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Search Results: 1 - 10 of 198066 matches for " N. Nafari "
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Can Holstein-Kondo lattice model be used as a candidate for the theory of high transition temperature superconductors
R Nourafkan,N Nafari
Iranian Journal of Physics Research , 2009,
Abstract: It is a common knowledge that the formation of electron pairs is a necessary ingredient of any theoretical work describing superconductivity. Thus, finding the mechanism of the formation of the electron pairs is of utmost importance. There are some experiments on high transition temperature superconductors which support the electron-phonon (e-ph) interactions as the pairing mechanism (ARPES), and there are others which support the spin fluctuations as their pairing mechanism (tunneling spectroscopy). In this paper, we introduce the Holstein-Kondo lattice model (H-KLM) which incorporates the e-ph as well as the Kondo exchange interaction. We have used the dynamical mean field theory (DMFT) to describe heavy fermion semiconductors and have employed the exact-diagonalization technique to obtain our results. The phase diagram of these systems in the parameter space of the e-ph coupling, g, and the Kondo exchange coupling, J, show that the system can be found in the Kondo insulating phase, metallic phase or the bi-polaronic phase. It is shown that these systems develop both spin gap and a charge gap, which are different and possess energies in the range of 1-100 meV. In view of the fact that both spin excitation energies and phonon energies lie in this range, we expect our work on H-KLM opens a way to formalize the theory of the high transition temperature superconductors .
Kondo lattice model at half-filling
R. Nourafkan,N. Nafari
Physics , 2007, DOI: 10.1088/0953-8984/20/25/255231
Abstract: The single- and two-channel Kondo lattice model consisting of localized spins interacting antiferromagnetically with the itinerent electrons, are studied using dynamical mean field theory. As an impurity solver for the effective single impurity Anderson model we used the exact diagonalization (ED) method. Using ED allowed us to perform calculations for low temperatures and couplings of arbitrary large strength. Our results for the single-channel case confirm and extend the recent investigations. In the two-channel case we find a symmetry breaking phase transition with increasing coupling strength.
Stabilized Spin-Polarized Jellium Model and Odd-Even Alternations in Jellium Metal Clusters
M. Payami,N. Nafari
Physics , 1997, DOI: 10.1063/1.477195
Abstract: In this paper, we have considered the mechanical stability of a jellium system in the presence of spin degrees of freedom and have generalized the stabilized jellium model, introduced by J. P. Perdew, H. Q. Tran, and E. D. Smith [Phys. Rev. B42, 11627 (1990)], to a spin-polarized case. By applying this generalization to metal clusters (Al, Ga, Li, Na, K, Cs), we gain additional insights about the odd-even alternations, seen in their ionization potentials. In this generalization, in addition to the electronic degrees of freedom, we allow the positive jellium background to expand as the clusters' polarization increases. In fact, our self-consistent calculations of the energetics of alkali metal clusters with spherical geometries, in the context of density functional theory and local spin density approximation, show that the energy of a cluster is minimized for a configuration with maximum spin compensation (MSC). That is, for clusters with even number of electrons, the energy minimization gives rise to complete compensation ($N_\uparrow=N_\downarrow$), and for clusters with odd number of electrons, only one electron remains uncompensated ($N_\uparrow-N_\downarrow=1$). It is this MSC-rule which gives rise to alternations in the ionization potentials. Aside from very few exceptions, the MSC-rule is also at work for other metal culsters (Al, Ga) of various sizes.
Quantum Hall effect in single wide quantum wells
M. Abolfath,L. Belkhir,N. Nafari
Physics , 1996, DOI: 10.1103/PhysRevB.55.10643
Abstract: We study the quantum Hall states in the lowest Landau level for a single wide quantum well. Due to a separation of charges to opposite sides of the well, a single wide well can be modelled as an effective two level system. We provide numerical evidence of the existence of a phase transition from an incompressible to a compressible state as the electron density is increased for specific well width. Our numerical results show a critical electron density which depends on well width, beyond which a transition incompressible double layer quantum Hall state to a mono-layer compressible state occurs. We also calculate the related phase boundary corresponding to destruction of the collective mode energy gap. We show that the effective tunneling term and the interlayer separation are both renormalised by the strong magnetic field. We also exploite the local density functional techniques in the presence of strong magnetic field at $\nu=1$ to calculate renormalized $\Delta_{SAS}$. The numerical results shows good agreement between many-body calculations and local density functional techniques in the presence of a strong magnetic field at $\nu=1$. we also discuss implications of this work on the $\nu=1/2$ incompressible state observed in SWQW.
