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                SEMINAR OF THE DYNAMICAL SYSTEMS GROUP
        Institute for Computational Sciences and Informatics
		CSI 929  (http://science.gmu.edu/physics/)
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    Properties of Intrinsic Localized Modes in Anharmonic Lattices

	  		Scott Bickham
		Complex Systems Theory Branch
		  Naval Research Laboratory

The recent discovery of intrinsic localized modes in one-dimensional 
perfect lattices { A. J. Sievers and S. Takeno, Phys. Rev. Lett., 
vol.61, 970 (1988)} has inspired a reexamination of lattice dynamics 
in systems with large amplitude excitations.  These intrinsic localized 
modes have widths on the order of a few lattice constants, hence the 
continuum approximation used in conventional soliton theory is not 
applicable and lattice discreteness effects are important.  In monatomic 
systems with positive quartic anharmonicity in the nearest-neighbor 
potential, both stationary and moving localized excitations are possible 
with vibrational frequencies lying above the plane wave spectrum.  The 
addition of cubic anharmonicity to this potential produces the additional 
features of a localized dc-expansion around the mode center and a 
decrease of the vibrational frequency with increasing local mode 
amplitude.  When more realistic nearest neighbor potentials such as 
Lennard-Jones and Born-Mayer are considered, the softening of the 
vibrational frequency due to the odd-order anharmonicities is strong 
enough to preclude the existence of intrinsic localized modes with 
frequencies above the plane wave spectrum; however, in a diatomic 
lattice, the intrinsic local mode frequency drops into the gap between 
the acoustic and optic branches { S. A. Kiselev, S. R. Bickham 
and A. J. Sievers, Phys. Rev. B, vol. 50, 9135 (1994)}.  Results of 
molecular dynamics simulations will be presented for monatomic and 
diatomic systems with several types of nearest-neighbor interactions in 
one, two and three dimensions.

                          Monday , October 30 1995
                                  5:30 pm
                          Room 206, Science & Tech. I
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