High resolution measurements of the temperature dependence of the roton energy of superfluid He-4

New measurements of the roton excitations in superfluid He-4 have been made using the IRIS spectrometer (energy resolution similar to 20 ge Vat the ro ton energy) at the ISIS spallation neutron source. The roton energy was det ermined over a temperature range of 0.56 to 1.61 K. High resolution (energy resolution < 1 mu eV) neutron inelastic scattering measurements by Anderse n et al.(2) of the dependence of the He-4 roton energy on temperature have shown significant deviations from the expected behaviour,, the temperature variation was found to be discontinuous and slower than predicted by the es tablished roton-roton interaction theory due to Bedell, Pines and Zawadowsk i (BPZ).(3) We find that our results can be described well by the BPZ theor y over this temperature range and we do not observe the anomalous behaviour reported by Andersen et al. We offer some explanations as to the origin of this apparent disagreement.