Roton Quantum Evaporation

Brief report of EPSRC grant GR/L29149
Investigators: Adrian F.G. Wyatt and Charles D.H. Williams.

We have developed a source of R^- rotons and shown that R^- rotons can quantum evaporate ⁴He atoms. The kinematics of the experiment also demonstrates directly the most unusual character of these quasiparticles -- their momentum is anti-parallel to their group velocity. It is worth noting that this is real momentum, in contrast to momentum of a particle moving in a crystalline lattice which is uncertain to an integral number of reciprocal lattice vectors. This work was published in Science. It completes our demonstration that high-energy excitations in liquid helium propagate ballistically at their group velocity and all quantum evaporate in one-to-one processes with conservation of energy and parallel momentum.

Despite the clear demonstration of quantum evaporation it has proved much harder to determine the quantum efficiency of the process. This is due to the difficulty of measuring the absolute fluxes of phonons and rotons. An experiment which avoids this problem measures a beam of atoms incident on a slab of liquid ⁴He and the flux of atoms which emanate from the other side. We have combined our expertise in two areas -- the preparation of caesium as a non-wetted substrate for liquid ⁴He and the creation and detection of atom beams -- to invent this unique experiment. A cell has been made which is divided into two regions by a ring of caesium which separates two portions of superfluid helium with different chemical potentials. In this way we can create a very stable thin slab of liquid helium which is accessible from both sides. The preliminary transmission results are directly comparable with the recent theoretical results by Krotscheck et al..

We have developed the theory of R⁺ roton-phonon scattering and compared it with previous measurements of the attenuation of propagating ballistic rotons through liquid ⁴He at various temperatures. The wavevector-dependent scattering is in very reasonable agreement with the extended theory with no adjustable parameters.

We have further analysed previous experimental results to expose some very interesting points. Phonon quantum evaporation appears to come from the Bose-Einstein condensate and the recoil momentum perpendicular to the surface can be understood in terms of the condensate recoiling as a rigid body. The quantum evaporation by R⁺ rotons has been shown to depend on wavevector, and the ratio of the probabilities for evaporating ³He and ⁴He atoms by phonons has been determined. We find that R⁺ rotons cannot quantum evaporate 3He in remarkable contrast to the ⁴He case.

This grant has also facilitated a very productive collaboration between our group and that of Professor I. Adamenko in Kharkov. As a result, we now have a detailed understanding of propagating phonon beams, a new theory for the efficient creation of high-energy phonons from low-energy phonons in liquid ⁴He. With this theory we have predicted that the high-energy phonons will build-up to suprathermal densities.

See also: other research reports.