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Science 19 February 1999:
Vol. 283. no. 5405, pp. 1150 - 1152
DOI: 10.1126/science.283.5405.1150
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Reports

Direct Evidence for R- Rotons Having Antiparallel Momentum and Velocity

M. A. H. Tucker, A. F. G. Wyatt *

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.

School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK.
*   To whom correspondence should be addressed.

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Science. ISSN 0036-8075 (print), 1095-9203 (online)