POSSIBLE EVIDENCE OF THE EXISTENCE OF A SOUND-LIKE COLLECTIVE MODE INND2-XCEXCUO4

Sound velocities along high and low-symmetry directions are measured w ith the goal of determining the full set of elastic constants in Nd2-x CexCuO4. The velocity of quasitransverse mode (upsilon(t)) with wave v ectors q lying in the {010} plane shows anomalous behavior. Rotating q from the [100] to the [001] direction we registered, first, a strong lowering, and, then, a steplike increase (up to two times) of upsilon( t). There is no reasonable explanation of the observed peculiarities i n the framework of conventional elasticity theory and the effect can b e accounted for by the existence of a two-dimensional soundlike collec tive mode of electronic origin that strongly interacts with lattice vi brations. A proposed phenomenological model of the effect is in qualit ative agreement with the experimental data.