Transport experiments in twisted bilayer graphene have revealed multiple superconducting domes separated by correlated insulating states1–5. These properties are generally associated with strongly ...correlated states in a flat mini-band of the hexagonal moiré superlattice as was predicted by band structure calculations6–8. Evidence for the existence of a flat band comes from local tunnelling spectroscopy9–13 and electronic compressibility measurements14, which report two or more sharp peaks in the density of states that may be associated with closely spaced Van Hove singularities. However, direct momentum-resolved measurements have proved to be challenging15. Here, we combine different imaging techniques and angle-resolved photoemission with simultaneous real- and momentum-space resolution (nano-ARPES) to directly map the band dispersion in twisted bilayer graphene devices near charge neutrality. Our experiments reveal large areas with a homogeneous twist angle that support a flat band with a spectral weight that is highly localized in momentum space. The flat band is separated from the dispersive Dirac bands, which show multiple moiré hybridization gaps. These data establish the salient features of the twisted bilayer graphene band structure.Spectroscopic measurements using nano-ARPES on twisted bilayer graphene directly highlight the presence of the flat bands.
Superconductivity can occur under conditions approaching broken-symmetry parent states
. In bilayer graphene, the twisting of one layer with respect to the other at 'magic' twist angles of around 1 ...degree leads to the emergence of ultra-flat moiré superlattice minibands. Such bands are a rich and highly tunable source of strong-correlation physics
, notably superconductivity, which emerges close to interaction-induced insulating states
. Here we report the fabrication of magic-angle twisted bilayer graphene devices with highly uniform twist angles. The reduction in twist-angle disorder reveals the presence of insulating states at all integer occupancies of the fourfold spin-valley degenerate flat conduction and valence bands-that is, at moiré band filling factors ν = 0, ±1, ±2, ±3. At ν ≈ -2, superconductivity is observed below critical temperatures of up to 3 kelvin. We also observe three new superconducting domes at much lower temperatures, close to the ν = 0 and ν = ±1 insulating states. Notably, at ν = ± 1 we find states with non-zero Chern numbers. For ν = -1 the insulating state exhibits a sharp hysteretic resistance enhancement when a perpendicular magnetic field greater than 3.6 tesla is applied, which is consistent with a field-driven phase transition. Our study shows that broken-symmetry states, interaction-driven insulators, orbital magnets, states with non-zero Chern numbers and superconducting domes occur frequently across a wide range of moiré flat band fillings, including close to charge neutrality. This study provides a more detailed view of the phenomenology of magic-angle twisted bilayer graphene, adding to our evolving understanding of its emergent properties.
The coexistence of superconducting and correlated insulating states in magic-angle twisted bilayer graphene
prompts fascinating questions about their relationship. Independent control of the ...microscopic mechanisms that govern these phases could help uncover their individual roles and shed light on their intricate interplay. Here we report on direct tuning of electronic interactions in this system by changing the separation distance between the graphene and a metallic screening layer
. We observe quenching of correlated insulators in devices with screening layer separations that are smaller than the typical Wannier orbital size of 15 nanometres and with twist angles that deviate slightly from the magic angle of 1.10 ± 0.05 degrees. Upon extinction of the insulating orders, the vacated phase space is taken over by superconducting domes that feature critical temperatures comparable to those in devices with strong insulators. In addition, we find that insulators at half-filling can reappear in small out-of-plane magnetic fields of 0.4 tesla, giving rise to quantized Hall states with a Chern number of 2. Our study suggests re-examination of the often-assumed 'parent-and-child' relation between the insulating and superconducting phases in moiré graphene, and suggests a way of directly probing the microscopic mechanisms of superconductivity in strongly correlated systems.
The discovery of magic angle twisted bilayer graphene has unveiled a rich variety of superconducting, magnetic, and topologically nontrivial phases. Here, we show that the zero-field states at odd ...integer filling factors in h -BN nonaligned devices are consistent with symmetry broken Chern insulators, as is evidenced by the observation of the anomalous Hall effect near moiré cell filling factor ν = + 1 . The corresponding Chern insulator has a Chern number C = ± 1 and a relatively high Curie temperature of Tc ≈ 4.5 K . In a perpendicular magnetic field above B > 0.5 T we observe a transition of the ν = + 1 Chern insulator from Chern number C = ± 1 to C = 3 , characterized by a quantized Hall plateau with Ryx = h/3e2. These observations demonstrate that interaction-induced symmetry breaking leads to zero-field ground states that include almost degenerate and closely competing Chern insulators, and that states with larger Chern numbers couple most strongly to the B field. In addition, the device reveals strong superconducting phases with critical temperatures of up to Tc ≈ 3.5 K . By providing the first demonstration of a system that allows gate-induced transitions between magnetic and superconducting phases, our observations mark a major milestone in the creation of a new generation of quantum electronics.
