Shirzad.

At least 19 recordsLinked to original sources

arXiv2020

Black Hole Entropy and Boundary Conditions

It is well-known that in order to make the action well defined, one may employ different kinds of boundary conditions (BCs) accompanied by the appropriate Gibbons-Hawking-York (GHY) terms. In this paper we investigate the role of the selected BC and the corresponding GHY terms on the black hole (BH) entropy. Our result shows, regardless of the kind of BC, the BH entropy in all cases is the same as one obtained under Dirichlet BC from Wald formula or semi-classical approximation method. We considered the Schwarzschild solution for $f(R)$-gravity and general relativity (GR) in standard dimensions as special models.

gr-qc↗

arXiv2018

Phononic Casimir corrections for Graphene resonator

By calculating a Casimir energy for the acoustic phonons of Graphene, we find some temperature-dependent corrections for the pretension of a Graphene sheet suspended on a trench. We obtain values of the order of few mN/m for these corrections in fully as well as doubly clamped Graphene on a narrow trench with one nanometer width, at room temperature. These values are considerable compared to the experimental values, and can increase the fundamental resonance frequency of the Graphene. The values of these corrections increase by increasing the temperature, and so they can be utilized for tuning the Graphene pretension.

cond-mat.mes-hall↗

arXiv2018

On the classification of consistent boundary conditions for $ \mathit{f}(\mathit{R})$-Gravity

Using a completely covariant approach, we discuss the role of boundary conditions (BCs) and the corresponding Gibbons--Hawking--York (GHY) terms in $ \mathit{f}(\mathit{R}) $-gravity in arbitrary dimensions. We show that $ f(\mathit{R}) $-gravity, as a higher derivative theory, is not described by a degenerate Lagrangian, in its original form. Hence, without introducing additional variables, one can not obtain consistent BCs, even by adding the GHY terms (except for $f(\mathit{R})=R$). However, following the Ostrogradsky approach, we can introduce a scalar field in the framework of Brans-Dicke formalism to the system to have consistent BCs by considering appropriate GHY terms. In addition to the Dirichlet BC, the GHY terms for both Neumann and two types of mixed BCs are derived. We show the remarkable result that the $f(\mathit{R})$-gravity is itself compatible with one type of mixed BCs, in $D$ dimension, i.e. it doesn't require any GHY term. For each BC, we rewrite the GHY term in terms of Arnowit-Deser-Misner (ADM) variables.

gr-qc↗

arXiv2017

Casimir energy for acoustic phonons in graphene

We find the Casimir energy, at finite temperature, for acoustic phonons in a Graphene sheet suspended over a rectangular trench, and the corresponding Casimir forces are interpreted as correction terms to the built-in tensions of the Graphene. We show that these corrections generally break the tensional isotropy of the membrane, and can increase or decrease the membrane tension. We demonstrate that for a narrow rectangular trench with side-lengths in the order of few nanometers and few micrometers, these temperature corrections are expected to be noticeable ($\sim 10^{-4} N/m$) at the room temperature. These corrections would be even more considerable by increasing the temperature, and can be applied for adjusting the built-in tension of the Graphene. Consequently we introduce a corrected version for the fundamental resonance frequency of the Graphene resonator.

cond-mat.mes-hall↗

arXiv2017

Casimir temperature correction to the Bosonic string mass

Through the Casimir effect, we obtain a temperature correction to the ground state mass of the Bosonic string. Accordingly, we show that higher than the temperature $\approx 1/2π$, the Tachyon attain real mass, and also we find massive Graviton for nonzero temperatures. As another consequence, we demonstrate that for temperatures higher than the Hagedorn temperature, the string would find again a Tachyonic state.

hep-th↗

arXiv2017

Fermionic Casimir effect in Graphene

We investigate the Fermionic Casimir effect at finite temperature for two parallel chain of adatoms in a Graphene sheet, and the corresponding Casimir force is interpreted as an interaction between the adatom chains. We apply useful techniques to find asymptotically explicit expressions for the Casimir energy, for small as well as large temperatures (with respect to the effective temperature of the Graphene). We obtain a value in the order of $10^{-2} N/m$ for the Casimir force between (per unit length of) the adatom chains being one nanometer apart, which is considerably noticeable e.g. in comparison to the experimental values for built-in tension of a suspended Graphene sheet.

cond-mat.mes-hall↗

arXiv2017

Quantum vacuum effects for a massive Bosonic string in background B-field

We study the Casimir effect for a bosonic string extended between D-branes, and living in a flat space with an antisymmetric background B-field. We find the Casimir energy as a function of theB-field, and the mass-parameter of the string, and accordingly we obtain a B-dependence correction term to the ground-state mass of the string. We show that for sufficiently large B-field, the ground state of the string contains real (i.e. non-Tachyonic) particles.

