High-resolution roton spectra around the superfluid transition temperature in liquid 4He

We show for the first time high-resolution excitation spectra of bulk superfluid and liquid 4He obtained in a wide energy range (0–3 meV) in a broad temperature interval 0.6–2.5 K. The experiments were performed at 0.4 bar constant liquid pressure on the time-of-flight crystal analyser neutron spectrometer IRIS at the ISIS Facility. There is a real gap in resolution between neutron spin echo or exact-backscattering spectroscopy and other conventional time-of-flight or triple-axis-spectrometer measurements, which at the moment can only be covered by IRIS. The unique combination of high resolution and wide dynamic range accessible on this inverted geometry spectrometer allowed for a non-conventionally detailed line-shape analysis of the roton excitations and made possible the separation of the roton and the higher-energy broad components. We provide the high-resolution evidence of a remainder single-phonon response at the extrapolated roton energy above Tλ. We conclude that the superfluid phase transition manifests itself in the strong slowdown of the variation of the single-phonon line shape with increasing temperature, rather than in the total disappearance of the signal.