Two interacting particles in a disordered chain II: Critical statistics   and maximum mixing of the one body states
  For two particles in a disordered chain of length $L$ with on-site interaction $U$, a duality transformation maps the behavior at weak interaction onto the behavior at strong interaction. Around the fixed point of this transformation, the interaction yields a maximum mixing of the one body states. When $L \approx L_1$ (the one particle localization length), this mixing results in weak chaos accompanied by multifractal wave functions and critical spectral statistics, as in the one particle problem at the mobility edge or in certain pseudo-integrable billiards. In one dimension, a local interaction can only yield this weak chaos but can never drive the two particle system to full chaos with Wigner-Dyson statistics. 
Xavier Waintal (1), Dietmar Weinmann (2) and Jean-Louis Pichard (1)   ((1) CEA Saclay, France (2) Augsburg, Germany)