INTRINSIC LOCALIZED MODES IN THE BULK AND AT THE SURFACE OF ANHARMONIC DIATOMIC CHAINS

Intrinsic localized vibrational modes in a diatomic anharmonic chain a re discussed in detail. We consider a finite (even or odd) number of p articles interacting with nearest-neighbor harmonic and cubic and quar tic anharmonic potentials. For suitable values of the potential parame ters we find two odd parity modes localized about light and heavy atom s, respectively, with frequencies above the top of the optical branch. In the gap there are localized modes arising from the top of the acou stic branch with odd parity if they are centered on a heavy atom and e ven parity if centered on a light atom. There are also two localized m odes originating from the bottom of the optical branch. The odd parity mode is centered on a light atom, while the even parity mode is cente red on a heavy atom. In addition, we have found two types of surface m odes. One is the anharmonic version of the surface mode occurring in a finite diatomic harmonic chain with frequency in the gap, and the oth er is entirely due to the anharmonicity. The latter mode has its frequ ency above the top of the optical branch. A comparison with other work is given.