406 research outputs found

### Binary Quantum Turbulence Arising from Countersuperflow Instability in Two-Component Bose-Einstein Condensates

We theoretically study the development of quantum turbulence from two
counter-propagating superfluids of miscible Bose-Einstein condensates by
numerically solving the coupled Gross-Pitaevskii equations. When the relative
velocity exceeds a critical value, the counter-superflow becomes unstable and
quantized vortices are nucleated, which leads to isotropic quantum turbulence
consisting of two superflows. It is shown that the binary turbulence can be
realized experimentally in a trapped system.Comment: 5 pages, 3 figure

### Stochastic cosmology, theories of perturbations and Lifshitz gravity

We review some works of E M Lifshitz connected with gravity and cosmology and
also some later works, connected with his ideas. The main topics of this review
are the stochastic cosmology of an anisotropic universe and of an isotropic
universe with the scalar field, the quasi-isotropic (gradient) expansion in
cosmology and Horava-Lifshitz gravity and cosmology.Comment: 30 pages, to appear in Physics - Uspekhi. arXiv admin note: text
overlap with arXiv:0901.3775 by other author

### The Shear Viscosity to Entropy Density Ratio of Trapped Fermions in the Unitarity Limit

We extract the shear viscosity to entropy density ratio \eta/s of cold
fermionic atoms in the unitarity limit from experimental data on the damping of
collective excitations. We find that near the critical temperature \eta/s is
roughly equal to 1/2 in units of \hbar/k_B. With the possible exception of the
quark gluon plasma, this value is closer to the conjectured lower bound
1/(4\pi) than any other known liquid.Comment: published versio

### Countersuperflow instability in miscible two-component Bose-Einstein condensates

We study theoretically the instability of countersuperflow, i.e., two
counterpropagating miscible superflows, in uniform two-component Bose-Einstein
condensates. Countersuperflow instability causes mutual friction between the
superfluids, causing a momentum exchange between the two condensates, when the
relative velocity of the counterflow exceeds a critical value. The momentum
exchange leads to nucleation of vortex rings from characteristic density
patterns due to the nonlinear development of the instability. Expansion of the
vortex rings drastically accelerates the momentum exchange, leading to a highly
nonlinear regime caused by intervortex interaction and vortex reconnection
between the rings. For a sufficiently large interaction between the two
components, rapid expansion of the vortex rings causes isotropic turbulence and
the global relative motion of the two condensates relaxes. The maximum vortex
line density in the turbulence is proportional to the square of the relative
velocity.Comment: 9 pages, 6 figure

### Complex inflaton field in quantum cosmology

We investigate the cosmological model with the complex scalar
self-interacting inflaton field non-minimally coupled to gravity. The different
geometries of the Euclidean classically forbidden regions are represented. The
instanton solutions of the corresponding Euclidean equations of motion are
found by numerical calculations supplemented by the qualitative analysis of
Lorentzian and Euclidean trajectories. The applications of these solutions to
the no-boundary and tunneling proposals for the wave function of the Universe
are studied. Possible interpretation of obtained results and their connection
with inflationary cosmology is discussed. The restrictions on the possible
values of the new quasi-fundamental constant of the theory-non-zero classical
charge-- are obtained. The equations of motion for the generalized cosmological
model with complex scalar field are written down and investigated. The
conditions of the existence of instanton solutions corresponding to permanent
values of an absolute value of scalar field are obtained.Comment: 34 pages with 2 gif figures, mprocl.sty, To appear in International
Journal of Modern Physics

### Instability and Chaos in Non-Linear Wave Interaction: a simple model

We analyze stability of a system which contains an harmonic oscillator
non-linearly coupled to its second harmonic, in the presence of a driving
force. It is found that there always exists a critical amplitude of the driving
force above which a loss of stability appears. The dependence of the critical
input power on the physical parameters is analyzed. For a driving force with
higher amplitude chaotic behavior is observed. Generalization to interactions
which include higher modes is discussed.
Keywords: Non-Linear Waves, Stability, Chaos.Comment: 16 pages, 4 figure

### Three dimensionality of pulsed second-sound waves in He II

Three dimensionality of 3D pulsed second sound wave in He II emitted from a
finite size heater is experimentally investigated and theoretically studied
based on two-fluid model in this study. The detailed propagation of 3D pulsed
second sound wave is presented and reasonable agreement between the
experimental and theoretical results is obtained. Heater size has a big
influence on the profile of 3D second sound wave. The counterflow between the
superfluid and normal fluid components becomes inverse in the rarefaction of 3D
second sound wave. The amplitude of rarefaction decreases due to the
interaction between second sound wave and quantized vortices, which explains
the experimental results about second sound wave near [Phys. Rev. Lett. 73,
2480 (1994)]. The accumulation of dense quantized vortices in the vicinity of
heater surface leads to the formation of a thermal boundary layer, and further
increase of heating duration results in the occurrence of boiling phenomena.
PACS numbers: 67.40.Pm 43.25.+y 67.40.BzComment: 30 pages, 9 figures. Physical Review B, Accepte

### Synchronous Quantum Gravity

The implications of restricting the covariance principle within a Gaussian
gauge are developed both on a classical and a quantum level. Hence, we
investigate the cosmological issues of the obtained Schr\"odinger Quantum
Gravity with respect to the asymptotically early dynamics of a generic
Universe. A dualism between time and the reference frame fixing is then
inferred.Comment: 8 pages, Proceedings of the II Stueckelberg worksho

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