Quantum cascade phenomenon in natural atomic superlattices formed in   Bi2Sr2CaCu2O8 single crystals
  Natural atomic superlattices are formed in certain strongly anisotropic layered compound. Here we study interlayer transport in single crystals of Bi2Sr2CaCu2O8 cuprates, which represent stacks of atomic scale intrinsic Josephson junctions. A series of resonant dips in conductance is observed at condition when bremsstrahlung and recombination bands in non-equilibrium spectrum of Josephson junctions overlap. The phenomenon is explained in terms of self-detection of a new type of collective strongly non-equilibrium state in stacked Josephson junctions, bearing certain resemblance with operation of a Quantum Cascade Laser. Conclusions are supported by in-situ generation-detection experiments, providing evidence for cascade amplification of recombination radiation, and by numerical simulations. 
V.M. Krasnov