167 research outputs found
Homogeneous nucleation of dislocations as bifurcations in a periodized discrete elasticity model
A novel analysis of homogeneous nucleation of dislocations in sheared
two-dimensional crystals described by periodized discrete elasticity models is
presented. When the crystal is sheared beyond a critical strain , the
strained dislocation-free state becomes unstable via a subcritical pitchfork
bifurcation. Selecting a fixed final applied strain , different
simultaneously stable stationary configurations containing two or four edge
dislocations may be reached by setting during different time
intervals . At a characteristic time after , one or two dipoles
are nucleated, split, and the resulting two edge dislocations move in opposite
directions to the sample boundary. Numerical continuation shows how
configurations with different numbers of edge dislocation pairs emerge as
bifurcations from the dislocation-free state.Comment: 6 pages, 4 figures, to appear in Europhys. Let
Echo in Optical Lattices: Stimulated Revival of Breathing Oscillations
We analyze a stimulated revival (echo) effect for the breathing modes of the
atomic oscillations in optical lattices. The effect arises from the dephasing
due to the weak anharmonicity being partly reversed in time by means of
additional parametric excitation of the optical lattice. The shape of the echo
response is obtained by numerically simulating the equation of motion for the
atoms with subsequent averaging over the thermal initial conditions. A
qualitative analysis of the phenomenon shows that the suggested echo mechanism
combines the features of both spin and phonon echoes.Comment: 13 pages, 3 figure
Null Energy Condition Violation and Classical Stability in the Bianchi I Metric
The stability of isotropic cosmological solutions in the Bianchi I model is
considered. We prove that the stability of isotropic solutions in the Bianchi I
metric for a positive Hubble parameter follows from their stability in the
Friedmann-Robertson-Walker metric. This result is applied to models inspired by
string field theory, which violate the null energy condition. Examples of
stable isotropic solutions are presented. We also consider the k-essence model
and analyse the stability of solutions of the form .Comment: 27 pages, references added, accepted for publication in Phys. Rev.
Spectroscopic investigations of dispersion-shifted fiber with thin active Bi-doped ring and high nonlinear refractive index
A germanium-doped silica-core fiber with an active region in the form of a thin ring of silica doped with bismuth ions was fabricated. Bismuth doping in the ring surrounding the core allows to stabilize bismuth in silica glass, and it does not impose any restrictions on the composition of the core. The bismuth concentration in the ring is less than 0.2 wt.%. The GeO2 concentration in the core is more than 15 mol.%. A high germanium concentration in the core allows to shift the zero dispersion wavelength to 1860 nm and to obtain a high nonlinear refractive index (n2 more than 3,2*10-20 m2/W). Spectroscopic investigations were carried out in the visible and near infrared (800-1700 nm) spectral range. Despite the small concentration of bismuth, we observed the absorption and luminescence characteristic bands, confirming the presence of bismuth active centers in silica glass. Upon pumping at 1350 nm the on/off gain spectrum was measured on a 20-m fiber. The gain was observed throughout investigated range of 1430-1530 nm. The maximal gain of ~9.5 dB was obtained near 1430 nm. The results of the spectroscopic investigations of the fiber with a thin active Bi-doped ring showed prospects of the creation and application of such fiber type for laser and nonlinear optics
AGE AND GEODYNAMIC SETTINGS OF MIASKITE-PEGMATITES, CARBONATITES, AND Nb-Zr MINERALIZATION OF THE ILMENO-VISHNEVOGORSKY ALKALINE COMPLEX, SOUTH URAL (U-Pb AND Rb-Sr ISOTOPE DATA)
The Ilmeno-Vishnevogorsky miaskite-phenite-carbonatite complex (IVС), located in the Urals folded region, is a representative of "linear type" carbonatite complexes, with which Nb-Zr deposits are associated. The ore potential of linear carbonatite complexes, the time and duration of ore-forming processes, and the geodynamic setting remain the subject of discussion. In order to estimate the age and duration of ore formation processes in the IVC, the U-Pb dating of zircon (LA-ICP-MS) from miaskite-pegmatites, as well as Rb-Sr dating of ore-bearing (pyrochlore-bearing) varieties of miaskite-pegmatites and carbonatites (ID-TIMS) were carried out. The U-Pb dating of zircon from the IVC miaskite-pegmatites showed the Permian age (280±8 Ma, P1 – age of the main zircon generation). The Rb-Sr-mineral isochron of miaskite-pegmatite showed a close age of 274±5 Ma (P1), confirming the results of U-Pb-dating of miaskite-pegmatites. The Rb-Sr dating of pyrochlore carbonatite varieties (sovites II) showed an age of 250±3 Ma (T1). The results of Rb-Sr- and U-Pb dating indicate that the processes of pegmatite- and carbonatite formation and the associated Zr-Nb ore formation in IVC occurred at the late collisional and postcollisional stage of development of the Ural folded region. Based on the data obtained, the geodynamic model for the IVC formation was proposed
FRW Cosmology with Non-positively Defined Higgs Potentials
We discuss the classical aspects of dynamics of scalar models with
non-positive Higgs potentials in the FRW cosmology. These models appear as
effective local models in non-local models related with string field theories.
After a suitable field redefinition these models have the form of local Higgs
models with a negative extra cosmological term and the total Higgs potential is
non-positively defined and has rather small coupling constant. The
non-positivity of the potential leads to the fact that on some stage of
evolution the expansion mode gives place to the mode of contraction, due to
that the stage of reheating is absent. In these models the hard regime of
inflation gives place to inflation near the hill top and the area of the slow
roll inflation is very small. Meanwhile one can obtain enough e-foldings before
the contraction to make the model under consideration admissible to describe
inflation.Comment: 40 pages, 20 figures, typos correcte
Bosons in a Lattice: Exciton-Phonon Condensate in Cu2O
We explore a nonlinear field model to describe the interplay between the
ability of excitons to be Bose-condensed and their interaction with other modes
of a crystal. We apply our consideration to the long-living para-excitons in
Cu2O. Taking into account the exciton-phonon interaction and introducing a
coherent phonon part of the moving condensate, we derive the dynamic equations
for the exciton-phonon condensate. These equations can support localized
solutions, and we discuss the conditions for the moving inhomogeneous
condensate to appear in the crystal. We calculate the condensate wave function
and energy, and a collective excitation spectrum in the semiclassical
approximation; the inside-excitations were found to follow the asymptotic
behavior of the macroscopic wave function exactly. The stability conditions of
the moving condensate are analyzed by use of Landau arguments, and Landau
critical parameters appear in the theory. Finally, we apply our model to
describe the recently observed interference and strong nonlinear interaction
between two coherent exciton-phonon packets in Cu2O.Comment: 34 pages, LaTeX, four figures (.ps) are incorporated by epsf.
Submitted to Phys. Rev.
Squeezing of Atoms in a Pulsed Optical Lattice
We study the process of squeezing of an ensemble of cold atoms in a pulsed
optical lattice. The problem is treated both classically and
quantum-mechanically under various thermal conditions. We show that a dramatic
compression of the atomic density near the minima of the optical potential can
be achieved with a proper pulsing of the lattice. Several strategies leading to
the enhanced atomic squeezing are suggested, compared and optimized.Comment: Latex, 9 pages, 10 figures, submitted to PR
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