Non-Abelian gauge potentials are quite relevant in subatomic physics, but they are relatively rare in a condensed matter context. Here we report the experimental evidence for non-Abelian gauge potentials in twisted graphene bilayers by scanning tunneling microscopy and spectroscopy. At a magic twisted angle, $\theta \approx {\left(1.11\pm 0.05\right)}^{\circ }$, a pronounced sharp peak, which arises from the nondispersive flat bands at the charge neutrality point, is observed in the tunneling density of states due to the action of the non-Abelian gauge fields. Moreover, we observe confined electronic states in the twisted bilayer, as manifested by regularly spaced tunneling peaks with energy spacing $\delta E\approx v_{\mathrm{F}}/D\approx 70\mathrm{meV}$ (here $v_{\mathrm{F}}$ is the Fermi velocity of graphene and $D$ is the period of the moiré patterns). This indicates that the non-Abelian gauge potentials in twisted graphene bilayers confine low-energy electrons into a triangular array of quantum dots following the modulation of the moiré patterns. Our results also directly demonstrate that the Fermi velocity in twisted bilayers can be tuned from about ${10}^6\mathrm{m}/\mathrm{s}$ to zero by simply reducing the twisted angle of about $2^{\circ }$.

• Received 16 April 2015
• Revised 12 June 2015

DOI:https://doi.org/10.1103/PhysRevB.92.081406

Source: http://link.aps.org/doi/10.1103/PhysRevB.92.081406