Oscillations localized near impurity layers in a layered crystal

It is shown that the interaction between vibrational modes polarized parall el and perpendicular to the layers in a strongly anisotropic layered crysta l is very weak for frequencies above the van Hove frequency, which correspo nds to a transition from closed isofrequency surfaces to surfaces which are open in the weak-coupling direction. Such mode quasisplitting results in q uasi-two-dimensional behavior of phonons polarized along layers and quasi-o ne-dimensional behavior of phonons polarized in a direction perpendicular t o the layers and localized near light or strongly coupled impurity atoms or monolayers. (C) 2001 American Institute of Physics.