Light induced transitions of valley Chern numbers and flat bands in a non-twisted moire graphene-hexagonal boron nitride superlattice

TL;DR

This work investigates light-macrodynamics in a non-twisted graphene–hBN moiré bilayer, demonstrating that high-frequency circularly polarized light can drive Floquet topological phases in an experimentally accessible platform. The authors construct a Floquet Hamiltonian from the equilibrium model, using $H_{ ext{G–hBN}} \approx H_{ ext{G}} + V_{ ext{hBN}}$ and a vector potential $\mathbf{A}(t)=A(\sin(\omega_0 t)\hat{x}+\cos(\omega_0 t)\hat{y})$, and analyze the system with Floquet copies $m,n\in\{-1,0,1\}$. They compute valley Chern numbers via Fukui discretization, finding that driving frequency $\omega_0$ and amplitude $A$ induce multiple topological transitions, including bands attaining $|\mathcal{C}_n|=2$, and that increasing $A$ can flatten bands, suggesting emergent correlations. The results indicate that untwisted moiré materials can emulate twisted moiré light phenomena and offer a tunable route to Floquet-engineered topology with potential experimental realization.

Abstract

Motivated by the rich topology and interesting quasi-band structure of twisted moire materials subjected to light, we study a non-twisted moire material under the influence of light. Our work is in part motivated by a desire to find an easier-to-synthesize platform that can help experimentally elucidate the interesting physics of moiré materials coupled to light. Similar to twisted moire materials, we uncover rich topology and interesting band flattening effects, which we summarize in relevant plots such as a topological phase diagram. Our work demonstrates that much of the interesting phenomenology of twisted moire materials under the influence of electromagnetic waves seems to be generically present even in more experimentally accessible untwisted moire platforms, which remain highly tunable by light.

Figures (8)

• Figure 1: A cartoon that demonstrates the moire pattern of Graphene (gray)/hBN (yellow and blue) and a moire superlattice vector
• Figure 2: An illustration of the moiré Brillouin zone (BZ) of both $K$ and $K'$ valley with the path used for the band structure calculation. Note that the ratio between graphene and hBN Brillouin zones has been greatly altered for illustrative purpose.
• Figure 3: The band structure for hBN/G Moire system for $K,K'$ approximations in black and dashed red respectively.
• Figure 4: A cartoon demonstration of irradiating a circularly polarized light on the Graphene/hBN system
• Figure 5: The $K$-valley band structure of G-hBN under the influence of circularly polarized light for (left) $\omega_0=4.5$ and (right) 8.5 eV at fixed $Aa_0 = 0.2$. The Chern numbers for the low-energy hole (electron) bands are also shown. We stress that, although due to resolution, it may appear that bands 3 and 4 overlap, they do not, which makes the computed Chern numbers well-defined.