It has been proposed [1,2] and numerically demonstrated [3] that a large amplitude vibration in a perfect 1-D lattice can localize because of anharmonicity. More detailed analytical and numerical investigations of classical 1-D anharmonic chains made possible by simple eigenvalue generating recursion relations have revealed a variety of stable intrinsic localized modes with frequencies outside of the plane wave bands [3-8]. Recently quantum mechanical aspects of ILMs (Intrinsic Localized Modes) have been considered [9,10]. Some progress also has been reported for higher dimensional classical crystal lattices with simple nearest-neighbor model interactions [11,12]. Particularly important has been the recognition that diatomic crystal potentials like the Born-Mayer-Coulomb in 1-D produces an intrinsic gap mode (IGM) between the optic and acoustic branches instead of an ILM above the plane wave spectrum [13]. The possibility of IGMs in 3-D anharmonic lattices with realistic potentials has remained elusive. One approach has been to focus on crystal surfaces and edges where harmonic localization already plays an important role [14-16]. In this paper we demonstrate with molecular dynamics simulations that, for sufficiently large vibrational amplitude, anharmonicity can stabilize an IGM in a 3-D uniform diatomic crystal with rigid ion NaI potential arranged in either the fcc or zinc blende structure. By developing a self-consistent numerical technique for finding an intrinsic localized mode eigenvector, we have been able to show that for a given gap mode amplitude with the same potential that the localization is much stronger for a Td symmetry site when compared to an Oh one.
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Last modified: April 3, 1997