High-Order Moiré Bands and Correlated States in Twisted Double-Bilayer Graphene at Small Twist Angles

Engineering moiré bands has established graphene-based superlattices as a highly tunable platform for investigating exotic quantum phases. Most of the focus has been concentrated on the lowest moiré band, and higher-order moiré bands remain largely unexplored because they are typically entangled with other bands and difficult to isolate. A promising route to access this regime is to design systems where high-order moiré bands become energetically isolated and experimentally accessible. Here, we observe multiple well-separated moiré minibands in twisted double-bilayer graphene (tDBG) devices at twist angles ranging from 0.45° to 0.82°. These bands show temperature stability, and multiband features are well captured in theoretically calculated band structures. We further identify correlated insulating states at half-integer fillings (7/2 and 5/2) that persist down to zero magnetic field and display valley-polarized characteristics. These findings suggest that strong correlation in small-angle tDBG extends beyond the lowest moiré band and can persist into higher-order minibands.