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Search Results: 1 - 10 of 462 matches for " Davood Momeni "
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Exact solution for the massless Cylindrically Symmetric Scalar Field in General Relativity, with Cosmological Constant
Davood Momeni,H. Miraghaei
Physics , 2009, DOI: 10.1142/S0217751X09046369
Abstract: In this article we present a new exact solution for scalar field with cosmological constant in cylindrical symmetry. Associated cosmological models is discussed. One of them describes a Cyclic universe.
On existence of a possible Lorentz invariant modified gravity in Weitzenb?ck spacetime
Davood Momeni,Ratbay Myrzakulov
Physics , 2015, DOI: 10.1007/s10509-015-2546-6
Abstract: Modified gravity which was constructed by torsion scalar $T$, namely $f(T)$ doesn't respect Lorentz symmetry. As an attempt to make a new torsion based modified gravity with Lorentz invarianve, recently $f(T,\mathcal{B})$ introduced where $B=2\nabla_{\mu}T^{\mu}$ \citep{Bahamonde:2015zma}. We would argue, even when all is constructed and done in a self-consistent form, if you handle them properly,we observe that there is no Lorentz invariant teleparallel equivalent of $f(R)$ gravity. All we found is that the $f(R)$ gravity in which $R$ must be computed in Weitzenb\"{o}ck spacetime, using Weitzenb\"{o}ck's connection, nor Levi-Civita connections is the only possible Lorentz invariant type of modified gravity. Consequently, $f(T)$ gravity can not obey Lorentz symmetry not only in its orthodoxica form but even in this new framework $f(T,\mathcal{B})$.
Design and Realization of an S-Band Microwave Low-Noise Amplifier for Wireless RF Subsystems
Ardavan Rahimian,Davood Momeni Pakdehi
Computer Science , 2014,
Abstract: This study undertakes the theoretical design, CAD modeling, realization, and performance analysis of a microwave low-noise amplifier (LNA) which has been accurately developed for operation at 3.0 GHz (S-band). The objective of this research is to thoroughly analyze and develop a reliable microstrip LNA intended for a potential employment in wireless communication systems, and satellite applications. The S-band microwave LNA demonstrates the appropriateness to develop a high-performance and well-established device realization for wireless RF systems. The microwave amplifier simulations have been conducted using the latest version of the AWR Design Environment software.
More on Superconductors via Gauge/Gravity Duality with Nonlinear Maxwell Field
Davood Momeni,Muhammad Raza,Ratbay Myrzakulov
Journal of Gravity , 2013, DOI: 10.1155/2013/782512
Abstract: We have developed the recent investigations on the second-order phase transition in the holographic superconductor using the probe limit for a nonlinear Maxwell field strength coupled to a massless scalar field. By analytical methods, based on the variational Sturm-Liouville minimization technique, we study the effects of the spacetime dimension and the nonlinearity parameter on the critical temperature and the scalar condensation of the dual operators on the boundary. Further, as a motivated result, we analytically deduce the DC conductivity in the low and zero temperatures regime. Especially in the zero temperature limit and in two dimensional toy model, we thoroughly compute the conductivity analytically. Our work clarifies more features of the holographic superconductors both in different space dimensions and on the effect of the nonlinearity in Maxwell's strength field. 1. Introduction In the recent years, using the holographic picture of the world, the AdS/CFT (anti de Sitter/conformal field theory) correspondence [1–3] has been applied to study some strongly correlated systems in condensed matter physics, especially for strongly coupled systems with the scale-invariance. Particularly, people studied the low temperature, quantum critical systems near critical point (see, e.g., [4, 5] and references therein). The critical phenomena, which happen here, is a second-order phase transition from normal phase to the superconducting phase, in which below a specific temperature , the DC conductivity becomes infinite. Such second-order phase transitions happen in the high-temperature superconductors and can be described very well by the AdS/CFT dictionary [6, 7]. From the classical and phenomenological point of view, superconductivity, in the high-temperature type II superconductors, modeled by a phenomenological Landau-Ginzburg Lagrangian. This Lagrangian contains a general complex value scalar field , plays the role of a condensate in a superconductive phase. Basically, to have a scalar condensation in the boundary quantum field theory using CFT on the boundary of the bulk, Hartnoll et al. [8] introduced a abelian gauge field and a typical conformal coupled charged complex scalar field in the bulk black hole background. The conformal mass is above the Breitenlohner-Freedman (BF) bound [9]. To solve the negative mass problem, finally Gubser [10] showed that the vector potential modifies the mass term of scalar field and we have a possibility to have hairy black holes in some parts of the parameter space. The full description of the superconductivity in the
Holographic superconductors in a model of non-relativistic gravity
M. R. Setare,Davood Momeni,N. Majd
Physics , 2010, DOI: 10.1007/JHEP05(2011)118
Abstract: We have studied holographic superconductors with spherical symmetry in the Ho$\check{\textbf{r}}$ava-Lifshitz gravity by using a semi analytical method, and also we have calculated the critical temperature and shown when the condensation will appear in a similar pattern as in the Einstein-Gauss- Bonnet gravity. We have computed the dependency of the conductivity as a function of frequency in this new non-relativistic model of quantum gravity.
