The amplitude dependence of the velocity of nonlinear second sound in helium II near Tλ

The amplitude dependence of the velocity of nonlinear second sound in Helium II, as given by the Khalatnikov nonlinear coefficient, was measured near Tλ using a single-pulse travel-time technique. Using nonplanar pulses which were amplified and focused by a spherical reflector, the confirmation of Khalanikov's second order two fluid model prediction was extended to within 7 mK of Tλ, much closer to the critical temperature than previously reported. This study was restricted to single pulses of second sound in order to ensure that the measurement pertained only to an undisturbed, vortex-tangle-free sample of Helium II