Scattering of Atomic Beams in Semiquantum Gases

Abstract

A low density system at rather high temperature whose particles obey the classical Boltzmann-Maxwell statistics and which however exhibits fundamentally quantum-mechanical macroscopic properties is usually called a semiquantum gas. The most typical semiquantum gases which are actively being studied in experiment are spin-polarized atomic hydrogen H↓ and deuterium D↓, gaseous ³He↑, the system of ³He-atoms dissolved in superfluid ⁴He etc. Quantitatively one can expect a rarefied Boltzmann gas to display a quantum-mechanical behavior within the temperature range¹:

$$ {{\varepsilon }_{d}} < < T < < {{\hbar }^{2}}/mr_{0}^{2},Nr_{0}^{3} < < 1, $$

((1))

where ∈ _d is the quantum degeneracy temperature, m the mass of a particle, r ₀ the interaction range (of the order of an atomic size) and N the atomic density. Under this condition a gas may possess a highly nontrivial spectrum of collective excitation (like spin waves^1–5). Moreover a very important mechanism of inelastic interaction between a particle in the gas and collective fluctuations of thermodynamical variables comes into effect if the criterion (1) is fulfilled⁶. Microscopically such an interaction may be interpreted as the inelastic scattering of paramagnetic particles with thermally excited phonons, spin modes and entropy fluctuations in the semiquantum gas. E.g. under certain conditions the exchange interaction between a particle of a gas and fluctuations of macroscopic magnetization strongly affects the thermal conductivity and viscosity of a spin-polarized semiquantum gas⁷.