Semiclassical reduction for magnetic Schroedinger operator with periodic   zero-range potentials and applications
  The two-dimensional Schroedinger operator with a uniform magnetic field and a periodic zero-range potential is considered. For weak magnetic fields we reduce the spectral problem to the semiclassical analysis of one-dimensional Harper-like operators. This shows the existence of parts of Cantor structure in the spectrum for special values of the magnetic flux. 
Bernard Helffer, Konstantin Pankrashkin