Interlaced dense point and absolutely continuous spectra for   Hamiltonians with concentric-shell singular interactions
  We analyze the spectrum of the generalized Schrodinger operator in $L^2(R^\nu) \nu \geq 2$, with a general local, rotationally invariant singular interaction supported by an infinite family of concentric, equidistantly spaced spheres. It is shown that the essential spectrum consists of interlaced segments of the dense point and absolutely continuous character, and that the relation of their lengths at high energies depends on the choice of the interaction parameters; generically the p.p. component is asymptotically dominant. We also show that for $\nu=2$ there is an infinite family of eigenvalues below the lowest band. 
P. Exner, M. Fraas