CROSSOVER PHENOMENA IN THE EXCITATION STATISTICS OF DISORDERED MANY-PARTICLE SYSTEMS

The energy level separation distribution of a fermionic disordered man y-particle system is studied. For a non-interacting system, a crossove r is observed from a Wigner (GOE) distribution for the first and secon d excitations to a different form which depends on the number of parti cles for low-lying excitations and then to a Poisson distribution for high excitations. This behaviour also holds for weakly interacting sys tems, while for stronger interactions a transition of the high excitat ions to a Wigner distribution is observed.