Topological flat bands in a family of multilayer graphene moiré lattices.

Moiré materials host a wealth of intertwined correlated and topological states of matter, all arising from flat electronic bands with nontrivial quantum geometry. A prominent example is the family of alternating-twist magic-angle graphene stacks, which exhibit symmetry-broken states at rational fillings of the moiré band and superconductivity close to half filling. Here, we introduce a second family of twisted graphene multilayers made up of twisted sheets of M- and N-layer Bernal-stacked graphene flakes.

Electronic interactions in Dirac fluids visualized by nano-terahertz spacetime interference of electron-photon quasiparticles.

Ultraclean graphene at charge neutrality hosts a quantum critical Dirac fluid of interacting electrons and holes. Interactions profoundly affect the charge dynamics of graphene, which is encoded in the properties of its electron-photon collective modes: surface plasmon polaritons (SPPs). Here, we show that polaritonic interference patterns are particularly well suited to unveil the interactions in Dirac fluids by tracking polaritonic interference in time at temporal scales commensurate with the electronic scattering.

Dynamically tunable moiré exciton Rydberg states in a monolayer semiconductor on twisted bilayer graphene.

Moiré excitons are emergent optical excitations in two-dimensional semiconductors with moiré superlattice potentials. Although these excitations have been observed on several platforms, a system with dynamically tunable moiré potential to tailor their properties is yet to be realized. Here we present a continuously tunable moiré potential in monolayer WSe, enabled by its proximity to twisted bilayer graphene (TBG) near the magic angle.

Symmetry-Broken Chern Insulators in Twisted Double Bilayer Graphene.

Twisted double bilayer graphene (tDBG) has emerged as a rich platform for studying strongly correlated and topological states, as its flat bands can be continuously tuned by both a perpendicular displacement field and a twist angle. Here, we construct a phase diagram representing the correlated and topological states as a function of these parameters, based on measurements of over a dozen tDBG devices encompassing two distinct stacking configurations. We find a hierarchy of symmetry-broken states that emerge sequentially as the twist angle approaches an apparent optimal value of θ ≈ 1.

Graphene-Based Analog of Single-Slit Electron Diffraction.

This work reports the experimental demonstration of single-slit diffraction exhibited by electrons propagating in encapsulated graphene with an effective de Broglie wavelength corresponding to their attributes as massless Dirac fermions. Nanometer-scale device designs were implemented to fabricate a single-slit followed by five detector paths. Predictive calculations were also utilized to readily understand the observations reported.

Mixed-dimensional moiré systems of twisted graphitic thin films.

Moiré patterns formed by stacking atomically thin van der Waals crystals with a relative twist angle can give rise to notable new physical properties. The study of moiré materials has so far been limited to structures comprising no more than a few van der Waals sheets, because a moiré pattern localized to a single two-dimensional interface is generally assumed to be incapable of appreciably modifying the properties of a bulk three-dimensional crystal. Here, we perform transport measurements of dual-gated devices constructed by slightly rotating a monolayer graphene sheet atop a thin bulk graphite crystal.

Gate-Tunable Proximity Effects in Graphene on Layered Magnetic Insulators.

The extreme versatility of van der Waals materials originates from their ability to exhibit new electronic properties when assembled in close proximity to dissimilar crystals. For example, although graphene is inherently nonmagnetic, recent work has reported a magnetic proximity effect in graphene interfaced with magnetic substrates, potentially enabling a pathway toward achieving a high-temperature quantum anomalous Hall effect. Here, we investigate heterostructures of graphene and chromium trihalide magnetic insulators (CrI, CrBr, and CrCl).

Unusual magnetotransport in twisted bilayer graphene.

SignificanceWhen two sheets of graphene are twisted to the magic angle of 1.1, the resulting flat moiré bands can host exotic correlated electronic states such as superconductivity and ferromagnetism. Here, we show transport properties of a twisted bilayer graphene device at 1.

Unraveling Strain Gradient Induced Electromechanical Coupling in Twisted Double Bilayer Graphene Moiré Superlattices.

Moiré superlattices of 2D materials with a small twist angle are thought to exhibit appreciable flexoelectric effect, though unambiguous confirmation of their flexoelectricity is challenging due to artifacts associated with commonly used piezoresponse force microscopy (PFM). For example, unexpectedly small phase contrast (≈8°) between opposite flexoelectric polarizations is reported in twisted bilayer graphene (tBG), though theoretically predicted value is 180°. Here a methodology is developed to extract intrinsic moiré flexoelectricity using twisted double bilayer graphene (tDBG) as a model system, probed by lateral PFM.

Competing correlated states and abundant orbital magnetism in twisted monolayer-bilayer graphene.

Flat band moiré superlattices have recently emerged as unique platforms for investigating the interplay between strong electronic correlations, nontrivial band topology, and multiple isospin 'flavor' symmetries. Twisted monolayer-bilayer graphene (tMBG) is an especially rich system owing to its low crystal symmetry and the tunability of its bandwidth and topology with an external electric field. Here, we find that orbital magnetism is abundant within the correlated phase diagram of tMBG, giving rise to the anomalous Hall effect in correlated metallic states nearby most odd integer fillings of the flat conduction band, as well as correlated Chern insulator states stabilized in an external magnetic field.

Hierarchical nanosheets built from superatomic clusters: properties, exfoliation and single-crystal-to-single-crystal intercalation.

