Controlled Excitation of Rotons in Superfluid Helium with an Optical Centrifuge

We experimentally demonstrate a controlled transfer of angular momentum to roton pairs in superfluid helium. The control is executed with an optical centrifuge and detected with coherent time- and frequency-resolved Raman scattering. We show that the sign of the Raman shift, and hence the orientation of the angular momentum transferred from the laser field to the rotons, is dictated by the centrifuge. The magnitude of the shift reflects the two-roton energy and indicates that the centrifuge-induced roton pairs are far from the equilibrium with the quantum bath. The observed decay of the coherent Raman signal suggests that the decoherence is governed by the scattering on thermal rotons and phonons. The demonstrated method offers ways of examining microscopic origins of superfluidity by controlling collective excitations in superfluids.

Quantum liquids

This article is dedicated to Roger A Cowley and his seminal contributions to our understanding of quantum liquids, both liquid4He and3He. Roger Cowley's neutron scattering measurements of the collective and independent particle response of liquid4He were made at Chalk River Laboratories in 1965-74 chiefly with A D B (Dave) Woods. They measured the phonon-roton (P-R) mode energy, intensity and width with new precision. Particularly, they extended the measurements to higher wave vector and identified both collective and single particle response regimes. They showed that the P-R mode terminated at a finite energy as predicted by Pitaeskii rather than continuing as predicted by Feynman and Feynman and Cohen. They determined both the single P-R mode and multimode contributions to the dynamics. They made direct comparison with theory which Roger understood well. They observed the Bose-Einstein condensate (BEC) fraction in liquid4He for the first time. This appears to be the first ever observation of BEC in any Bose gas or liquid. Roger Cowley's pioneering measurements of the density excitations of liquid3He were made at the Institut Laue Langevin (ILL) in the period 1973-80. Roger, Reinhard Scherm, W G (Bill) Stirling and collaborators showed for the first time that the density response of this highly neutron absorbing liquid could indeed be observed with neutrons. They documented with others the dynamic response as a function of temperature and pressure stimulating extensive theoretical and experimental interest that continues today.

Observation of dynamic atom-atom correlation in liquid helium in real space

Liquid ⁴He becomes superfluid and flows without resistance below temperature 2.17 K. Superfluidity has been a subject of intense studies and notable advances were made in elucidating the phenomenon by experiment and theory. Nevertheless, details of the microscopic state, including dynamic atom–atom correlations in the superfluid state, are not fully understood. Here using a technique of neutron dynamic pair-density function (DPDF) analysis we show that ⁴He atoms in the Bose–Einstein condensate have environment significantly different from uncondensed atoms, with the interatomic distance larger than the average by about 10%, whereas the average structure changes little through the superfluid transition. DPDF peak not seen in the snap-shot pair-density function is found at 2.3 Å, and is interpreted in terms of atomic tunnelling. The real space picture of dynamic atom–atom correlations presented here reveal characteristics of atomic dynamics not recognized so far, compelling yet another look at the phenomenon.

Liquid helium can be treated as an ideal gas or a condensed liquid and displays intriguing features like Bose–Einstein condensation. Here the authors show that roton excitation reveals information on real space dynamic atom-atom correlations in superfluid helium, which could be used to benchmark models.

Direct evidence for R- rotons having antiparallel momentum and velocity

Experimental evidence is presented that shows that the momentum of a R- roton (a particle-like excitation in liquid helium-4) is antiparallel to its velocity. Although this is anticipated from the negative slope of the dispersion curve for these excitations, it has only been possible to test since the development of a source of ballistic R- rotons. The backward refraction of the quantum evaporation process, which is the signature of antiparallel momentum and velocity, is observed.