A Fast Pulsed Source of Ballistic R⁻ Rotons in Superfluid ⁴He

Abstract

Superfluid ⁴ He is unique in having well-defined excitations (R ⁻ rotons) with momentum oppositely directed to their velocity. If a beam of R ⁻ rotons can be produced, it could be unambiguously detected by quantum evaporation because the atoms will emerge in the opposite quadrant to that for atoms evaporated by R ⁺ rotons and phonons. Previous work shows that a heated metal film which is immersed in superfluid ⁴ He only creates phonons and R ⁺ rotons. A sponge-like heater does appear to produce R ⁻ rotons but, because it has a long time constant, it cannot be used in time of flight studies. We have developed a source that produces fast pulses of R ⁻ rotons suitable for time of flight measurements. The method uses interactions between R ⁺ rotons to create R ⁻ rotons, so a transient high density of R ⁺ rotons in a small confined volume is needed. The source appears to operate as we expect from a model of the evolution of the R ⁺ and R ⁻ roton populations.