On the nature of the negative-conductivity resonance in a superlattice   Bloch oscillator
  Adding a high-frequency ac component to the bias field of a superlattice induces a synchronous modulation of the velocity with which the electrons traverse the Brillouin zone. In the presence of inelastic scattering, the k-space velocity modulation causes a wave-like bunching of the electrons in k-space, which in turn introduces a high-frequency component in the real-space current, synchronous with the drive field, but phase-shifted relative to the latter. For a drive frequency equal to the Bloch frequency, the phase shift is less than 90 degrees (implying a positive ac conductivity), increasing to 90 degrees in the limit of a vanishing scattering (a purely reactive current). If the drive frequency is lowered below the Bloch frequency, and the inelastic scattering rate remains sufficiently low, the phase shift can increase beyond 90 degrees, implying a negative ac conductivity, which peaks at a drive frequency not far below the Bloch frequency. 
Herbert Kroemer