arXiv subjects

M. Zubair

Publications and source records attributed to M. Zubair.

At least 37 records · Page 2Linked to original sources

arXiv2021

Relativistic compact stars in Tolman spacetime via an anisotropic approach

In this present work, we have obtained a singularity-free spherically symmetric stellar model with anisotropic pressure in the background of Einstein's general theory of relativity. The Einstein's field equations have been solved by exploiting Tolman {\em ansatz} [Richard C Tolman, Phys. Rev. 55:364, 1939] in $(3+1)$-dimensional space-time. Using observed values of mass and radius of the compact star PSR J1903+327, we have calculated the numerical values of all the constants from the boundary conditions. All the physical characteristics of the proposed model have been discussed both analytically and graphically. The new exact solution satisfies all the physical criteria for a realistic compact star. The matter variables are regular and well behaved throughout the stellar structure. Constraints on model parameters have been obtained. All the energy conditions are verified with the help of graphical representation. The stability condition of the present model has been described through different testings.

gr-qc↗

arXiv2021

An anisotropic version of Tolman VII solution in $f(R,T)$ gravity via Gravitational Decoupling MGD Approach

In this work, we have adopted gravitational decoupling by Minimal Geometric Deformation (MGD) approach and have developed an anisotropic version of well-known Tolman VII isotropic solution in the framework of $f(R, T)$ gravity, where $R$ is Ricci scalar and $T$ is trace of energy momentum tensor. The set of field equations has been developed with respect to total energy momentum tensor, which combines effective energy momentum tensor in $f(R,T)$ gravity and additional source $ϕ_{ij}$. Following MGD approach, the set of field equations has been separated into two sections. One section represents $f(R, T)$ field equations, while the other is related to the source $ϕ_{ij}$. The matching conditions for inner and outer geometry have also been discussed and an anisotropic solution has been developed using mimic constraint for radial pressure. In order to check viability of the solution, we have considered observation data of three different compact star models, named PSR J1614-2230, PSR 1937+21 and SAX J1808.4-3658 and have discussed thermodynamical properties analytically and graphically. The energy conditions are found to be satisfied for the three compact stars. The stability analysis has been presented through causality condition and Herrera's cracking concept, which ensures physical acceptability of the solution.

gr-qc↗

arXiv2020

Solar System Tests in Modified Teleparallel Gravity

In this paper, we study different Solar System tests in a modified Teleparallel gravity theory based on an arbitrary function $f(T,B)$ which depends on the scalar torsion $T$ and the boundary term $B$. To do this, we first find new perturbed spherically symmetric solutions around Schwarzschild for different power-law forms of the arbitrary Lagrangian. Then, for each model we calculated the photon sphere, perihelion shift, deflection of light, Cassini experiment, Shapiro delay and the gravitational redshift. Finally, we confront these computations with different known experiments from these Solar System tests to put different bounds on the mentioned models. We then conclude that $f(T,B)$ is compatible with these Solar System experiments with a wide range of parameters which are relevant for cosmology.

gr-qc↗

arXiv2020

Generalized scaling law for exciton binding energy in two-dimensional materials

Binding energy calculation in two-dimensional (2D) materials is crucial in determining their electronic and optical properties pertaining to enhanced Coulomb interactions between charge carriers due to quantum confinement and reduced dielectric screening. Based on full solutions of the Schrödinger equation in screened hydrogen model with a modified Coulomb potential ($1/r^{β-2}$), we present a generalized and analytical scaling law for exciton binding energy, $E_β = E_{0}\times \big (\,aβ^{b}+c\big )\, (μ/ε^{2})$, where $β$ is a fractional-dimension parameter accounted for the reduced dielectric screening. The model is able to provide accurate binding energies, benchmarked with the reported Bethe-Salpeter Equation (BSE) and experimental data, for 58 mono-layer 2D and 8 bulk materials respectively through $β$. For a given material, $β$ is varied from $β$ = 3 for bulk 3D materials to a value lying in the range 2.55$-$2.7 for 2D mono-layer materials. With $β_{\text{mean}}$ = 2.625, our model improves the average relative mean square error by 3 times in comparison to existing models. The results can be used for Coulomb engineering of exciton binding energies in the optimal design of 2D materials.

physics.app-ph↗

arXiv2020

Strong Gravitational Lensing for Photon Coupled to Weyl Tensor in Kiselev Black Hole

