High Resolution Measurements of the Temperature Dependence of the Roton Energy of Superfluid 4He

New measurements of the roton excitations in superfluid 4He have been made using the IRIS spectrometer (energy resolution ∼20μeV at the roton energy) at the ISIS spallation neutron source. The roton energy was determined over a temperature range of 0.56 to 1.61 K. High resolution (energy resolution <1μeV) neutron inelastic scattering measurements by Andersen et al.2 of the dependence of the 4He 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 established roton-roton interaction theory due to Bedell, Pines and Zawadowski (BPZ).3 We find that our results can be described well by the BPZ theory 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.