مجله پژوهش فیزیک ایران
سال پانزدهم شماره 1 (بهار 1394)

In recent work the structural، electronic and optical properties of BSb compound in bulk and surface (110) states have been studied. Calculations have been performed using Full-Potential Augmented Plane Wave (FP-LAPW) method by WIEN2k code in Density Functional Theory (DFT) framework. The structural properties of the bulk such as lattice constant، bulk module and elastic constants have been investigated using four different approximations. The band gap energy of the bulk and the (110) surface of BSb were obtained about 1. 082 and 0. 38 eV respectively. Moreover the surface energy، the work function، the surface relaxation، surface state and the band structure of BSb (110) were investigated using symmetric and stoichiometric 15 layers slabs with the vacuum of 20 Bohr. In addition، the real and imaginary parts of the dielectric function of the bulk and the BSb (110) slab were calculated and compared to each other. Our obtained results have a good agreement with the available results.

Ion Beam Induced Luminescence (IBIL) is a useful IBA technique which could be utilized to obtain information about the nature of chemical bonds in materials. Regarding the probed area، this non-destructive and fast technique is a suitable complementary one for MicroPIXE. Since most minerals are luminescent، IBIL is an applicable analytical technique in mineralogy. In this research work، to characterize a Lapis lazuli sample، a 2. 7 MeV proton beam is utilized. After data collection and analysis of the results obtained from both techniques of IBIL and MicroPIXE، elemental maps of the sample were developed. Comparison of the results with other available ones in the literature indicates the capability and accuracy of the combination of the two complementary techniques for characterization of minerals as well as precious historical objects

In this paper، we have investigated the Penrose tiling photonic quasi crystal fiber with 10-fold symmetry. Due to low birefringence of this fiber، we have proposed two different asymmetric structure so as so obtain high birefringence and low beat length. By modifying radius of the air holes near core and changing the shape of air holes in cladding (from circle to elliptic)، these new structures have been introduced. As a consequence، we have obtained a high birefringence of 710-3، confinement loss 102 dB/Km and a beat length of less than 210-3 mm.

In this paper، the electron temperature and ion density of laser-produced plasma in nanosecond regime in different pressure of ambient gas have been studied by using single Langmuir probe. Using the current-voltage characteristics curve of probe، the value of electron temperature in different pressure (range of 10-5×10-5 mbar) is in the range of 2-29 eV and also the ion density estimated about 1013 cm-3 in millimeter scale and 1011 cm-3 in centimeters scale from target to probe.. It is important that regardless of I-V characteristic of Langmuir probe، having time of maxim current signal of probe at certain distance from target surface، it would be possible to obtain the particles speed by time of flight method. The results show that the electrons and ions speeds are about 108 cm/s and 106cm/s respectively. Also the results of this experiment are Comparable with other techniques such as Shadowgraphy in the similar experimental conditions.

Positron annihilation lifetime spectroscopy method with valuation of non-destructive investigation of material، provides information about electron density، defect concentration، type of defects and atoms around the defects. The stability of the system was tested with a source. The time resolution of the whole system has been derived about 365 ps at FWHM. Then n- and p-type silicon samples were irradiated with a 10 MeV electron beam with dosage of 3، 12، and 30 kGy. Three components were fitted to the lifetime spectra using the PAScual program. The first component is related to the positron annihilation in the positron source which was obtained 186 ps. The second component is related to the positron annihilation lifetime in the sample bulk which was obtained 218 ps. The last lifetime component which is related to the positron annihilation in defect was small and sample dependent

Stability of synchronous state is a fundamental problem in synchronization. We study Matrix Measure as an approach for investigating of stability of synchronous states of chaotic maps on complex networks. Matrix Measure is a measure which depends on network structure. Using this measure and comparing with synchronization threshold which depends on the function of the map، show us how the synchronous state can be stabilized. We use these methods for networks with different parameters and topologies. Our numerical calculation shows that synchronous states on more dense networks are more stable. Network’s size is another effective parameter that order of value and extent of stability interval is determined by network’s size. Our results also show that among dense networks، Random and Scale-Free networks have larger stability interval of coupling strength. Finally، we use Error Function to test a prediction of Matrix Measure approach.