The ambition of the present work is to highlight the phenomena of strong gravitational lensing and deflection angle for the photons coupling with Weyl tensor in a Kiselev black hole. Here, we have extended the prior work of Chen and Jing \cite{1} for Schwarzschild black hole to Kiselev black hole. For this purpose, the equation of motion for the photons coupled to Weyl tensor, null geodesic and equation of photon sphere in a Kiselev black hole spacetime have been formulated. It is found that the equation of motion of the photons depends not only on the coupling between photon and Weyl tensor, but also on the polarization direction of the photons. There is a critical value of the coupling parameter $α$ for existence of the marginally circular photon orbit outside the event horizon, which depends on the parameters of black hole and the polarization direction of photons. Further, the polarization directions of coupled photon and the coupling parameter $α$, both modify the features of the photon sphere, the angle of deflection and the functions $(\bar{a}$ and $\bar{b})$ for the strong gravitational lensing in Kiselev black hole spacetime. In addition to this, the observable gravitational lensing quantities and the shadows of the Kiselev black hole spacetime are presented in detail.

gr-qc↗

arXiv2020

Anisotropic Tolman V Solution by Minimal Gravitational Decoupling Approach

In this paper, we consider well known Tolman V perfect fluid solution and extend it to its an anisotropic version using gravitational decoupling by minimal geometric deformation approach and analyze the behavior of new version of the solution graphically. The effects of coupling constant on anisotropic factor has been measured and presented graphically. The matching conditions at the surface of inner and outer geometry have also been discussed. The viability of the solution has been studied by presenting the physical analysis of the solution.

gr-qc↗

arXiv2020

Inflationary Solution of Hamilton Jacobi Equations during Weak Dissipative Regime

In this paper, an elegant mathematical approach is introduced to solve the equations of warm inflationary model without using extra approximations other than slow-roll. This important inflationary method known as Hamilton-Jacobian formalism. Here tachyon field and the imperfect fluid are considered to be the cosmic ingredients to create inflation. A general formalism is developed for the considered inflationary model and further work is restricted to weak dissipative regime. A detailed analysis of the model is presented for three different choices of bulk and dissipative coefficients taking as constant as well as variable (function of Hubble parameter and inflaton). In each case, the involved model parameters are constrained to plot the physical acceptable range of scalar spectral index and tensor to scalar ratio. The parametric trajectories proved that the acquired results for all the three cases are compatible with Planck astrophysical data. Furthermore, the existence of warm inflation and slow-roll limit are also verified graphically.

gr-qc↗

arXiv2020

Noncommutative inspired wormholes admitting conformal motion involving minimal coupling

In this manuscript, we explore the existence of wormhole solutions exhibiting spherical symmetry in a modified gravity namely $f(R,T)$ theory by involving some aspects of non-commutative geometry. For this purpose, we consider the anisotropic matter contents along with the well-known Gaussian and Lorentizian distributions of string theory. For the sake of simplicity in analytic discussions, we take a specific form of $f(R,T)$ function given by $f(R,T)=R+λT$. For both these non-commutative distributions, we get exact solutions in terms of exponential and hypergeometric functions. By taking some suitable choice of free parameters, we investigate different interesting aspects of these wormhole solutions graphically. We also explored the stability of these wormhole models using equilibrium condition. It can be concluded that the obtained solutions are stable and physically viable satisfying the wormhole existence criteria. Lastly, we discuss the constraints for positivity of the active gravitational mass for both these distributions.

gr-qc↗

arXiv2019

Transport in armchair graphene nanoribbons and in ordinary waveguides

We study dc and ac transport along armchair graphene nanoribbons using the ${\bf k\cdot p}$ spectrum and eigenfunctions and general linear-response expressions for the conductivities. Then we contrast the results with those for transport along ordinary waveguides. In all cases we assess the influence of elastic scattering by impurities, describe it quantitatively with a Drude-type contribution to the current previously not reported, and evaluate the corresponding relaxation time for long- and short-range impurity potentials. We show that this contribution dominates the response at very low frequencies. In both cases the conductivities increase with the electron density and show cusps when new subbands start being occupied. As functions of the frequency the conductivities in armchair graphene nanoribbons exhibit a much richer peak structure than in ordinary waveguides: in the former intraband and interband transitions are allowed whereas in the latter only the intraband ones occur. This difference can be traced to that between the corresponding spectra and eigenfunctions.

cond-mat.mes-hall↗

arXiv2018

Static spherically symmetric wormholes in generalized $f(R,ϕ)$ gravity

In this paper, we examine static spherically symmetric wormhole solutions in generalized $f(R,ϕ)$ gravity. To do this, we consider three different kinds of fluids: anisotropic, barotropic and isotropic. We explore different $f(R,ϕ)$ models and inspect the energy conditions for all of those three fluids. It is found that under some models in this theory, it is possible to obtain wormhole solutions without requiring exotic matter. The discussion about the conditions where the standard energy conditions (WEC and NEC) are valid for the fluids is discussed in details. From our results and for our cases, we conclude that for anisotropic and isotropic fluids, realistic wormhole geometries satisfying the energy conditions can be constructed.

gr-qc↗

arXiv2018

Thermodynamics in a Modified Gravity Involving Higher-Order Torsion Derivative Terms

