Nonlinear second sound in superfluid He-4 under pressure

The pressure dependence of the nonlinearity coefficient a of the roton seco nd sound in superfluid He-II at pressures up to 25 bar has been studied. It has been found that with increasing pressure the temperature T-alpha, at w hich the coefficient a goes through zero, is lowered from 1.88 to 1.58 K. I t means that at all pressures up to the freezing of the superfluid liquid t here exists a wide range of temperatures T-alpha < T < T-lambda where the n onlinearity coefficient is negative: alpha < 0. In propagating compressiona l wave of second sound, it corresponds to a positive oscillation of the tem perature in He-II, and a temperature discontinuity (shock) should appear at the back side of the pulse. On generating a bipolar second sound pulse wit h shock in its middle, the width of the pulse does not change with distance . (C) 2000 Elsevier Science B.V. All rights reserved.