Bilayer graphene is an attractive platform for studying new two-dimensional electron physics, because its flat energy bands are sensitive to out-of-plane electric fields and these bands magnify electron-electron interaction effects. Theory predicts a variety of interesting broken symmetry states when the electron density is at the carrier neutrality point, and some of these states are characterized by spontaneous mass gaps, which lead to insulating behaviour. These proposed gaps are analogous to the masses generated by broken symmetries in particle physics, and they give rise to large Berry phase effects accompanied by spontaneous quantum Hall effects. Although recent experiments have provided evidence for strong electronic correlations near the charge neutrality point, the presence of gaps remains controversial. Here, we report transport measurements in ultraclean double-gated bilayer graphene and use source-drain bias as a spectroscopic tool to resolve a gap of ∼2 meV at the charge neutrality point. The gap can be closed by a perpendicular electric field of strength ∼15 mV nm(-1), but it increases monotonically with magnetic field, with an apparent particle-hole asymmetry above the gap. These data represent the first spectroscopic mapping of the ground states in bilayer graphene in the presence of both electric and magnetic fields.

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At the charge neutrality point, bilayer graphene (BLG) is strongly susceptible to electronic interactions and is expected to undergo a phase transition to a state with spontaneously broken symmetries. By systematically investigating a large number of single-and double-gated BLG devices, we observe a bimodal distribution of minimum conductivities at the charge neutrality point. Although σ min is often approximately 2-3 e 2 ∕h (where e is the electron charge and h is Planck's constant), it is several orders of magnitude smaller in BLG devices that have both high mobility and low extrinsic doping. The insulating state in the latter samples appears below a transition temperature T c of approximately 5 K and has a T ¼ 0 energy gap of approximately 3 meV. Transitions between these different states can be tuned by adjusting disorder or carrier density.topological states | anomalous hall | spontaneous quantum Hall states | electron-electron interactions | layer antiferromagnets B ilayer graphene (BLG) has provided a fascinating new platform for both post-silicon electronics and exotic many-body physics (1-23). Because its conduction and valence bands touch at two points in momentum space and have approximately quadratic dispersion accompanied by momentum-space pseudospin textures with vorticity J ¼ 2, charge-neutral BLG is likely to have a broken-symmetry ground state in the absence of disorder (6-11, 15-18, 24-26). Theoretical work on the character of the ground state in neutral BLG has examined a variety of distinct but related pseudospin ferromagnet states-including gapped anomalous Hall (5,6,19),[16][17][18]22), and current loop states (26)-that break time-reversal symmetry, and gapless nematic states, which alter Dirac point structure and reduce rotational symmetry (6-11, 15-18, 24, 25). The pseudospin degree of freedom reflects the presence of two low-energy carbon sites per unit cell that are localized in different layers. Experimental work has confirmed the strong role of interactions, but has been equivocal in specifying ground-state properties. In particular, both gapped and gapless states have been reported (19-23) in suspended BLG. The low-temperature minimum conductivity at the charge neutrality point (CNP), σ min , has ranged from approximately 0.05 to 250 μS. These orders-of-magnitude differences between σ min values measured in apparently similar samples have been baffling.In this paper we attempt to shed light on these ambiguous findings by systematically examining a large number of single-and double-gated BLG samples, with mobility values ranging from 500 to 2;000 cm 2 ∕V·s for substrate-supported samples and 6,000 to 350,000 for suspended samples. We find a surprisingly constant σ min value of approximately 2-3 e 2 ∕h for a majority of the devices (here, e is the electron charge and h is Planck's constant), independent of their mobility and of the presence or absence of substrates. However, for T below approximately 5 K, the best devices form an insulating state with an energy gap of approximately 2-3...

Electrochemically induced Fenton (electro-Fenton) reaction was used for efficient and controllable preparation of hydroxyl radicals, leading to the generation of luminescent quantum dots through etching of as-exfoliated MoS2 nanosheets. Morphologic changes of MoS2 nanosheets during the electro-Fenton reaction were monitored using transmission electron microscopy, showing that etching of MoS2 nanosheets induced by hydroxyl radicals resulted in rapid homogeneous fracturing of the sheets into small dots via a transition of nanoporous morphology. The as-generated dots with vertical dimensional thickness of ca. 0.7 nm and plane size of ca. 5 nm were demonstrated to be MoS2 quantum dots (MoS2-QDs), and their photoluminescence properties were explored based on quantum confinement, edge effect, and intrinsic characteristics. Moreover, the degree of etching and the concomitant porosity of MoS2 nanosheets could be conveniently tuned via the electro-Fenton reaction time, resulting in a new morphology of nanoporous MoS2 nanosheets, with potential new applications in various significant areas.

We report pronounced magnetoconductance oscillations observed on suspended bilayer and trilayer graphene devices with mobilities up to 270,000 cm²/V s. For bilayer devices, we observe conductance minima at all integer filling factors ν between 0 and -8, as well as a small plateau at ν=1/3. For trilayer devices, we observe features at ν=-1, -2, -3, and -4, and at ν∼0.5 that persist to 4.5 K at B=8 T. All of these features persist for all accessible values of Vg and B, and could suggest the onset of symmetry breaking of the first few Landau levels and fractional quantum Hall states.