The present study is elaborated to investigate the validity of thermodynamical laws in a modified teleparallel gravity based on higher-order derivatives terms of torsion scalar. For this purpose, we consider spatially flat FRW model filled with perfect fluid matter contents. Firstly, we explore the possibility of existence of equilibrium as well as non-equilibrium picture of thermodynamics in this extended version of teleprallel gravity. Here, we present the first law and the generalized second law of thermodynamics (GSLT) using Hubble horizon. It is found that non-equilibrium description of thermodynamics exists in this theory with the presence of an extra term called as entropy production term. We also establish GSLT using the logarithmic corrected entropy. Further, by taking the equilibrium picture, we discuss validity of GSLT at Hubble horizon for two $F$ different models. Using Gibbs law and the assumption that temperature of matter within Hubble horizon is similar to itself, we use different cases for choices scale factors to discuss the GSLT validity graphically in all scenarios. It is found that the GSLT is satisfied for a specified range of free parameters in all cases.

physics.gen-ph↗

arXiv2018

Quantum magnetotransport in a bilayer MoS2: influence of a perpendicular electric field

We first derive the energy dispersion of bilayer MoS$_{2}$ in the presence of a perpendicular electric field $E_z$. We show that the band gap and layer splitting can be controlled by the field $E_z$. Away from the $k$ point, the intrinsic SOC splitting increases in the conduction band but is weakly affected in the valence band. We then analyze the band structure in the presence of a perpendicular magnetic field $B$ and the field $E_z$, including spin and valley Zeeman terms, and evaluate the Hall and longitudinal conductivities. We discuss the numerical results as functions of the fields $B$ and $E_z$ for finite temperatures. The field $B$ gives rise to a significant spin splitting in the conduction band, to a beating in the Shubnikov-de Haas (SdH) oscillations when it's weak, and to their splitting when it's strong. The Zeeman terms and $E_{z}$ suppress the beating and change the positions of the beating nodes of the SdH oscillations at low $B$ fields and enhance their splitting at high $B$ fields. Similar beating patterns are observed in the spin and valley polarizations at low $B$ fields. Interestingly, a $90\%$ spin polarization and a $100\%$ square-wave-shaped valley polarization are observed at high $B$ fields. The Hall-plateau sequence depends on $E_z$. These findings may be pertinent to future spintronic and valleytronic devices.

cond-mat.mes-hall↗

arXiv2018

Magneto-optical properties of bilayer transition metal dichalcogenides

In transition metal dichalcogenides the spin-orbit interaction affects differently the conduction and valence band energies as functions of $k$ and the band gap is large. Consequently, when a perpendicular magnetic field $B$ is applied the conduction and valence band Landau levels are also different and this leads to a splitting of the interband optical absorption lines in both the absence and presence of an external electric field $E_{z}$. When $B$ and $E_{z}$ are present the peaks in the imaginary part of the Hall conductivity give two distinct contributions of opposite sign to the interband spectrum. The real part of the right- and left-handed interband conductivity, however, retains its two-peak structure but the peaks are shifted in energy and amplitude with respect to each other in contrast with graphene. The response of the intraband conductivity is significantly modified when the Fermi energy $E_{F}$ and the field $B$ are varied. Its optical spectral weight is found to increase with $E_{F}$ in contrast with the decrease observed in graphene. Further, the position and amplitude of the intraband response depends on the field $B$. The absorption peaks vary linearly with $B$ for all fields similar to bilayer graphene for low fields but in contrast with the high-field $\sqrt{B}$ dependence in it.

cond-mat.mes-hall↗

arXiv2018

Energy Constraints and Phenomenon of Cosmic Evolution in $f(T,B)$ Framework

We investigate the cosmological evolution in a new modified teleparallel theory, called $f(T,B)$ gravity, which is formulated by connecting both $f(T)$ and $f(R)$ theories with a boundary term $B$. Here, $T$ is the torsion scalar in teleparallel gravity and $R$ is the scalar curvature. For this purpose, we assume flat Friedmann-Robertson-Walker (FRW) geometry filled with perfect fluid matter contents. We study two cases in this gravity: One is for a general function of $f(T,B)$, and the other is for a particular form of it given by the term of $-T+F(B)$. We also formulate the general energy constraints for these cases. Furthermore, we explore the validity of the bounds on the energy conditions by specifying different forms of$f(T,B)$ and $F(B)$ function obtained by the reconstruction scheme for de Sitter, power-law, the $Λ$CDM and Phantom cosmological models. Moreover, the possible constraints on the free model parameters are examined with the help of region graphs. In addition, we explore the evolution of the effective equation of state (EoS) $ω_{eff}$ for the universe and compare theoretical results with the observational data. It is found that the effective EoS represents the phantom phase or the quintessence one in the accelerating universe in all of the cases consistent with the observational data.

