Propagation of "cool" pulses in liquid He II

We applied pulses of heat to liquid He and found that below the λ transition pulses of lower-than-ambient temperature propagate in the He II. This "cool" pulse is attributed to the temperature change that accompanies a compressive first-sound pulse. The temperature pulse is negative because the thermal expansion of He II is negative near the λ transition. © 1971 The American Physical Society.