**
Power spectrum
**

Power spectrum of odd-parity anharmonic gap modes in a diatomic
lattice. The numerical simulations are for 601 particles. Panels (a)
and (b) show the spectra of the gap modes with the amplitudes
**A/d**=0.1 and **A/d**=0.2,
respectively. They are obtained over the time intervals
**T**=256**T**_{m}, where
**T**_{m} is the vibrational period of
highest frequency plane wave mode.
Because the symmetry of the odd-parity mode should eliminate even
harmonics in the power spectrum of the gap mode vibration, the results
in panel (a) indicate that the localized mode is translating between one site
and its neighbor in the next unit cell. In moving, the mode takes on
an even-parity character for part of the time and even harmonics in
the power spectrum are permitted in this case. This slow
transformation from one site to the next also produces the quasi-dc
component in panel (a). The extra noise observed for panel (b) is
attributed to the emission of plane waves by the gap mode.

Last modified: August 12, 1997