Critical Quantum Chaos in 2D Disordered Systems with Spin-Orbit Coupling

We examine the validity of the recently proposed semi-Poisson level spacing distribution function P(S), which characterizes `critical quantum chaos', in 2D disordered systems with spin-orbit coupling. At the Anderson transition we show that the semi-Poisson P(S) can describe closely the critical distribution obtained with averaged boundary conditions, over Dirichlet in one direction with periodic in the other and Dirichlet in both directions. We also obtain a sub-Poisson linear number variance $\Sigma_{2}(E)\approx \chi_{0}+ \chi E$, with asymptotic value $\chi\approx0.07$. The obtained critical statistics, intermediate between Wigner and Poisson, is relevant for disordered systems and chaotic models.