4,007 research outputs found
Fluctuations of the vortex line density in turbulent flows of quantum fluids
We present an analytical study of fluctuations of the Vortex Line Density
(VLD) in turbulent
flows of quantum fluids. Two cases are considered. The first one is the
counterflowing (Vinen) turbulence, where the vortex lines are disordered, and
the evolution of quantity obeys the Vinen equation. The second
case is the quasi-classic turbulence, where vortex lines are believed to form
the so called vortex bundles, and their dynamics is described by the HVBK
equations. The latter case, is of a special interest, since a number of recent
experiments demonstrate the dependence for spectrum VLD,
instead of law, typical for spectrum of vorticity. In
nonstationary situation, in particular, in the fluctuating turbulent flow there
is a retardation between the instantaneous value of the normal velocity and the
quantity . This retardation tends to decrease in the accordance
with the inner dynamics, which has a relaxation character. In both cases the
relaxation dynamics of VLD is related to fluctuations of the relative velocity,
however if for the Vinen case the rate of temporal change for
is directly depends on , for the HVBK dynamics it
depends on . As a result, for the
disordered case the spectrum coincides with the spectrum . In the
case of the bundle arrangement, the spectrum of the VLD varies (at different
temperatures) from to dependencies. This
conclusion may serve as a basis for the experimental determination of what kind
of the turbulence is implemented in different types of generation.Comment: 8 pages, 29 reference
Thermodynamic inequalities in superfluid
We investigate general thermodynamic stability conditions for the superfluid.
This analysis is performed in an extended space of thermodynamic variables
containing (along with the usual thermodynamic coordinates such as pressure and
temperature) superfluid velocity and momentum density. The stability conditions
lead to thermodynamic inequalities which replace the Landau superfluidity
criterion at finite temperatures.Comment: 7 pages, 1 figur
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
Relativistic effects in two-particle emission for electron and neutrino reactions
Two-particle two-hole contributions to electroweak response functions are
computed in a fully relativistic Fermi gas, assuming that the electroweak
current matrix elements are independent of the kinematics. We analyze the
genuine kinematical and relativistic effects before including a realistic
meson-exchange current (MEC) operator. This allows one to study the
mathematical properties of the non-trivial seven-dimensional integrals
appearing in the calculation and to design an optimal numerical procedure to
reduce the computation time. This is required for practical applications to CC
neutrino scattering experiments, where an additional integral over the neutrino
flux is performed. Finally we examine the viability of this model to compute
the electroweak 2p-2h response functions.Comment: Major revision (shortened). 22 pages, 18 figure
2p-2h excitations in neutrino scattering: angular distribution and frozen approximation
We study the phase-space dependence of 2p-2h excitations in neutrino
scattering using the relativistic Fermi gas model. We follow a similar approach
to other authors, but focusing in the phase-space properties, comparing with
the non-relativistic model. A careful mathematical analysis of the angular
distribution function for the outgoing nucleons is performed. Our goals are to
optimize the CPU time of the 7D integral to compute the hadron tensor in
neutrino scattering, and to conciliate the different relativistic and non
relativistic models by describing general properties independently of the
two-body current. For some emission angles the angular distribution becomes
infinite in the Lab system, and we derive a method to integrate analytically
around the divergence. Our results show that the frozen approximation, obtained
by neglecting the momenta of the two initial nucleons inside the integral of
the hadron tensor, reproduces fairly the exact response functions for constant
current matrix elements.Comment: 8 pages, 4 figures. Contribution to 16th International Workshop on
Neutrino Factories and Future Neutrino Beam Facilities, 25-30 August, 2014.
Held at University of Glasgow, United Kingdo
Weight bases of Gelfand-Tsetlin type for representations of classical Lie algebras
This paper completes a series devoted to explicit constructions of
finite-dimensional irreducible representations of the classical Lie algebras.
