Valley polarization in biased bilayer graphene using circularly-polarized light

Abstract

There exist two inequivalent local minima in graphene's electronic band structure known as valleys or Dirac points which are labelled K and K'. The valley index is binary and the concept of using this two-state system to perform logical operations is known as valleytronics. Achieving a population imbalance between the K and K' valleys is a critical first step for any valleytronic device. A valley polarization can be induced in biased bilayer graphene using circularly-polarized light. Right-hand circularly-polarized light couples strongly to the K valley, while light of the opposite helicity couples strongly to K'. In this thesis, we present a detailed theoretical study of valley polarization in biased bilayer graphene. We show that a nearly perfect valley polarization can be achieved with the proper choices of external bias and centre frequency of the exciting pulse. We find that the optimal operating frequency \omega is given by \hbar\omega=2a, where 2a is the potential energy difference between the graphene layers (the external bias). We also find that the valley polarization originates not from the Dirac points themselves, but rather from a ring of states surrounding each. Our calculations indicate that intervalley scattering and thermal effects complicate the simple picture that the valley polarization is maximized for \hbar\omega=2a. Intervalley scattering via optical phonons greatly reduces the valley polarization for high-frequency pulses, while thermal populations significantly reduce the valley polarization for small external biases. A valley-polarized system exhibits an anomalous Hall effect whose sign depends on the valley index. When operating under optimal conditions, we find that the induced Hall conductivity can be comparable in magnitude to the longitudinal conductivity. In addition, we find that the Hall conductivity is largely insensitive to thermal effects, suggesting that experiments could be performed at room temperature without a significant reduction in signal. This work provides insight into the origin of valley polarization in bilayer graphene and will aid experimentalists seeking to study valley polarization in the lab.