Lei Ying

Multistability, chaos, and random signal generation in semiconductor superlattices

Historically, semiconductor superlattices, artificial periodic structures of different semiconductor materials, were invented with the purpose of engineering or manipulating the electronic properties of semiconductor devices. A key application lies in generating radiation sources, amplifiers, and detectors in the unusual spectral range of subterahertz and terahertz (0.1-10 THz), which cannot be readily realized using conventional radiation sources, the so-called THz gap. Efforts in the past three decades have demonstrated various nonlinear dynamical behaviors including chaos, suggesting the potential to exploit chaos in semiconductor superlattices as random signal sources (e.g., random number generators) in the THz frequency range. We consider a realistic model of hot electrons in semiconductor superlattice, taking into account the induced space charge field. Through a systematic exploration of the phase space we find that, when the system is subject to an external electrical driving of a single frequency, chaos is typically associated with the occurrence of multistability. That is, for a given parameter setting, while there are initial conditions that lead to chaotic trajectories, simultaneously there are other initial conditions that lead to regular motions. Transition to multistability, i.e., the emergence of multistability with chaos as a system parameter passes through a critical point, is found and argued to be abrupt. Multistability thus presents an obstacle to utilizing the superlattice system as a reliable and robust random signal source. However, we demonstrate that, when an additional driving field of incommensurate frequency is applied, multistability can be eliminated, with chaos representing the only possible asymptotic behavior of the system. In such a case, a random initial condition will lead to a trajectory landing in a chaotic attractor with probability 1, making quasiperiodically driven semiconductor superlattices potentially as a reliable device for random signal generation to fill the THz gap. The interplay among noise, multistability, and chaos is also investigated.

Effect of geometrical rotation on conductance fluctuations in graphene quantum dots

Conductance fluctuations are ubiquitous in quantum transport through nanoscale devices, and how to modulate or control the fluctuation patterns is of considerable interest. We use two-terminal graphene devices as a prototypical system and articulate a scheme based on geometrical rotation of the device to effectively modulate the conductance fluctuations. To facilitate a systematic calculation of the conductance as a function of the Fermi energy and the rotation angle, we use a layer-by-layer based, recursive non-equilibrium Greens function approach, which is demonstrated to be computationally extremely efficient. Our study indicates that relative rotation of the device, which is experimentally feasible, can markedly affect the degree of conductance fluctuations, and we provide physical explanations of this behavior based on the emergence of edge states.