Gate-dependent pseudospin mixing in graphene/boron nitride moire superlattices

Electrons in graphene are described by relativistic Dirac-Weyl spinors with a two-component pseudospin(1-12). The unique pseudospin structure of Dirac electrons leads to emerging phenomena such as the massless Dirac cone(2), anomalous quantum Hall effect(2,3), and Klein tunnelling(4,5) in graphene. The capability to manipulate electron pseudospin is highly desirable for novel graphene electronics, and it requires precise control to differentiate the two graphene sublattices at the atomic level. Graphene/boron nitride moire superlattices, where a fast sublattice oscillation due to boron and nitrogen atoms is superimposed on the slow moire period, provides an attractive approach to engineer the electron pseudospin in graphene(13-18). This unusual moire superlattice leads to a spinor potential with unusual hybridization of electron pseudospins, which can be probed directly through infrared spectroscopy because optical transitions are very sensitive to excited state wavefunctions. Here, we perform micro-infrared spectroscopy on a graphene/boron nitride heterostructure and demonstrate that the moire superlattice potential is dominated by a pseudospin-mixing component analogous to a spatially varying pseudomagnetic field. In addition, we show that the spinor potential depends sensitively on the gate-induced carrier concentration in graphene, indicating a strong renormalization of the spinor potential from electron-electron interactions.