The emergence of flat bands and correlated behaviors in "magic angle" twisted bilayer graphene (tBLG) has sparked tremendous interest, though many aspects of the system are under intense debate. Here we report observation of both superconductivity and the Mott-like insulating state in a tBLG device with a twist angle of ~0.93º, which is smaller than the magic angle by 15%. At an electron concentration of ±5 electrons/moiré unit cell, we observe a narrow resistance peak with an activation energy gap ~0.1 meV, indicating the existence of an additional correlated insulating state. This is consistent with theory predicting the presence of a high-energy band with an energetically flat dispersion. At a doping of ±12 electrons/moiré unit cell we observe a resistance peak due to the presence of Dirac points in the spectrum. Our results reveal that the "magic" range of tBLG is in fact larger than what is previously expected, and provide a wealth of new information to help decipher the strongly correlated phenomena observed in tBLG.

Antiferromagnetic insulators (AFMI) are robust against stray fields, and their intrinsic dynamics could enable ultrafast magneto-optics and ultrascaled magnetic information processing. Low dissipation, long distance spin transport and electrical manipulation of antiferromagnetic order are much sought-after goals of spintronics research. Here, we report the first experimental evidence of robust long-distance spin transport through an AFMI, in our case the gate-controlled, canted antiferromagnetic (CAF) state that appears at the charge neutrality point of graphene in the presence of an external magnetic field. Utilizing gate-controlled quantum Hall (QH) edge states as spin-dependent injectors and detectors, we observe large, non-local electrical signals across a 5 µm-long, insulating channel only when it is biased into the ν=0 CAF state. Among possible transport mechanisms, spin superfluidity in an antiferromagnetic state gives the most consistent interpretation of the non-local signal's dependence on magnetic field, temperature and filling factors. This work also demonstrates that graphene in the QH regime is a powerful model system for fundamental studies of ferromagnetic and antiferromagnetic spintronics.An important goal of spintronics research is to identify mechanisms that minimize dissipation in devices that seek to exploit the action of spin currents. In magnetic insulators, spin-currents can be carried dissipatively by magnon quasiparticles 1-3 . In the case of systems with easy plane magnetic order, they can also be carried collectively in the form of dissipationless spin supercurrents 4-9 . While magnon transport is much less efficient in an ideal antiferromagnetic insulators (AFMI) than that in ferromagnetic insulators in the absence of a thermal gradient, spin superfluidity is theoretically expected to be a possibility in both cases.

As the Fermi level and band structure of two-dimensional materials are readily tunable, they constitute an ideal platform for exploring the Lifshitz transition, a change in the topology of a material's Fermi surface. Using tetralayer graphene that host two intersecting massive Dirac bands, we demonstrate multiple Lifshitz transitions and multiband transport, which manifest as a nonmonotonic dependence of conductivity on the charge density n and out-of-plane electric field D, anomalous quantum Hall sequences and Landau level crossings that evolve with n, D, and B.

As a high mobility two-dimensional semiconductor with strong structural and electronic anisotropy, atomically thin black phosphorus (BP) provides a new playground for investigating the quantum Hall (QH) effect, including outstanding questions such as the functional dependence of Landau level (LL) gaps on magnetic field B, and possible anisotropic fractional QH states. Using encapsulated few-layer BP transistors with mobility up to 55 000 cm/(V s), we extracted LL gaps over an exceptionally wide range of B for QH states at filling factors -1 to -4, which are determined to be linear in B, thus resolving a controversy raised by its anisotropy. Furthermore, a fractional QH state at ν ≈ -4/3 and an additional feature at -0.56 ± 0.1 are observed, underscoring BP as a tunable 2D platform for exploring electron interactions.

We study proximity-induced spin-orbit coupling (SOC) in bilayer graphene/few-layer WSe 2 heterostructure devices. Contact mode atomic force microscopy (AFM) cleaning yields ultra-clean interfaces and high-mobility devices. In a perpendicular magnetic field, we measure the quantum Hall effect to determine the Landau level structure in the presence of out-of-plane Ising and inplane Rashba SOC. A distinct Landau level crossing pattern emerges when tuning the charge density and displacement field independently with dual gates, originating from a layer-selective SOC proximity effect. Analyzing the Landau level crossings and measured inter-Landau level energy gaps yields the proximity induced SOC energy scale. The Ising SOC is » 2.2 meV, 100 times higher than the intrinsic SOC in graphene, while its sign is consistent with theories predicting a dependence of SOC on interlayer twist angle. The Rashba SOC is ~15 meV. Finally, we infer the magnetic field dependence of the inter-Landau level Coulomb interactions. These ultraclean bilayer graphene/WSe 2 heterostructures provide a high mobility system with the potential to realize novel topological electronic states and manipulate spins in nanostructures. AUTHOR INFORMATION Corresponding Author

