Sound velocities along high- and low-symmetry directions are measured with the goal of determining the full set of elastic constants in ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_4$. The velocity of quasitransverse mode (${\mathit{v}}_{\mathit{t}}$) with wave vectors 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 ${\mathit{v}}_{\mathit{t}}$. There is no reasonable explanation of the observed peculiarities in the framework of conventional elasticity theory and the effect can be accounted for by the existence of a two-dimensional soundlike collective mode of electronic origin that strongly interacts with lattice vibrations. A proposed phenomenological model of the effect is in qualitative agreement with the experimental data.

Close

PrevNext

View figure in article