The existent of the intrinsic electric dipole moments (EDM) lead to CP violation in a physical system. In the non-relativistic and point like limits، the effects of them in atoms are canceled which is well-known as Schiff screening effects. It is why that the energy shift due to the EDM is proportional to the expectation value of which vanishes in non-relativistic limit. In this paper، using Foldy-Wouthuysen representation we remove the odd terms (those terms mix the positive and negative energy solutions) up to order and then study the Schiff screening effects.

An investigation isundertaken to study the transmission of the electromagnetic waves through inhomogeneous over-dense plasma. The transmission is due to the excitation of the surface modes of plasma or plasmons. The construction of the plasma is in such a way that the charge density of the plasma linearly increases and passes through its critical value and then linearly decreases. The dissipative effects are considered for the plasma. The amplitude of the electromagnetic waves and the reflection coefficient are calculated and then the distribution of the electric field amplitude of the entire structure is obtained and discussed. In this way، itis shown that the considered structure facilitates the anomalous wave transmission through over-dense plasma medium.

In this paper، the spectral behavior of a two-cell phase conjugated mirror (PCM) with a two-pass Nd: YAG amplifier has been analyzed experimentally and theoretically. For this purpose، amplifier input intensity for the three different states of laser oscillator with one، two and three longitudinal modes have been investigated. The numerical model is based on 2-dimension equations of stokes back-scattered intensity and acoustic wave، for two-cell generator-amplifier phase conjugated mirror with proper boundary conditions and simultaneously solving the rate equations of Nd: YAG optical amplifier. Results of the Fourier analysis of the amplified intensity show considerable differences between Fourier amplitudes of the amplifier equipped with PCM and a two-pass amplifier with a conventional turning mirror under the same output energy. The amplifier with PCM has a completely filtered and different spectral behavior with clearly reduction of the beating between Fourier components of the input optical field.

The classical J1-J2 Heisenberg model on bipartite lattice exhibits «Neel» order. However if the AF interactions between the next nearest neighbor (nnn) are increased with respect to the nearest neighbor (nn)، the frustration effect arises. In such situations، new phases such as ordered phases with coplanar or spiral ordering and disordered phases such as spin liquids can arise. In this paper we use the self-consistent Gaussian approximation to study the J1-J2 Heisenberg model in honeycomb and diamond lattices. We find the spin liquid phases such as ring-liquid and pancake-liquid in honeycomb lattice. Also for diamond lattice we show that the degeneracy of ground state can be lifted by thermal fluctuations through the order by disorder mechanism.

In this paper، we study the electronic conductance of molecular nanowires by considering the electron hopping between the first and second neighbors with the help Green’s function method at the tight-binding approach. We investigate three types of structures including linear uniform and periodic chains as well as poly (p-phenylene) molecule which are embedded between two semi-infinite metallic leads. The results show that in the second neighbor approximation، the resonance، anti-resonance and Fano phenomena occur in the conductance spectra of these structures. Moreover، a new gap is observed at edge of the lead energy band wich its width depends on the value of the electron hopping energy between the second neighbors. In the systems including intrinsic gap، this hopping energy shifts the gap in the energy spectra.

In this work، density functional calculations and Boltzmann semiclassical theory of transport are used to investigate structural، electronic، and thermoelectric properties of AgSbSe2 crystal. According to the published experimental measurements، five more likely structures of this compound are considered and their structural and electronic properties are calculated and compared together. Then، thermoelectric properties (electrical conductivity، electronic contribution to the thermal conductivity، power factor، and Seebeck coefficient) of three more stable structures are investigated in the constant relaxation time approximation. Finally، the calculated temperature dependence of Seebeck coefficient is compared with the corresponding experimental measurements of others.

We have deposited Fe/Ag/Fe multilayer by physical vapor deposition (PVD) method in different Ag thickness as spacer working in a vacuum of 2×10-6mbar. The structural properties، magnetic response of the samples at low temperatures and room temperature was investigated by XRD، physical properties measurement system (PPMS) and vibrating sample magnetometer (VSM) respectively. Hysteresis loops show that the easy axis of magnetization is in plane of the film. Also the magnetization and Hc of samples decrease with increasing temperature.