INDIRECT OBSERVATION OF A SOUNDLIKE COLLECTIVE MODE IN ELECTRONIC CUPRATES

We report the results of sound velocity measurements in Nd2CuO4 and Pr 2CuO4 single crystals. An unexpected effect is found, which consists i n unusual behavior of the velocity of the quasitransverse mode v(1) at out-of-CuO2 plane direction of the wave vector q. Rotating q from the [100] or [110] to the [001] direction we registered, first, a strong lowering, and, then, a steplike increase of v(t). The observed feature s can be described by a phenomenological model, based on the suggestio n about the existence of a sound like collective mode coupled to u(zz) lattice deformation. All components of the elastic moduli tensor, bul k moduli and Debye temperatures are obtained.