Metrics details Abstract Mono- and bilayer graphene have generated tremendous excitement owing to their unique and potentially useful electronic properties 1. Suspending single-layer graphene flakes above the substrate 2 , 3 has been shown to greatly improve sample quality, yielding high-mobility devices with little charge inhomogeneity. Here we report the fabrication of suspended bilayer graphene devices with very little disorder. We observe quantum Hall states that are fully quantized at a magnetic field of 0. This resistance is predominantly affected by the perpendicular component of the applied field, and the extracted energy gap is significantly larger than expected for Zeeman splitting. These findings indicate that the broken-symmetry states arise from many-body interactions and underscore the important part that Coulomb interactions play in bilayer graphene.
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Metrics details Abstract Mono- and bilayer graphene have generated tremendous excitement owing to their unique and potentially useful electronic properties 1. Suspending single-layer graphene flakes above the substrate 2 , 3 has been shown to greatly improve sample quality, yielding high-mobility devices with little charge inhomogeneity.
Here we report the fabrication of suspended bilayer graphene devices with very little disorder. We observe quantum Hall states that are fully quantized at a magnetic field of 0.
This resistance is predominantly affected by the perpendicular component of the applied field, and the extracted energy gap is significantly larger than expected for Zeeman splitting.
These findings indicate that the broken-symmetry states arise from many-body interactions and underscore the important part that Coulomb interactions play in bilayer graphene. Download PDF Main The linear dispersion of graphene near its Fermi energy gives rise to low-energy excitations that behave as massless Dirac fermions 1.
Here we report the fabrication of high-quality suspended bilayer graphene devices Fig. Figure 1: Characterization of suspended bilayer samples S3 blue and S4 red at zero magnetic field. Inset: Zoom-in on the low-temperature behaviour. The pronounced dip in the conductivity at very low densities may be enhanced by a disorder-induced gap.
Full size image We focus first on the behaviour of our samples in zero magnetic field. These numbers represent a modest improvement of approximately a factor of two over unsuspended bilayers, but it remains unclear why the mobility is this low given the indications of sample quality discussed above, the low magnetic field at which we observe quantum Hall plateaus and the high mobilities observed in suspended monolayers 2 , 3.
It is predicted 26 that the mobility of bilayer graphene should be more than an order of magnitude smaller than that of monolayer graphene. This discrepancy was not observed in unsuspended samples 24 , but mobility in such samples may be limited by disorder associated with the substrate.
It is also worthwhile to comment on the possibility that the sharp dip in conductivity at low n is enhanced by a small energy gap that opens owing to disorder-induced differences in carrier density between the top and bottom layers of the flake We next discuss the magnetic-field-dependent behaviour of our samples.
Figure 2: Splitting of the eightfold-degenerate Landau level in suspended bilayers. The conversion between back-gate voltage and density for each sample was calibrated using this type of measurement. The numbers indicate filling factor. Quantum Hall plateaus associated with the broken-symmetry quantum Hall states are apparent. We focus, however, on two-terminal devices because they are more homogeneous see Supplementary Information. The magnetic field at which these effects emerge is over an order of magnitude smaller than has been reported for monolayers 6 , 7 , 12 , 13 , 14 , 15 , Broken-symmetry states could arise from several causes, including spin splitting due to the Zeeman effect 12 , strain-induced lifting of valley degeneracy 29 , the opening of an energy gap due to a potential difference between the two layers or Coulomb interactions 17 , In our samples, the proximity of Vpeak to zero back-gate voltage makes it unlikely that we observe an energy gap due to chemical doping It has recently been shown 30 that large-scale ripples appear in suspended graphene membranes when they are cooled from to K, but room-temperature scanning electron micrographs of our suspended flakes do not show prominent corrugations Fig.
The interaction energy due to Coulomb effects in bilayer graphene is expected to be two orders of magnitude stronger than spin splitting caused by the Zeeman effect 17 , 18 , so the observed broken-symmetry states are unlikely to be associated with Zeeman splitting. We therefore tentatively attribute the symmetry breaking to Coulomb interactions.
Our data do not fit a Kosterlitz—Thouless-type transition, nor do the flakes show activated behaviour over the full temperature range of our measurements.
Maximum resistance of sample S3 at the charge-neutrality point as a function of magnetic field and temperature. Inset: Zoom-in on the low-temperature curves. We do not observe saturation of the resistance for temperatures down to mK. This can be explained if we assume that the LLs are broadened by disorder. In such a scenario, a constant offset in magnetic field Boff is needed to resolve distinct quantum Hall states. Inset: Two-terminal conductance as a function of density and magnetic field.
Inset: Schematic showing the relative orientation between field and sample. The resistance depends primarily on Bperp, contradicting the expected behaviour for a Zeeman gap. The gap is several times larger than expected for Zeeman splitting, and tilted-field experiments provide further evidence that the broken-symmetry states probably arise from many-body effects rather than Zeeman splitting.
Rmax B,T is primarily dictated by the perpendicular component of field Bperp Fig. Methods Suspended bilayer graphene devices are fabricated using a method similar to that described in ref. Briefly, mechanical exfoliation of highly oriented pyrolytic graphite grade ZYA, SPI Supplies is used to deposit few-layer graphene flakes on a Si substrate coated with a nm layer of SiO2. Bilayer flakes are identified using an optical microscope, on the basis of contrast between the flake and the surrounding substrate.
Electrical leads are then patterned using electron-beam lithography, followed by thermal evaporation of 3 nm of Cr and nm of Au, and subsequent liftoff in warm acetone. Samples are quickly transferred to methanol and dried using a critical-point dryer.
Finished samples are transferred to the measurement system as quickly as possible, and are typically used without further cleaning or current annealing. Electronic transport measurements have been made on multiple samples, using standard a. The Si substrate serves as a global back gate, which is used to vary the carrier density in the bilayer. References 1 Castro Neto, A. The electronic properties of graphene.
GIANT INTRINSIC CARRIER MOBILITIES IN GRAPHENE AND ITS BILAYER PDF
Library subscriptions will be modified accordingly. Giant intrinsic carrier mobilities in graphene and its bilayer. Series I Physics Physique Fizika. Jaszczak 4and A. We have studied temperature dependences of electron transport in graphene and its bilayer and found extremely low electron-phonon scattering rates that set the fundamental limit on possible charge carrier mobilities at room temperature. Weyl fermions are observed in a solid.
Broken-symmetry states and divergent resistance in suspended bilayer graphene