ABSTRACT
Energy-conserving interactions which reverse the energy flow can occur by way of a N process or a U process. Reversal of the energy flow in a 3-phonon N process can occur in several different ways: (1) noncollinear processes exhibiting extreme elastic anisotropics, (2) dispersive noncollinear transitions to or from a region of an acoustic branch exhibiting negative group velocity, (3) collinear acoustic-to-optical transitions. Processes (1) and (2) will be numerically illustrated for copper by simple graphical methods applied to a lattice dynamical model. Process (3) will be illustrated graphically for a collinear interaction in an alkali halide. These and other examples presented represent the variety of energy-flow-reversing N and U processes that can be realized in dispersive single crystals.