The single-particle and many-body properties of twisted bilayer graphene (TBG) can be dramatically different from those of a single graphene layer, particularly when the two layers are rotated ...relative to each other by a small angle (θ ≈ 1°), owing to the moiré potential induced by the twist. Here we probe the collective excitations of TBG with a spatial resolution of 20 nm, by applying mid-infrared near-field optical microscopy. We find a propagating plasmon mode in charge-neutral TBG for θ = 1.1−1.7°, which is different from the intraband plasmon in single-layer graphene. We interpret it as an interband plasmon associated with the optical transitions between minibands originating from the moiré superlattice. The details of the plasmon dispersion are directly related to the motion of electrons in the moiré superlattice and offer an insight into the physical properties of TBG, such as band nesting between the flat band and remote band, local interlayer coupling, and losses. We find a strongly reduced interlayer coupling in the regions with AA stacking, pointing at screening due to electron–electron interactions. Optical nano-imaging of TBG allows the spatial probing of interaction effects at the nanoscale and potentially elucidates the contribution of collective excitations to many-body ground states.Moiré potentials substantially alter the electronic properties of twisted bilayer graphene at a magic twist angle. A propagating plasmon mode, which can be observed with optical nano-imaging, is associated with transitions between the moiré minibands.
Spin-orbit coupling is a fundamental mechanism that connects the spin of a charge carrier with its momentum. In the optical domain, an analogous synthetic spin-orbit coupling is accessible by ...engineering optical anisotropies in photonic materials. Both yield the possibility of creating devices that directly harness spin and polarization as information carriers. Atomically thin transition metal dichalcogenides promise intrinsic spin-valley Hall features for free carriers, excitons and photons. Here we demonstrate spin- and valley-selective propagation of exciton-polaritons in a monolayer of MoSe
that is strongly coupled to a microcavity photon mode. In a wire-like device we trace the flow and helicity of exciton-polaritons expanding along its channel. By exciting a coherent superposition of K and K' tagged polaritons, we observe valley-selective expansion of the polariton cloud without either an external magnetic field or coherent Rayleigh scattering. The observed optical valley Hall effect occurs on a macroscopic scale, offering the potential for applications in spin-valley-locked photonic devices.
Because of their ultrafast intrinsic dynamics and robustness against stray fields, antiferromagnetic insulators1–3 are promising candidates for spintronic components. Therefore, long-distance, ...low-dissipation spin transport and electrical manipulation of antiferromagnetic order are key research goals in antiferromagnetic spintronics. Here, we report experimental evidence of robust spin transport through an antiferromagnetic insulator, in our case the gate-controlled state that appears in charge-neutral graphene in a magnetic field4–6. Utilizing quantum Hall edge states as spin-dependent injectors and detectors, we observe large, non-local electrical signals across charge-neutral channels that are up to 5 μm long. The dependence of the signal on magnetic field, temperature and filling factor is consistent with spin superfluidity1,2,4,7–10 as the spin-transport mechanism. This work demonstrates the utility of graphene in the quantum Hall regime as a powerful model system for fundamental studies in antiferromagnetic spintronics.
Exciton-polaritons in semiconductor microcavities constitute the archetypal realization of a quantum fluid of light. Under coherent optical drive, remarkable effects such as superfluidity, dark ...solitons or the nucleation of vortices have been observed, and can be all understood as specific manifestations of the condensate collective excitations. In this work, we perform a Brillouin scattering experiment to measure their dispersion relation Formula: see text directly. The results, such as a speed of sound which is apparently twice too low, cannot be explained upon considering the polariton condensate alone. In a combined theoretical and experimental analysis, we demonstrate that the presence of an excitonic reservoir alongside the polariton condensate has a dramatic influence on the characteristics of the quantum fluid, and explains our measurement quantitatively. This work clarifies the role of such a reservoir in polariton quantum hydrodynamics. It also provides an unambiguous tool to determine the condensate-to-reservoir fraction in the quantum fluid, and sets an accurate framework to approach ideas for polariton-based quantum-optical applications.
We demonstrate ionic liquid (IL) gating of suspended few-layer MoS2 transistors, where ions can accumulate on both exposed surfaces. Upon application of IL, all free-standing samples consistently ...display more significant improvement in conductance than substrate-supported devices. The measured IL gate coupling efficiency is up to 4.6 × 1013 cm–2 V–1. Electrical transport data reveal contact-dominated electrical transport properties and the Schottky emission as the underlying mechanism. By modulating IL gate voltage, the suspended MoS2 devices display metal–insulator transition. Our results demonstrate that more efficient charge induction can be achieved in suspended two-dimensional (2D) materials, which with further optimization, may enable extremely high charge density and novel phase transition.
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