Semiclassical Theory for Parametric Correlation of Energy Levels
  Parametric energy-level correlation describes the response of the energy-level statistics to an external parameter such as the magnetic field. Using semiclassical periodic-orbit theory for a chaotic system, we evaluate the parametric energy-level correlation depending on the magnetic field difference. The small-time expansion of the spectral form factor $K(\tau)$ is shown to be in agreement with the prediction of parameter dependent random-matrix theory to all orders in $\tau$. 
Taro Nagao, Petr Braun, Sebastian M\"uller, Keiji Saito, Stefan   Heusler and Fritz Haake