Large-amplitude oscillation dynamics and domain suppression in a   superlattice Bloch oscillator
  We analyze the current dynamics of a superlattice Bloch oscillator under conditions where, in addition to the dc bias field causing a negative differential ac conductivity, a similarly strong ac field is present, at a frequency somewhat below the Bloch frequency associated with the dc field. The differential conductivity at the ac drive frequency is then negative, but, because of the strong non-linearities of the system, the dc differential conductivity is modified by the ac field. For a sufficiently strong ac field, there will be a dc bias range inside which the dc differential conductivity has turned positive, while the ac conductivity remains negative. The phenomenon should lend itself to the suppression of the domain-like space charge instabilities that are normally associated with a negative dc differential conductivity. 
Herbert Kroemer