مجله پژوهش فیزیک ایران
سال چهاردهم شماره 3 (پاییز 1393)

The existing theoretical approaches have been unable to confront the large corpus of outstanding data on the cuprate superconductor in a persuasive and unified manner. There is a general opinion now growing that these theories miss some essential points. We propose one based on multichannel Kondo effect that is physically justifiable and confronts the data adequately. In addition to earlier data، the definitive proof of involvement of this effect has been now provided by the numerical agreement of the characteristic Kondo temperatures with the fluctuation frequencies at which Eliashberg function peaks. It is، therefore، suggested that the theory of multichannel Kondo effect be accepted as the correct theory of underdoped cuprate.

Bianchi type-VI0 space time with strange quark matter attached to string cloud in Nordtvedt [1] general scalar tensor theory of gravitation with the help of a special case proposed by Schwinger [2] is obtained. The field equations have been solved by using the anisotropy feature of the universe in the Bianchi type-VI0 space time. Some important features of the model، thus obtained، have been discussed

Weld area and weld depth/width ratio can be considered to be of the most important geometrical factors in pulsed laser welding. The effects of carbon dioxide and oxygen additions to the argon shielding gas on the weld properties in pulsed laser welding of low carbon steel is investigated. Presence of carbon dioxide and oxygen up to 10 and 15 percent respectively decreases the weld geometrical factors. But، at higher levels of additions، the weld geometrical factors will increase. It is observed that the plasma plume temperature decreases from 6000K to 5500K with the addition of 15% carbon dioxide but increases to 7700K with 25% carbon dioxide addition. Increase in laser absorption coefficient، laser energy absorption، formation of oxide layer on the work-piece surface، exothermic reactions and their competitive effects can be considered as the competing phenomena involved in such a behavior in the weld profile.

Laser irradiation parameters، especially the laser fluence and the number of pulses are very important factors affecting microstructures formation and improvement of the surface characteristics in different medical، electronic and the other industrial applications. Information about the fluence domain and the number of pulses for the formation of the structures is very important and determines the desirable or unwanted effects of the laser irradiation on the surfaces regarding the desired applications. In this paper Polyethersulfone films were irradiated with a XeCl laser at fluences above the ablation threshold. The effects of the laser fluence and the number of pulses on the formation of different microstructures on the surface were investigated.

In this work we report the synthesis and characterization of hybrid nanostructures of multiwall carbon nanotubes (MWCNT) and cadmium sulphide (CdS) nanoparticles. In a solution of thioacetamide and cadmium sulphide، purified MWCNT are added to prepare the CNT/CdS hetrostructure. XRD diagrams، SEM images، and also photoluminescence spectra of the prepared samples are analyzed. SEM images show the CdS nanoparticles of 17nm size coated on the CNTs. Visible photoluminescence with peaks at 435 nm and 535 nm، obtained from the prepared CNT/CdS، is discussed. The synthesis technique can be simply extended to other luminescent dopants، and the possibility of making optoelectronic devises based on the CNTs is confirmed by this method

Density functional approach was used to study the isotropic- nematic (I-N) transition and calculate the values of freezing parameters of the Gay- Berne liquid crystal model. New direct and pair correlation functions of a molecular fluid with Gay- Berne pair potential were used. These new functions were used in density functional theory as input to calculate the isotropic- nematic transition densities for elongation at various reduced temperatures. It was observed that the isotropic- nematic transition densities increase as the temperature increases. It was found that the new direct correlation function is suitable to study the isotropic- nematic transition of Gay- Berne liquids. Comparison to other works showed qualitative agreement

A numerical study is presented to investigate the electronic transport properties through a synthetic DNA molecule based on a quasiperiodic arrangement of its constituent nucleotides. Using a generalized Green''s function technique، the electronic conduction through the poly (GACT) -poly (CTGA) DNA molecule in a metal/DNA/metal model structure has been studied. Making use of a renormalization scheme we transform the Hamiltonian of double-stranded DNA (dsDNA) molecule to an effective Hamiltonian corresponding to a one-dimensional chain in which the effective on-site energies are arranged as a quasiperiodic lattice according to Fibonacci sequence. The room temperature current-voltage characteristic of dsDNA has been investigated in this Fibonacci model and compared with those corresponding to poly (GACT) -poly (CTGA) DNA molecule. Our results indicate the main effect of the quasiperiodic arrangement of the nucleotides as the Fibonacci sequence on the electronic spectrum structure of the dsDNA is that the energy band gaps of the molecule have a tendency for suppression.

