Roton-roton scattering in liquidHe4: A direct observation

Abstract

We have directly observed the effects of roton-roton scattering by crossing two beams of ballistic rotons in liquid "^He at a temperature of -0.1 K. One beam is weak and is used as the probe. Its attenuation is measured as it passes through the stronger scattering beam which is predominantly perpendicular to the probe beam. The beams are pulsed and the dispersion of the roton velocities enables the momenta of the rotons to be resolved by time-of-flight measurements. It appears that the scattering is strongly wave-vector dependent.PACS numbers: 67.40.-w, 63.2aHpThe excitation model of liquid "^He envisages a ground state and excitations from this ground state. The excitations, phonons and rotons, are well defined with an inverse lifetime T<^(o, where (o is the energy of the excitation. The excitation picture is well established and the dispersion curve of the excitations has been measured by neutron scattering (see Fig. 1). The lifetime of the excitations is limited by scattering between the excitations and so the inverse lifetime is directly proportional to the prevailing density of excitations. Therefore T increases with temperature as the ambient excitation density increases. Various methods of measuring the lifetimes confirm this. ^"^ However, these measurements can only be made at high temperatures where the lifetime is short enough to be detectable. At low temperatures the lifetimes are orders of magnitude longer and this can only be seen by time-of-flight measurements.^The scattering measurements to date rely on a thermal distribution of excitations, so the excitation lifetimes which are obtained are necessarily averages over the energies of the excitations and the scattering angles. As it is likely that scattering cross sections are dependent on energy and angle, a more refined type of experiment is desirable. In principle one would like to have two well-15 10 v \--

New approach to the observation of the condensate fraction in superfluid helium-4

“…In order to make the mean free path of helium in the scattering chamber large enough, the chamber must be kept at about 0.3 K or less. This low temperature chamber would be an ideal environment for a bolometer [16,18,19], so that is probably the best choice for the atom detector. The use of bolometers for detecting low-energy helium atoms is well known [20] and has recently been reported [16] at the energies needed for the experiment proposed here.…”

Majorana surface states of superfluidHe3AandB

“…They were performed in the past on 4 He where roton-roton scattering is treated 45 and on the 3 He B-phase where the surface Andreev bound state is investigated. 46,47 Using this method, we may pick up Majorana QPs with a particular wave number.…”

Evidence of scattering of bulk elementary excitations in isotopically pure liquid helium-II at low temperatures

“…Forbes and Wyatt [29,30] give the first evidence on the scattering of BEEs. Their work [29] uses the technique crossed ballistic BEE beams to investigate the BEE scattering under the right angle, for which some details of employments of the new method were published earlier in [30]. Note that Wyatt et al's explanation [14][15][16][17][18][19][20] of their experimental results forbids any propagation for the first non-dispersive Zakharenko wave with energy $7 K in the BEE phonon branch owing to the phonon backflow effect measured by W.G.…”

“…Some experimental records presented here were taken from [28]. Forbes and Wyatt [29,30] give the first evidence on the scattering of BEEs. Their work [29] uses the technique crossed ballistic BEE beams to investigate the BEE scattering under the right angle, for which some details of employments of the new method were published earlier in [30].…”