physics.gen-ph↗

arXiv2018

Cosmic evolution in the background of non-minimal coupling in $f(R,T,R_{μν}T^{μν})$ Gravity

An accelerated expansion phase is being experienced by the universe due to the presence of an unknown energy component known as dark energy (DE). To find out the cosmic evolution scientists ever tried to modify Einstein's gravitational theory and its unexplored parts. We also look forward to address the same problem with a different approach based on interaction between matter and geometry. For this purpose we consider $f(R,T,Q)$ modified theory (where $R$ is the Ricci Scalar, $T$ is the trace of energy-momentum tensor (EMT) $T_{uv}$ and $Q=R_{uv}T^{uv}$ is interaction of EMT $T_{μν}$ and Ricci Tensor $R_{uv}$). We formulate modified field equations in the background of Friedmann-Lema$\hat{i}$tre-Robertson-Walker (FLRW) model which is defined as $ds^2=dt^2-a(t)^2(dx^2+dy^2+dz^2 )$, where $a(t)$ represents the scale factor. In this formalism energy density is found using covariant divergence of modified field equations. $ρ$ involves a contribution from non-minimal matter geometry coupling which helps to study different cosmic eras based on equation of state (EOS). Furthermore, we apply the energy bounds to constrain the model parameters establishing a pathway to discuss the cosmic evolution for best suitable parameters in accordance with recent observations.

physics.gen-ph↗

arXiv2017

Fractional Fowler-Nordheim Law for Field Emission from Rough Surface with Nonparabolic Energy Dispersion

The theories of field electron emission from perfectly planar and smooth canonical surfaces are well understood, but they are not suitable for describing emission from rough, irregular surfaces arising in modern nanoscale electron sources. Moreover, the existing models rely on Sommerfeld's free-electron theory for the description of electronic distribution which is not a valid assumption for modern materials with nonparabolic energy dispersion. In this paper, we derive analytically a generalized Fowler-Nordheim (FN) type equation that takes into account the reduced space-dimensionality seen by the quantum mechanically tunneling electron at a rough, irregular emission surface. We also consider the effects of non-parabolic energy dispersion on field-emission from narrow-gap semiconductors and few-layer graphene using Kane's band model. The traditional FN equation is shown to be a limiting case of our model in the limit of a perfectly flat surface of a material with parabolic dispersion. The fractional-dimension parameter used in this model can be experimentally calculated from appropriate current-voltage data plot. By applying this model to experimental data, the standard field-emission parameters can be deduced with better accuracy than by using the conventional FN equation.

physics.acc-ph↗

arXiv2017

Thermodynamics and cosmological reconstruction in $f(T,B)$ gravity

Recently, it was formulated a teleparallel theory called $f(T,B)$ gravity which connects both $f(T)$ and $f(R)$ under suitable limits. In this theory, the function in the action is assumed to depend on the torsion scalar $T$ and also on a boundary term related with the divergence of torsion, $B=2\nabla_μT^μ$. In this work, we study different features of a flat Friedmann-Lemaître-Robertson-Walker (FLRW) cosmology in this theory. First, we show that the FLRW equations can be transformed to the form of Clausius relation $\hat{T}_hS_{\rm eff}=-dE+WdV$, where $\hat{T}_h$ is the horizon temperature and $S_{\rm eff}$ is the entropy which contains contributions both from horizon entropy and an additional entropy term introduced due to the non-equilibrium. We also formulate the constraint for the validity of the generalised second law of thermodynamics (GSLT). Additionally, using a cosmological reconstruction technique, we show that both $f(T,B)$ and $-T+F(B)$ gravity can mimic power-law, de-Sitter and $Λ$CDM models. Finally, we formulate the perturbed evolution equations and analyse the stability of some important cosmological solutions.

gr-qc↗

arXiv2017

Generalized Fowler-Nordheim field-induced vertical electron emission model for two-dimensional materials

Current theoretical description of field-induced electron emission remains mostly bounded by the classic Fowler-Nordheim (FN) framework developed nearly one century ago. For the emerging class of two-dimensional (2D) materials, many basic assumptions of FN model become invalid due to their reduced dimensionality and exotic electronic properties. In this work, we develop analytical and semi-analytical models of field-induced vertical electron emission from the surface of 2D materials by explicitly taking into account the reduced dimensionality, non-parabolic energy spectrum, non-conserving in-plane electron momentum, finite-temperature and space-charge-limited effects. We show that the traditional FN law is no longer valid for 2D materials. The modified vertical field emission model developed here provides better agreement with experimental results. Intriguingly, a new high-field regime of \emph{saturated surface field emission} emerges due to the reduced dimensionality of 2D materials. A remarkable consequence of this saturated field emission effect is the absence of space-charge-limited current normally expected at high field in three-dimensional bulk material.

cond-mat.mes-hall↗