Tuning the properties of atomic crystals in the two-dimensional (2D) limit is synthetically challenging, but critical to unlock their potential in fundamental research and nanotechnology alike. 2D crystals assembled using superatomic blocks could provide a route to encrypt desirable functionality, yet strategies to link the inorganic blocks together in predetermined dimensionality or symmetry are scarce. Here, we describe the synthesis of anisotropic van der Waals crystalline frameworks using the designer superatomic nanocluster Co(py)CoSeL (py = pyridine, L = PhPN(Tol)), and ditopic linkers.

Switching 2D magnetic states via pressure tuning of layer stacking.

The physical properties of two-dimensional van der Waals crystals can be sensitive to interlayer coupling. For two-dimensional magnets, theory suggests that interlayer exchange coupling is strongly dependent on layer separation while the stacking arrangement can even change the sign of the interlayer magnetic exchange, thus drastically modifying the ground state. Here, we demonstrate pressure tuning of magnetic order in the two-dimensional magnet CrI.

Tunable crystal symmetry in graphene-boron nitride heterostructures with coexisting moiré superlattices.

In van der Waals (vdW) heterostructures consisting of atomically thin crystals layered on top of one another, lattice mismatch and rotation between the layers can result in long-wavelength moiré superlattices. These moiré patterns can drive notable band structure reconstruction of the composite material, leading to a wide range of emergent phenomena including superconductivity, magnetism, fractional Chern insulating states and moiré excitons. Here, we investigate devices consisting of monolayer graphene encapsulated between two crystals of boron nitride (BN), in which the rotational alignment of all three components is controlled.

Maximized electron interactions at the magic angle in twisted bilayer graphene.

The electronic properties of heterostructures of atomically thin van der Waals crystals can be modified substantially by moiré superlattice potentials from an interlayer twist between crystals. Moiré tuning of the band structure has led to the recent discovery of superconductivity and correlated insulating phases in twisted bilayer graphene (TBG) near the 'magic angle' of twist of about 1.1 degrees, with a phase diagram reminiscent of high-transition-temperature superconductors.

Tuning superconductivity in twisted bilayer graphene.

Materials with flat electronic bands often exhibit exotic quantum phenomena owing to strong correlations. An isolated low-energy flat band can be induced in bilayer graphene by simply rotating the layers by 1.1°, resulting in the appearance of gate-tunable superconducting and correlated insulating phases.

Dynamic band-structure tuning of graphene moiré superlattices with pressure.

Heterostructures can be assembled from atomically thin materials by combining a wide range of available van der Waals crystals, providing exciting possibilities for designer electronics . In many cases, beyond simply realizing new material combinations, interlayer interactions lead to emergent electronic properties that are fundamentally distinct from those of the constituent layers . A critical parameter in these structures is the interlayer coupling strength, but this is often not easy to determine and is typically considered to be a fixed property of the system.

Ambipolar Landau levels and strong band-selective carrier interactions in monolayer WSe.

Monolayers (MLs) of transition-metal dichalcogenides (TMDs) exhibit unusual electrical behaviour under magnetic fields due to their intrinsic spin-orbit coupling and lack of inversion symmetry. Although recent experiments have also identified the critical role of carrier interactions within these materials, a complete mapping of the ambipolar Landau level (LL) sequence has remained elusive. Here we use single-electron transistors (SETs) to perform LL spectroscopy in ML WSe, and provide a comprehensive picture of the electronic structure of a ML TMD for both electrons and holes.

Evolution of the electronic band structure of twisted bilayer graphene upon doping.

The electronic band structure of twisted bilayer graphene develops van Hove singularities whose energy depends on the twist angle between the two layers. Using Raman spectroscopy, we monitor the evolution of the electronic band structure upon doping using the G peak area which is enhanced when the laser photon energy is resonant with the energy separation of the van Hove singularities. Upon charge doping, the Raman G peak area initially increases for twist angles larger than a critical angle and decreases for smaller angles.

Pressure-induced commensurate stacking of graphene on boron nitride.

Combining atomically-thin van der Waals materials into heterostructures provides a powerful path towards the creation of designer electronic devices. The interaction strength between neighbouring layers, most easily controlled through their interlayer separation, can have significant influence on the electronic properties of these composite materials. Here, we demonstrate unprecedented control over interlayer interactions by locally modifying the interlayer separation between graphene and boron nitride, which we achieve by applying pressure with a scanning tunnelling microscopy tip.

van der Waals Heterostructures with High Accuracy Rotational Alignment.

We describe the realization of van der Waals (vdW) heterostructures with accurate rotational alignment of individual layer crystal axes. We illustrate the approach by demonstrating a Bernal-stacked bilayer graphene formed using successive transfers of monolayer graphene flakes. The Raman spectra of this artificial bilayer graphene possess a wide 2D band, which is best fit by four Lorentzians, consistent with Bernal stacking.

Local Spectroscopic Characterization of Spin and Layer Polarization in WSe_{2}.

We report scanning tunneling microscopy and scanning tunneling spectroscopy (STS) measurements of monolayer and bilayer WSe_{2}. We measure a band gap of 2.21±0.

Intrinsic disorder in graphene on transition metal dichalcogenide heterostructures.

Semiconducting transition metal dichalchogenides (TMDs) are a family of van der Waals bonded materials that have recently received interest as alternative substrates to hexagonal boron nitride (hBN) for graphene, as well as for components in novel graphene-based device heterostructures. We elucidate the local structural and electronic properties of graphene on TMD heterostructures through scanning tunneling microscopy and spectroscopy measurements. We find that crystalline defects intrinsic to TMDs induce substantial electronic scattering and charge carrier density fluctuations in the graphene.