Nonlinear second sound in He-II under pressure

The dependence of the nonlinearity coefficient alpha for the roton second s ound in superfluid He-II on pressure P is studied for the first time. It is found that as the value of P increases from saturated vapor pressure to 25 atm, the temperature T-alpha at which the coefficient alpha reverses its s ign decreases from 1.88 to 1.58 K, i.e., there exists a wide temperature in terval below T-lambda in He-II at all pressures, in which the nonlinearity coefficient alpha is negative, and a temperature discontinuity (shock wave) is formed at the trailing edge of the second sound compression wave. (C) 1 999 American Institute of Physics. [S1063-777X(99)00206-6].