Fermi-resonance-like phenomena in lattice dynamics of FCC La

Nonperturbative (resonance) anharmonic effects in the lattice dynamics of fcc La due to an integer ratio of phonon frequencies (3:1) in a certain point of the Brillouin zone are predicted. Depending on initial phonon phase values, an energy transfer between the "resonance" modes can be either regular or chaotic. It is shown that these energy-transfer processes can lead to low-frequency peaks in the inelastic neutron-scattering spectra of La.