49 research outputs found
Interaction of phonons at superfluid helium-solid interfaces
A new method of obtaining the interaction Hamiltonian of phonons at
superfluid helium-solid interface is proposed in the work. Equations of
hydrodynamic variables are obtained in terms of second quantization if helium
occupies a half-space. The contributions of all processes to the heat flux from
solid to superfluid helium are calculated based on the obtained Hamiltonian.
The angular distribution of phonons emitted by a solid is found in different
processes. It is shown that all the exit angles of superfuild helium phonons
are allowed. The obtained results are compared with experimental data and with
previous theoretical works.Comment: 9 pages, 3 figure
Pressure of thermal excitations in superfluid helium
We find the pressure, due to the thermal excitations of superfluid helium, at
the interface with a solid. The separate contributions of phonons, rotons
and rotons are derived. The pressure due to rotons is shown to be
negative and partially compensates the positive contribution of rotons,
so the total roton pressure is positive but several times less than the
separate and roton contributions. The pressure of the quasiparticle
gas is shown to account for the fountain effect in . An experiment is
proposed to observe the negative pressure due to rotons.Comment: 14 pages, 4 figure
Evolution of a pulse of noninteracting quasiparticles with dispersion and initial angular width
The evolution of a pulse of noninteracting quasiparticles, caused by their different velocities
and angular distribution of momenta, is studied theoretically. Equations are found that describe
the shape of the pulse surface at any time. The time of the beginning, end and duration of the density
of the quasiparticle energy flux is determined at a general spatial point. The quasiparticle energy
density is considered at all times and positions, and it is shown that the region of high energy
density, in the middle of the pulse, is equal to the initial energy density under certain conditions.
These theoretical results are discussed in relation to experimental data on the evolution of a pulse
of noninteracting phonons in superfluid helium
Volume element structure and roton-maxon-phonon excitations in superfluid helium beyond the Gross-Pitaevskii approximation
We propose a theory which deals with the structure and interactions of volume
elements in liquid helium II. The approach consists of two nested models linked
via parametric space. The short-wavelength part describes the interior
structure of the fluid element using a non-perturbative approach based on the
logarithmic wave equation; it suggests the Gaussian-like behaviour of the
element's interior density and interparticle interaction potential. The
long-wavelength part is the quantum many-body theory of such elements which
deals with their dynamics and interactions. Our approach leads to a unified
description of the phonon, maxon and roton excitations, and has noteworthy
agreement with experiment: with one essential parameter to fit we reproduce at
high accuracy not only the roton minimum but also the neighboring local maximum
as well as the sound velocity and structure factor.Comment: 9 pages, 6 figure
Short-Wave Excitations in Non-Local Gross-Pitaevskii Model
It is shown, that a non-local form of the Gross-Pitaevskii equation allows to
describe not only the long-wave excitations, but also the short-wave ones in
the systems with Bose-condensate. At given parameter values, the excitation
spectrum mimics the Landau spectrum of quasi-particle excitations in superfluid
Helium with roton minimum. The excitation wavelength, at which the roton
minimum exists, is close to the inter-particle interaction range. It is shown,
that the existence domain of the spectrum with a roton minimum is reduced, if
one accounts for an inter-particle attraction.Comment: 5 pages, 5 figures, UJP style; presented at Bogolyubov Kyiv
Conference "Modern Problems of Theoretical and Mathematical Physics",
September 15-18, 200
West Nile Fever in the Rostov Region: Ecological and Epidemiological Peculiarities of the Outbreak in 2010
This paper describes the outbreak of West Nile fever in the Rostov Region in 2010 and evaluates its ecological and epidemiological peculiarities. From 15th of July till 22nd of September 2010, detected were the 64 cases (1, 4800/0000) of the disease, which were characterized by vector-born mechanism of transmission. Peak of morbidity coincided with mass breeding of Culicidae, increase in the number of Culex mosquitoes, and reoccurring growth of Aedes mosquito population. Diffuse type of the epidemiological process, higher rates of the cases among urbanites, infected in the country-side area, were the characteristic features of that outbreak. West Nile virus antigen was detected by means of IFA in samples taken from An. maculipennis and Cx. pipiens mosquitoes, wild and synanthropic birds, Rh. rossicus ticks, house and wood mice, which facilitates identification of the core factors for the agent circulation and West Nile fever natural focus formation