Here the case of odd orthogonal Lie algebras (of type B) is considered (two
previous papers dealt with C and D types). A weight basis for each
representation of the Lie algebra o(2n+1) is constructed. The basis vectors are
parametrized by Gelfand--Tsetlin-type patterns. Explicit formulas for the
matrix elements of generators of o(2n+1) in this basis are given. The
construction is based on the representation theory of the Yangians. A similar
approach is applied to the A type case where the well-known formulas due to
Gelfand and Tsetlin are reproduced.Comment: 29 pages, Late
The frozen nucleon approximation in two-particle two-hole response functions
We present a fast and efficient method to compute the inclusive two-particle
two-hole (2p-2h) electroweak responses in the neutrino and electron
quasielastic inclusive cross sections. The method is based on two
approximations. The first neglects the motion of the two initial nucleons below
the Fermi momentum, which are considered to be at rest. This approximation,
which is reasonable for high values of the momentum transfer, turns out also to
be quite good for moderate values of the momentum transfer . The
second approximation involves using in the "frozen" meson-exchange currents
(MEC) an effective -propagator averaged over the Fermi sea. Within the
resulting "frozen nucleon approximation", the inclusive 2p-2h responses are
accurately calculated with only a one-dimensional integral over the emission
angle of one of the final nucleons, thus drastically simplifying the
calculation and reducing the computational time. The latter makes this method
especially well-suited for implementation in Monte Carlo neutrino event
generators.Comment: 8 pages, 5 figures and 1 tabl
Two-nucleon emission in neutrino and electron scattering from nuclei: the modified convolution approximation
The theoretical formalism of inclusive lepton-nucleus scattering in the
two-nucleon emission channel is discussed in the context of a simplified
approach, the modified convolution approximation. This allows one to write the
2p2h responses of the relativistic Fermi gas as a folding integral of two 1p1h
responses with the energies and momenta transferred to each nucleon. The idea
behind this method is to introduce different average momenta for the two
initial nucleons in the matrix elements of the two-body current, with the
innovation that they depend on the transferred energies and momenta. This
method treats exactly the two-body phase space kinematics, and reduces the
formulae of the response functions from seven-dimensional integrals over
momenta to much simpler three-dimensional ones. The applicability of the method
is checked by comparing with the full results within a model of electroweak
meson-exchange currents. The predictions are accurate enough, especially in the
low-energy threshold region where the average momentum approximation works the
best.Comment: 35 pages, 13 figure
Magnetic vortex-antivortex crystals generated by spin-polarized current
We study vortex pattern formation in thin ferromagnetic films under the
action of strong spin-polarized currents. Considering the currents which are
polarized along the normal of the film plane, we determine the critical current
above which the film goes to a saturated state with all magnetic moments being
perpendicular to the film plane. We show that stable square vortex-antivortex
superlattices (\emph{vortex crystals}) appears slightly below the critical
current. The melting of the vortex crystal occurs with current further
decreasing. A mechanism of current-induced periodic vortex-antivortex lattice
formation is proposed. Micromagnetic simulations confirm our analytical results
with a high accuracy.Comment: 12 pages, 11 figure
Destroying superfluidity by rotating a Fermi gas at unitarity
We study the effect of the rotation on a harmonically trapped Fermi gas at
zero temperature under the assumption that vortices are not formed. We show
that at unitarity the rotation produces a phase separation between a non
rotating superfluid (S) core and a rigidly rotating normal (N) gas. The
interface between the two phases is characterized by a density discontinuity
, independent of the angular velocity. The depletion
of the superfluid and the angular momentum of the rotating configuration are
calculated as a function of the angular velocity. The conditions of stability
are also discussed and the critical angular velocity for the onset of a
spontaneous quadrupole deformation of the interface is evaluated.Comment: 5 pages, 4 figures; comments added; 2 figures changed according to
new results; inset Fig.2 corrected; accepted for publication in Phys. Rev.
Let
- …