In rhombohedral-stacked few-layer graphene, the very flat energy bands near the charge neutrality point are unstable to electronic interactions, giving rise to states with spontaneous broken symmetries. Using transport measurements on suspended rhombohedral-stacked tetralayer graphene, we observe an insulating ground state with a large interaction-induced gap up to 80 meV. This gapped state can be enhanced by a perpendicular magnetic field, and suppressed by an interlayer potential, carrier density, or a critical temperature of ~ 40 K.Since 2004, graphene has rapidly become an extra-ordinary 2D electron system for low dimensional physics, as it hosts massless Dirac fermions exhibiting an unconventional quantum Hall effect with a with a Berry's phase of π[1,2]. More recently, the few-layer "cousins" of monolayer graphene (MLG) have also attracted significant attention, as they constitute highly unusual, fascinating 2D platforms [3][4][5][6]. Like MLG, they are atomically thin membranes with chiral charge carriers; however, they differ from MLG in band structure, crystal symmetries, and strength of electronic interactions, all of which have profound effects on their electronic properties[7-10].Few-layer graphene (FLG) has two natural stable allotropes which can be distinguished by Raman spectroscopy [11,12]. ABA or Bernal stacking is the most stable and abundant form found in 85% of natural graphite. ABC or rhombohedral stacking, which occurs in ~14% of bulk graphite, obeys inversion symmetry, and its dispersion can be approximated as simply E~k M , where k is the wave vector and M the number of layers [13]. Thus, rhombohedral-stacked FLG (r-FLG) is highly unusual in the very flat bands near the charge neutrality, which host large and even diverging (for M>2) density of states and extremely large electronic interactions. The interaction parameter, r s , also known as the Wigner-Seitz radius, is the ratio of the average electron Coulomb interaction energy to the Fermi energy, given by r s ∝ n −(M −1)/2 , where n is charge density and M is the power of the dispersion relation. For MLG, M=1, r s = e 2 / ε r !v F~2 .2 in suspended MLG, independent of density. Here, e is the electron charge, ε r is the background dielectric constant, ~1 for suspended graphene, ħ is reduced Planck's constant and v F ~ 10 6 m/s is the Fermi velocity. For bilayer graphene (BLG) and rhombohedral-stacked trilayer (r-TLG) and tetralayer (r-4LG) graphene, r s ∝ !!/! , n -1 , and !!/! , respectively. Close to the charge neutrality point, r s increases by 1-2 orders of magnitude when an extra layer is added. Indeed, interaction-induced gaps of ~ 2 meV in suspended BLG [14][15][16][17], and ~42 meV in suspended r-TLG [18,19] have been observed (see Table 1). Among theoretical proposals [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36], the experimental observations are most consistent with a layer antiferromagnetic state (LAF) with broken time reversal symmetry. *

Owing to the spin, valley, and orbital symmetries, the lowest Landau level in bilayer graphene exhibits multicomponent quantum Hall ferromagnetism. Using transport spectroscopy, we investigate the energy gaps of integer and fractional quantum Hall (QH) states in bilayer graphene with controlled layer polarization. The state at filling factor ν=1 has two distinct phases: a layer polarized state that has a larger energy gap and is stabilized by high electric field, and a hitherto unobserved interlayer coherent state with a smaller gap that is stabilized by large magnetic field. In contrast, the ν=2/3 quantum Hall state and a feature at ν=1/2 are only resolved at finite electric field and large magnetic field. These results underscore the importance of controlling layer polarization in understanding the competing symmetries in the unusual QH system of BLG.

Relativistic fermions in topological quantum materials are characterized by linear energy–momentum dispersion near band crossing points. Under magnetic fields, relativistic fermions acquire Berry phase of π in cyclotron motion, leading to a zeroth Landau level (LL) at the crossing point, a signature unique to relativistic fermions. Here we report the unusual interlayer quantum transport behavior resulting from the zeroth LL mode observed in the time reversal symmetry breaking type II Weyl semimetal YbMnBi2. The interlayer magnetoresistivity and Hall conductivity of this material are found to exhibit surprising angular dependences under high fields, which can be well fitted by a model, which considers the interlayer quantum tunneling transport of the zeroth LL's Weyl fermions. Our results shed light on the unusual role of zeroth LLl mode in transport.