Limit-cycle-induced frequency locking in self-sustained current oscillations in superlattices

The ac response of self-sustained current oscillations (SSCO's) in weakly coupled GaAs/AlAs superlattices (SL's) is derived based on the deformation of a limit cycle under an external ac driving force. Frequency locking into an integer fraction of the ac frequency is obtained in a periodic response in which a limit cycle deforms either with or without a topological change. This frequency locking is robust against the ac bias because a limit cycle can adjust itself. The results are verified both numerically and experimentally, indicating that SSCO's in SL's can be understood within the framework of the general concepts and principles of nonlinear physics.