Combined Minivalley and Layer Control in Twisted Double Bilayer Graphene

Deducing the internal interfaces of twisted multilayer graphene via moiré-regulated surface conductivity

The stacking state of atomic layers critically determines the physical properties of twisted van der Waals materials. Unfortunately, precise characterization of the stacked interfaces remains a great challenge as they are buried internally. With conductive atomic force microscopy, we show that the moiré superlattice structure formed at the embedded interfaces of small-angle twisted multilayer graphene (tMLG) can noticeably regulate surface conductivity even when the twisted interfaces are 10 atomic layers beneath the surface. Assisted by molecular dynamics (MD) simulations, a theoretical model is proposed to correlate surface conductivity with the sequential stacking state of the graphene layers of tMLG. The theoretical model is then employed to extract the complex structure of a tMLG sample with crystalline defects. Probing and visualizing the internal stacking structures of twisted layered materials is essential for understanding their unique physical properties, and our work offers a powerful tool for this via simple surface conductivity mapping.

Interlayer Electron-Hole Friction in Tunable Twisted Bilayer Graphene Semimetal

Charge-neutral conducting systems represent a class of materials with unusual properties governed by electron-hole (e-h) interactions. Depending on the quasiparticle statistics, band structure, and device geometry these semimetallic phases of matter can feature unconventional responses to external fields that often defy simple interpretations in terms of single-particle physics. Here we show that small-angle twisted bilayer graphene (SA TBG) offers a highly tunable system in which to explore interactions-limited electron conduction. By employing a dual-gated device architecture we tune our devices from a nondegenerate charge-neutral Dirac fluid to a compensated two-component e-h Fermi liquid where spatially separated electrons and holes experience strong mutual friction. This friction is revealed through the T^{2} resistivity that accurately follows the e-h drag theory we develop. Our results provide a textbook illustration of a smooth transition between different interaction-limited transport regimes and clarify the conduction mechanisms in charge-neutral SA TBG.

Scattering between Minivalleys in Twisted Double Bilayer Graphene

A unique feature of the complex band structures of moiré materials is the presence of minivalleys, their hybridization, and scattering between them. Here, we investigate magnetotransport oscillations caused by scattering between minivalleys-a phenomenon analogous to magnetointersubband oscillations-in a twisted double bilayer graphene sample with a twist angle of 1.94°. We study and discuss the potential scattering mechanisms and find an electron-phonon mechanism and valley conserving scattering to be likely. Finally, we discuss the relevance of our findings for different materials and twist angles.

Correlated electron-hole state in twisted double-bilayer graphene

[Figure: see text].

Strong Interminivalley Scattering in Twisted Bilayer Graphene Revealed by High-Temperature Magneto-Oscillations

Twisted bilayer graphene (TBG) provides an example of a system in which the interplay of interlayer interactions and superlattice structure impacts electron transport in a variety of nontrivial ways and gives rise to a plethora of interesting effects. Understanding the mechanisms of electron scattering in TBG has, however, proven challenging, raising many questions about the origins of resistivity in this system. Here we show that TBG exhibits high-temperature magneto-oscillations originating from the scattering of charge carriers between TBG minivalleys. The amplitude of these oscillations reveals that interminivalley scattering is strong, and its characteristic timescale is comparable to that of its intraminivalley counterpart. Furthermore, by exploring the temperature dependence of these oscillations, we estimate the electron-electron collision rate in TBG and find that it exceeds that of monolayer graphene. Our study demonstrates the consequences of the relatively small size of the superlattice Brillouin zone and Fermi velocity reduction on lateral transport in TBG.

Evidence of Orbital Ferromagnetism in Twisted Bilayer Graphene Aligned to Hexagonal Boron Nitride

We have previously reported ferromagnetism evinced by a large hysteretic anomalous Hall effect in twisted bilayer graphene (tBLG). Subsequent measurements of a quantized Hall resistance and small longitudinal resistance confirmed that this magnetic state is a Chern insulator. Here, we report that when tilting the sample in an external magnetic field, the ferromagnetism is highly anisotropic. Because spin-orbit coupling is weak in graphene, such anisotropy is unlikely to come from spin but rather favors theories in which the ferromagnetism is orbital. We know of no other case in which ferromagnetism has a purely orbital origin. For an applied in-plane field larger than 5 T, the out-of-plane magnetization is destroyed, suggesting a transition to a new phase.