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571.
572.
573.
In the present paper we have investigated the self-focusing behaviour of radially symmetrical rippled Gaussian laser beam
propagating in a plasma. Considering the nonlinearity to arise from relativistic phenomena and following the approach of Akhmanov
et al, which is based on the WKB and paraxial-ray approximation, the self-focusing behaviour has been investigated in some detail.
The effect of the position and width of the ripple on the self-focusing of laser beam has been studied for arbitrary large
magnitude of nonlinearity. Results indicate that the medium behaves as an oscillatory wave-guide. The self-focusing is found
to depend on the position parameter of ripple as well as on the beam width. Values of critical power has been calculated for
different values of the position parameter of ripple. Effects of axially and radially inhomogeneous plasma on self-focusing
behaviour have been investigated and presented here. 相似文献
574.
Y. Fagot-Revurat X. Bourdon M. Mehring R. K. Kremer 《Applied magnetic resonance》2000,19(3-4):363-372
The full temperature dependence of the electric field gradient tensor at the Na sites has been determined by nuclear magnetic resonance (NMR) in the temperature range 8–330 K in α-Nax 2O5 (x = 0.996). Above the spin-Peierls transition (T c = 34.7 K), only a single Na site is observed in agreement with the Pmmn space group proposed to describe this compound as the first example of a 1/4-filled ladder system. Below Tc, eight distinct quadrupolar23Na sites are observed according to the distortion wave vector kc = (1/2, 1/2, 1/4) previously reported. In addition, the opening of a spin gap is evidenced by a rapid drop of the magnetic hyperfine shift23K at Tc. The results are discussed in the context of a charge-order-driven spin-Peierls transition. 相似文献
575.
576.
In an SVZ-type approach sum rules for baryonic currents have been investigated, without radiative QCD corrections but with the inclusion of non-perturbative terms due to the non-vanishing vacuum expectation value of the quark-antiquark condensate. We give upper bounds for baryon masses, which allow a choice of optimal interpolating operators for low-lying baryon states. The results show that the mentioned vacuum expectation value not only sets the scale for parity splitting but also for the masses of the baryons directly. The alternative way of explicit symmetry breaking by quark masses has also been investigated in the same technical framework. 相似文献
577.
Li Rukang R. K. Kremer J. Maier 《Physica C: Superconductivity and its Applications》1992,200(3-4):344-348
A new layered cuprate compound with a nominal composition of GaSr2Y2−xCexCu2O9−δ has been prepared. It crystallizes in a tetragonal lattice with cell parameters: a = 3.812 Å, c = 28.16 Å. The structure of the compound belongs to the same family of 1222 phase and is derived from that of GaSr2LnCu2O7 by replacing the single Ln3+ layer with a double fluorite (Y, Ce)2O2 layer. Like other parent cuprate compounds of superconductors, the as-prepared samples showed antiferromagnetic and semiconducting behavior. After treatment under high oxygen pressure, the samples exhibited bulk superconductivity with transition temperatures between 12–14 K. 相似文献
578.
579.
Soddemann T Auernhammer GK Guo H Dünweg B Kremer K 《The European physical journal. E, Soft matter》2004,13(2):141-151
Experiments on a variety of systems have shown that layered liquids are unstable under shear even if the liquid layers are planes of constant velocity. We investigate the stability of smectic-A like liquids under shear using Molecular Dynamics simulations and a macroscopic hydrodynamic theory (including the layer normal and the director as independent variables). Both methods show an instability of the layers, which sets in above a critical shear rate. We find a remarkable qualitative and reasonable quantitative agreement between both methods for the spatial homogeneous state and the onset of the instability.Received: 2 October 2003, Published online: 9 March 2004PACS:
61.30.Dk Continuum models and theories of liquid crystal structure - 61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling - 05.70.Ln Nonequilibrium and irreversible thermodynamics - 83.10.Rs Computer simulation of molecular and particle dynamics - 83.50.Ax Steady shear flows, viscometric flowTh.Soddemann : Present address: Rechenzentrum der Max-Planck-Gesellschaft, Boltzmannstrasse 2, 85748 Garching, Germany 相似文献
580.
Employing molecular dynamics simulations of a coarse-grained model, the equilibrium swelling behavior of a cross-linked polyelectrolyte gel has been investigated with a bead-spring defect-free network with diamond-like topology under good solvent conditions and close to the theta-point, the counterions were treated explicitly. To examine the validity range of simple scaling arguments we varied the amount of charges in the system, the length of the network chains, the strength of the electrostatic interactions for both solvent qualities over a wide range of parameters. We find that classical theories only apply in the limit of weak electrostatics and suggest corrections based on the microscopic interplay of the systems' components accessible in a simulation, which model successfully our measured data. 相似文献