Observation of One-Dimensional Second Sound in Superfluid Helium

A New collective mode of heat transport has been observed in superfluid helium at low temperatures and pressures. Unlike ordinary second sound, this new mode travels at essentially the acoustic-sound velocity and is characterized by the nearly collinear phonon relaxation arising from anomalous dispersion. The mode has been found to persist into the collisionless regime, $\omega \tau >1\mathrm{}$.