We study the level spacing statistics $P\left(s\right)$ in many-body Fermi systems and determine a critical two-body interaction strength $U_c$ at which a crossover from Poisson to Wigner-Dyson statistics takes place. Near the Fermi level, the results allow one to find a critical temperature $T_{\mathrm{ch}}$ above which quantum chaos and thermalization set in.

• Received 5 June 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.1837