Observational constraints on non-minimally coupled Galileon model
Mubasher Jamil,Davood Momeni,Ratbay Myrzakulov
Physics , 2013, DOI: 10.1140/epjc/s10052-013-2347-4
Abstract: As an extension of Dvali-Gabadadze-Porrati (DGP) model, the Galileon theory has been proposed to explain the "self-accelerating problem" and "ghost instability problem". In this Paper, we extend the Galileon theory by considering a non-minimally coupled Galileon scalar with gravity. The statefinder analysis, $Om(z)$ diagnostic and constraint the model parameters have been investigate , we find $\Omega_\text{m0}=0.263_{-0.031}^{+0.031}$, $n=1.53_{-0.37}^{+0.21}$ (at the $95\%$ confidence level) with $\chi^2_{\text{min}}=473.376$. Further we show that due to the SNe Ia + BAO data ,our model behaves like a phantom-like dark energy.
Investigation of particle and molecular extinction effects in remote sensing by ultraviolet DIAL in the lower atmosphere
Gholamreza Shayeganrad,Leila Mashhadi,Davood Momeni
Physics , 2009,
Abstract: This study presents theoretical investigation of the effects of particle and molecular extinction in horizontal remote sensing near the ground for several visibilities at UV wavelengths by neglecting the spatial inhomogeneity of aerosol in the atmosphere and taking into account the dependence of refracting on air temperature and pressure. Due to weak attenuation of oxygen and other gaseous atmospheric constituents in this region, we have only considered the effect of ozone in calculation. The results are important to estimate systematic errors in measuring gas concentration introduced by large wavelength separation in UV-DIAL. The total attenuation (km-1) at wavelengths is listed in the form of a table from 200 to 400 nm for several values of visibilities. It is found the aerosol attenuation in UV region varies quite smoothly with wavelength and therefore systematic error caused by aerosol scattering is negligible in remote sensing by UV-DIAL even with large wavelength separation. Moreover, it has been found that only aerosol extinction is dominant in lidar remote sensing in the lower atmosphere in UV region. In large altitude that aerosol concentration is lower; the molecular scattering is important especially for wavelengths larger than 310 nm.
New Modified Mimetic Gravity
Davood Momeni,Aziza Altaibayeva,Ratbay Myrzakulov
Physics , 2014, DOI: 10.1142/S0219887814500911
Abstract: A modified Mimetic gravity (MMG) is proposed as a generalization of general relativity. The model contain a physical metric which is function of an auxiliary (unphysical) metric and a Lyra's metric. We construct different kinds of conformally invariant models in different levels of the expansion parameter $\lambda$. This model phenomenologically has been extended to higher order forms. Cosmology of a certain class of such models has been investigated in details. A cosmological solution has been proposed in inhomogeneous form of scalar field. For homogenous case, energy conditions are widely investigated. We have shown that the system evaluated at intervals shorter than a certain time $T_c$ meets all the energy conditions.
Wormholes in a viable f(T) gravity
Mubasher Jamil,Davood Momeni,Ratbay Myrzakulov
Physics , 2012, DOI: 10.1140/epjc/s10052-012-2267-8
Abstract: In this paper, we derive some new exact solutions of static wormholes in f(T) gravity. We discuss independent cases of the pressure components including isotropic and anisotropic pressure. Lastly we consider radial pressure satisfying a barotropic equation of state. We also check the behavior of null energy condition (NEC) for each case and observe that it is violated for the anisotropic case, while it is satisfied for isotropic and barotropic cases.
Kaluza-Klein reduction and Bergmann-Wagoner bi-scalar general action of scalar-tensor gravity
Kazuharu Bamba,Davood Momeni,Ratbay Myrzakulov
Physics , 2014, DOI: 10.1142/S0219887815501066
Abstract: We examine the Kaluza-Klein (KK) dimensional reduction from higher-dimensional space-time and the properties of the resultant Bergmann-Wagoner general action of scalar-tensor theories. With the analysis of the perturbations, we also investigate the stability of the anti-de Sitter (AdS) space-time in the $D\in\mathcal{N}$-dimensional Einstein gravity with the negative cosmological constant. Furthermore, we derive the conditions for the dimensional reduction to successfully be executed and present the KK compactification mechanism.
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