hep-th↗

arXiv2014

Casimir-like corrections to the classical tensions of the strings and membranes

We find the Casimir-like energies for strings and membranes. We show that the related Casimir forces can be interpreted as quantum corrections to the classical tensions of the strings and membranes. We see that these corrections always increase the tensions of the circular string as well as spherical membrane, while for the straight string, rectangular and cylindrical membranes, these Casimir forces may increase or decrease the tensions. So we find that the quantum vacuum can break the (tensional) isotropy of the rectangular and cylindrical membranes. Also obtaining the nonzero-temperature Casimir energy, we find relations for the tensions at nonzero temperature.

hep-th↗

arXiv2014

Simplifying the algebra of first class constraints, examples on SO(3) and SO(4)

We discuss the problem of non abelian constrained systems and the origin of appearance of non abelian algebras. We show that it is possible, in principle, to change a non abelian system to an abelian one, at least locally. Our method is based on solutions of the differential equations due to the algebra of first class constraints. We suggest that multiplicative constraints should be regularized at each step of calculation. Some examples on SO(3) and SO(4) algebras are discussed. We show that they can be replaced by abelian constraints.

hep-th↗

arXiv2013

Quantization In Finite Volumes Using Symplectic Quantization Programm

We use the ideas of symplectic quantization for quantizing fields in finite volumes. We consider, as examples, the Klein-Gordon and electromagnetic fields in three dif- ferent boxes. As a second idea we consider the given boundary conditions as primary constrains. Consistency of primary constrains leads to infinite chains of constraints at the boundaries. Without solving the equation of motion, we impose the set of con- straints on suitable expansions of the fields. We show that if the new set of variables, such as Fourier modes, are chosen appropriately, imposing the constraints omits a few number of canonical pairs. Hence, the reduced phase space, with canonical pairs as coordinates, is achieved.

hep-th↗

arXiv2012

Symplectic Quantization of Massive Bosonic string in background B-field

We give the details of symplectic quantization for a system containing second class constraints. This method is appropriate for imposing infinite series of constraints due to the boundary conditions. We use this method for massive bosonic strings in a background B-field and find the correct expansions of the fields in terms of the physical modes. We have found a canonical basis for this model.

hep-th↗

arXiv2011

Constraint structure of the three dimensional massive gravity

Constraint analysis of the three-dimensional massive gravity, the so- called new massive gravity, is studied in the Palatini formalism. We show that amongst 6 components of the metric, 2 are dynamical, which is compatible with the existence of one vector massive graviton in the linearized theory (Fierz-Pauli theory).

hep-th↗

arXiv2006

Perfect and Imperfect Gauge Fixing

Gauge fixing may be done in different ways. We show that using the chain structure to describe a constrained system, enables us to use either a perfect gauge, in which all gauged degrees of freedom are determined; or an imperfect gauge, in which some first class constraints remain as subsidiary conditions to be imposed on the solutions of the equations of motion. We also show that the number of constants of motion depends on the level in a constraint chain in which the gauge fixing condition is imposed. The relativistic point particle, electromagnetism and the Polyakov string are discussed as examples and perfect or imperfect gauges are distinguished.

hep-th↗

arXiv2006

The Reduced Phase Space of An Open String in The Background B-Field

The problem of an open string in background $B$-field is discussed. Using the discretized model in details we show that the system is influenced by infinite number of second class constraints. We interpret the allowed Fourier modes as the coordinates of the reduced phase space. This enables us to compute the Dirac brackets more easily. We prove that the coordinates of the string are non-commutative at the boundaries. We argue that in order to find the Dirac bracket or commutator algebra of the physical variables, one should not expand the fields in terms of the solutions of the equations of motion. Instead, one should impose the set of constraints in suitable coordinates.

hep-th↗

arXiv2005

BFT Method for Mixed Constrained Systems and Chern-Simons Theory

We show that the BFT embedding method is problematic for mixed systems (systems possessing both first and second class constraints). The Chern-Simons theory as an example is worked out in detail. We give two methods to solve the problem leading to two different types of finite order BFT embedding for Chern-Simons theory.

hep-th↗

arXiv2004

The Difficulty of Symplectic Analysis with Second Class Systems

Using the basic concepts of chain by chain method we show that the symplectic analysis, which was claimed to be equivalent to the usual Dirac method, fails when second class constraints are present. We propose a modification in symplectic analysis that solves the problem.

hep-th↗

arXiv2004

The BFT Method With Chain Structure

We have constructed a modified BFT method that preserves the chain structure of constraints. This method has two advantages: first, it leads to less number of primary constraints such that the remaining constraints emerge automatically; and second, it gives less number of independent gauge parameters. We have applied the method for bosonized chiral Schwiger model. We have constructed a gauge invariant embedded Lagrangian for this model.

hep-th↗

arXiv2003

Finite Order BFFT Method

We have proposed a method in the context of BFFT approach that leads to truncation of the infinite series regarded to constraints in the extended phase space, as well as other physical quantities (such as Hamiltonian). This has been done for cases where the matrix of Poisson brackets among the constraints is symplectic or constant. The method is applied to Proca model, single self dual chiral bosons and chiral Schwinger models as examples.

hep-th↗