Introduction to Noncommutative Algebraic Geometry (First Draft)
Manizheh Nafari
Mathematics , 2015,
Abstract: This Lecture Notes is meant to introduce noncommutative algebraic geometry tools (which were invented by M. Artin, W. Schelter, J. Tate, and M. Van den Bergh in the late 1980s) and also graded skew Clifford algebras (which were introduced by T. Cassidy and M. Vancliff).
Roughness of undoped graphene and its short-range induced gauge field
N. Abedpour,M. Neek-Amal,Reza Asgari,F. Shahbazi,N. Nafari,M. Reza Rahimi Tabar
Physics , 2007, DOI: 10.1103/PhysRevB.76.195407
Abstract: We present both numerical and analytical study of graphene roughness with a crystal structure including $500 \times 500$ atoms. The roughness can effectively result in a random gauge field and has important consequences for its electronic structure. Our results show that its height fluctuations in small scales have scaling behavior with a temperature dependent roughness exponent in the interval of $ 0.6 < \chi < 0.7 $. The correlation function of height fluctuations depends upon temperature with characteristic length scale of $ \approx 90 {\AA}$ (at room temperature). We show that the correlation function of the induced gauge field has a short-range nature with correlation length of about $\simeq 2-3 {\AA}$. We also treat the problem analytically by using the Martin-Siggia-Rose method. The renormalization group flows did not yield any delocalized-localized transition arising from the graphene roughness. Our results are in good agreement with recent experimental observations.
Phase diagram of Holstein-Kondo lattice model at half-filling
Reza Nourafkan,Nasser Nafari
Physics , 2008, DOI: 10.1103/PhysRevB.79.075122
Abstract: We study the Kondo lattice model which is modified by the Holstein term, involving both the Kondo exchange coupling and the electron-phonon coupling constants, characterized by $J$ and $g$, respectively. The model is solved by employing the dynamical mean-field theory in conjunction with exact diagonalization technique. A zero temperature phase diagram of symmetry unbroken states at half filling is mapped out which exhibits an interplay between the two interactions and accounts for both spin and charge fluctuations. When the Kondo exchange coupling is dominant the system is in Kondo insulator state. Increasing $g$ for small values of $J$ leads to a Kondo insulator-metal transition. Upon further enhancement of $g$ a transition to the bipolaronic insulating phase takes place. Also a small region with non-Fermi liquid behavior is found near the Kondo insulator-metal transition.
Graded Skew Clifford Algebras that are Twists of Graded Clifford Algebras
Manizheh Nafari,Michaela Vancliff
Mathematics , 2012,
Abstract: We prove that if $A$ is a regular graded skew Clifford algebra and is a twist of a regular graded Clifford algebra $B$ by an automorphism, then the subalgebra of $A$ generated by a certain normalizing sequence of homogeneous degree-two elements is a twist of a polynomial ring by an automorphism, and is a skew polynomial ring. We also present an example that demonstrates that this can fail when $A$ is not a twist of $B$.
Mona Nafari,Mansour Nejati Jahromi,Gholam HoseinSheisi
International Journal of New Computer Architectures and their Applications , 2011,
Abstract: In this paper, a reversible data hiding scheme has been proposed which is based on correlation of subsample images. The proposed method modifies the blocks of subsampled image to prepare vacant positions for data embedding. The PSNR of the stego image produced by the proposed method is guaranteed to be above 47.5 dB, while the embedding capacity is at least, almost 6.5 times higher than that of the Kim et al. techniques with the same PSNR. This technique has the capability to control the capacity-PSNR. Experimental results support that the proposed method exploits the correlation of blocked sub-sampled image, outperforms the prior works in terms of larger capacity and stego image quality both. On various test images, we demonstrate the validity of our proposed method by comparing to other existing reversible data hiding algorithms.
Surface Polaron Formation in the Holstein model
Reza Nourafkan,Massimo Capone,Nasser Nafari
Physics , 2009, DOI: 10.1103/PhysRevB.80.155130
Abstract: The effect of a solid-vacuum interface on the properties of a strongly coupled electron-phonon system is analyzed using dynamical mean-field theory to solve the Holstein model in a semi-infinite cubic lattice. Polaron formation is found to occur more easily (i.e., for a weaker electron-phonon coupling) on the surface than in the bulk. On the other hand, the metal-insulator transition associated to the binding of polarons takes place at a unique critical strength in the bulk and at the surface.
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