Doublon relaxation in the Bose-Hubbard model
  Decay of a high-energy double occupancy state, doublon, in a narrow-band lattice requires creation of a coherent many-particle excitation. This leads to an exponentially long relaxation time of such a state. We show that, if the average occupation number is sufficiently small, the corresponding exponent may be evaluated exactly. To this end we develop the quasiclassical approach to calculation of the high-order tree-level decay amplitudes. 
A.L. Chudnovskiy, D.M. Gangardt, and A. Kamenev
cond-mat.quant-gas hep-th