LOCALIZATION AND EQUIPARTITION OF ENERGY IN THE BETA-FPU CHAIN - CHAOTIC BREATHERS

The evolution towards equipartition in the beta-FPU chain is studied c onsidering as initial condition the highest frequency mode. Above an a nalytically derived energy threshold, this zone-boundary mode is shown to be modulationally unstable and to give rise to a striking localiza tion process. The spontaneously created excitations have strong simila rity with moving exact breathers solutions. But they have a finite lif etime and their dynamics is chaotic. These chaotic breathers are able to collect very efficiently the energy in the chain. Therefore their s ize grows in time and they can transport a very large quantity of ener gy, These features can be explained analyzing the dynamics of perturbe d exact breathers of the FPU chain. In particular, a close connection between the Lyapunov spectrum of the chaotic breathers and the Floquet spectrum of the exact ones has been found. The emergence of chaotic b reathers is convincingly explained by the absorption of high frequency phonons whereas a breather's metastabiljty is for the first time iden tified. The lifetime of the chaotic breather is related to the time ne cessary for the system to reach equipartition. The equipartition time turns out to be dependent on the system energy density epsilon only. M oreover, such time diverges as epsilon(-2) in the limit epsilon --> 0 and vanishes as epsilon(-1/4) for epsilon --> infinity. (C) 1998 Elsev ier Science B.V.