Direct Evidence for R Rotons Having Antiparallel Momentum and Velocity

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Science  19 Feb 1999:
Vol. 283, Issue 5405, pp. 1150-1152
DOI: 10.1126/science.283.5405.1150

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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.

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