Interaction of Alfven waves with plasma inhomogeneity generates phase mixing which can cause the dissipation of Alfven waves. We investigated the dissipation of standing Alfven waves due to phase mixing at the presence of steady flow and sheared magnetic field in solar spicules. Moreover، the transition region between chromosphere and corona was considered. Our numerical simulation showed that the phase mixing and dissipation rate of Alfven waves are enhanced relative to viscosity and resistivity gradients. Comparison of the results of our models with and without these gradients illustrated a significant difference between them. In other words، with these assumptions، Alfven waves may transfer the photospheric energy to the corona during timescales corresponding to the observed lifetimes of spicules. It should be noted that the results of our numerical simulation were in good agreement with observational scaling law obtained by Kuridze et al. [1]

In this paper، using high order perturbative series expansion method، the critical exponents of the order parameter and susceptibility in transition from ferromagnetic to disordered phases for 1D quantum Ising model in transverse field، with ferromagnetic nearest neighbor and anti-ferromagnetic next to nearest neighbor interactions، are calculated. It is found that for small value of the frustrating second neighbor coupling (k<0. 4)، the exponents are the same as those of the classical 2D Ising model (B=1/8 and Y=7/8)، but a deviation from 2D classical Ising universality class is observed for larger values of frustration (0. 4< K<0. 5). The reason for this deviation is the “disorder line” which becomes tangent to the Ferromagnetic phase boundary line at K=0. 5

We want to study thermal corrections on the speed of light at low temperature considering temperature dependence of photon vacuum polarization tensor at two-loop level in the standard QED. It is found that the heat bath behaves as a dispersive medium for the propagation of light and reduces its speed proportional to the second order of temperature. Similarities and differences، in light of already known calculations which are based on Euler-Heisenberg Lagrangian and/or those using temperature dependent electromagnetic properties of the medium are discussed.

A simple relation between the optical electronegativity، energy gap، refractive index and electronic polarizability is given for ternary chalcopyrite semiconductors. Energy gap has been evaluated from the optical electronegativity whereas refractive index and electronic polarizability values have been evaluated from the energy gap by proposing a linear relation between them. The calculated values are in fair agreement with the experimental values and earlier researchers. This work highlights the significance of interrelation between these parameters.

We analyze three-body decays of and. Under the factorization approach، there are tree level diagrams for these decay modes and the transition matrix element of decay is factorized into a form factor multiplied by decay constant and form factor multiplied into weak vertices form factor. The transition matrix element of decay is also factorized into a form factor multiplied into weak vertices form factor. We investigate these decays by using the Dalitz plot technique. First، we use the general form of this technique and calculate the branching ratios of the and decays and obtain the values of and، while the experimental results are less than and respectively. Then we assume that، in the decays، because the mass of the is too heavy against the K-mesons. So the momentum of the can have a small amount. Namely، the carries small momentum. In the probability summation of the amplitudes، the momentum of the، instead of starting from its maximum value، starts from a state in which two light mesons make an angle of degrees. Using our assumption، we calculate the branching ratios of and decays and obtain and respectively. In addition، we estimate the branching ratios for different angles between the two light mesons and get that when the angle between them is، in fact they decay back to back، and these values become and، which are the best answers close to the experimental values. The branching ratios obtained by applying our assumption are compatible with the experimental results.

A metallic coupler is proposed to interface a silicon on insulator (SOI) waveguide with a narrow hybrid plasmonic waveguide (200× 200 nm). The device operation is investigated and optimized to attain the best tradeoff between the mode confinement and the propagation loss. Calculations reveal that a high confinement and low loss of the energy is achieved from a silicon slab waveguide into the dielectric slot of area 200×200 nm2 and a coupling efficiency of 70% (0.8 dB) at the 1.55 μm telecommunication wavelength can be achieved.