Untitled

Slipko

et al. 2007

Self Cite

The problem of the creation of high-energy phonons (h-phonons) by a pulse of low-energy phonons (l-phonons) moving from a heater to a detector in superfluid helium, is solved. The rate of h-phonon creation is obtained and it is shown that created h-phonons occupy a much smaller solid angle in momentum space, than the l-phonons. An analytical expression for the creation rate of h-phonon, along the symmetry axis of a pulse, are derived. It allows us to get useful approximate analytical expressions for the creation rate of h-phonons. The time dependences of the parameters which describe the l-phonon pulse are obtained. This shows that half of the initial energy of l-phonon pulse can be transferred into h-phonons. The results of the calculations are compared with experimental data and we show that this theory explains a number of experimental results. The value of the momentum, which separates the l-and h-phonon subsystems, is found.

“…We note that momentum dependence of n 1 , when q 1 0 = , was ob- tained in Ref. 9 with the approximate distribution function (10); the rate is similar to that presented here.…”

Section: Dependence Of the Rate N 1 On All Parameterssupporting

confidence: 85%

Section: The Kinetic Equation For the Distribution Function Of H-phononsmentioning

confidence: 99%

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Creation of high-energy phonons by four-phonon processes in anisotropic phonon systems of HeII

Slipko

et al. 2007

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“…The theory of this unique phenomenon was proposed in [21,22]. A further development of the theory [23][24][25][26][27][28], shows that in strongly anisotropic phonon systems there exists an asymmetry between processes of creation and decay for the h phonons. Such an asymmetry causes the distribution function of h phonons in the anisotropic phonon system to be S times greater than that in the Bose-Einstein distribution.…”

Section: >>mentioning

confidence: 99%

“…is the number of phonons with momentum p 1 created per unit time as the result of four-phonon (4pp) interactions; defines the transition probability density for 4pp processes, which have been calculated [25]; W b and W d each represent a set of three solid angles, one for each momentum over which the function is integrated, i.e., W bi and W di (i = 2,3,4). These maximum angles are determined by the anisotropy of the phonon system and the angles in the 4pp interactions.…”

Section: The Inequalities For Determining Of the Stationary Distributmentioning

confidence: 99%

On the suprathermal distribution in an anisotropic phonon system in He II

Wyatt

2003

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The equation that describes the suprathermal distribution of high-energy phonons (h phonons) created in anisotropic phonon systems in superfluid helium is obtained. The solution of this equation enables the derivation of the value of suprathermal ratio S as the ratio of the actual distribution to the Bose-Einstein one, its dependences on the momentum of the h phonons, the anisotropy parameters, and the temperature of the low-energy phonons from which the h phonons are created. We analyze this equation to obtain an estimate of the value of the ratio between the h-phonon number density in anisotropic and isotropic phonon systems and draw conclusions about the dependence of S on the relevant parameters.

Asymmetry of relaxation processes and the creation of high-energy phonons in the anisotropic phonon systems of He II

Wyatt

2002

A study is made of relaxation processes in anisotropic phonon systems in superfluid helium. The rates of creation and annihilation of high-energy (h) phonons in all possible processes in an anisotropic phonon system are obtained as functions of momentum, temperature, and the anisotropy parameter. The physical causes of the asymmetry in the creation and annihilation of h phonons are elucidated. All of the processes of interaction of h phonons with low-energy phonons (l phonons) and h phonons with each other are investigated, and the role of each of these processes in the formation of the distribution function of h phonons is determined. All of the processes that do not conserve the total number of h phonons are taken into account in constructing an equation describing the change in energy density of the h phonons in the main beam, and a quasistationary solution of this equation is found. The solution of this equation can be used to find the energy density of the created h phonons at any point in space at an arbitrary time and the energy density of l phonons in the main beam.