From chaotic to disordered systems - a periodic orbit approach
  We apply periodic orbit theory to a quantum billiard on a torus with a variable number N of small circular scatterers distributed randomly. Provided these scatterers are much smaller than the wave length they may be regarded as sources of diffraction. The relevant part of the spectral determinant is due to diffractive periodic orbits only. We formulate this diffractive zeta function in terms of a N*N transfer matrix, which is transformed to real form. The zeros of this determinant can readily be computed. The determinant is shown to reproduce the full density of states for generic configurations if N>3. We study the statistics exhibited by these spectra. The numerical results suggest that the spectra tend to GOE statistics as the number of scatterers increases for typical members of the ensemble. A peculiar situation arises for configurations with four scatterers and kR tuned to kR=y_{0,1}\approx 0.899, where the statistics appears to be perfectly Poissonian. 
Per Dahlqvist (Stockholm)