Homogeneity of bilayer graphene

Freitag, F.; Weiß, Markus; Maurand, Romain; Trbovic, Jelena; Schönenberger, Christian

doi:10.1016/j.ssc.2012.09.001

Cited by 18 publications

(44 citation statements)

References 26 publications

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“…2,3 Recent experiments on suspended bilayer graphene, [4][5][6][7] which is free of substrate effects, reveal a gapped state at and around the charge neutrality point. The state may be of topological origin 8 due to the observed 4,6 conductance of the order of e 2 /h and may exhibit an anomalous quantum Hall effect, i.e.…”

Section: Introductionmentioning

confidence: 99%

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d-wave superconductivity on the honeycomb bilayer

2012

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We introduce a microscopic model on the honeycomb bilayer, which in the small-momentum limit captures the usual (quadratic dispersion in kinetic term) description of bilayer graphene. In the limit of strong interlayer hopping it reduces to an effective honeycomb monolayer model with also third neighbor hopping. We study interaction effects in this effective model focusing on possible superconducting instabilities. We find d x 2 −y 2 superconductivity in the strong coupling limit of an effective tJ-model-like description that gradually transforms into d + id time-reversal symmetry breaking superconductivity at weak couplings. In this limit the small momentum order parameter expansion is (kx + iky) 2 [or (kx − iky) 2 ] in both valleys of the effective low-energy description. The relevance of our model and investigation for the physics of bilayer graphene is also discussed.

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Section: Introductionmentioning

confidence: 99%

“…The state may be of topological origin 8 due to the observed 4,6 conductance of the order of e 2 /h and may exhibit an anomalous quantum Hall effect, i.e. a quantum Hall effect at zero magnetic field.…”

Section: Introductionmentioning

confidence: 99%

d-wave superconductivity on the honeycomb bilayer

2012

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“…Bilayer graphene was exfoliated from natural graphite onto oxidized silicon wafers, and contacted with Cr/Au electrodes using e-beam lithography. The bilayer was suspended by partly removing the oxide in a buffered hydrofluoric acid wet etch and subsequent critical point drying, as described in [22]. The data shown here were obtained on a four-terminal sample in Hall-cross geometry that had been shaped by an Ar/O 2 plasma dry etching process (figure 1).…”

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confidence: 99%

Spin symmetry of the bilayer graphene ground state

et al. 2013

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We show nonlinear transport experiments on clean, suspended bilayer graphene that reveal a gap in the density of states. Looking at the evolution of the gap in magnetic fields of different orientation, we find that the groundstate is a spin-ordered phase. Of the three possible gapped groundstates that are predicted by theory for equal charge distribution between the layers, we can therefore exclude the quantum anomalous Hall phase, leaving the layer antiferromagnet and the quantum spin Hall phase as the only possible gapped groundstates for bilayer graphene.

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“…1,2,[14][15][16][17][18][19][20][21][22][23] Unfortunately, suspended graphene allows only rather small devices to be realized (larger devices are mechanically unstable), and even more stringent limitations on the experiments have been posed so far by the inability to controllably realize high-quality multi-terminal devices. 22,[24][25][26][27] To address this last issue, here we discuss a strategy to fabricate multi-terminal suspended devices with an electronic mean-free path exceeding the device size, and demonstrate the high quality of these devices through different transport measurements. In particular, we report the observation of a negative multiterminal resistance due to the ballistic transport on suspended bilayer graphene, and very high quality quantum Hall data that include the independent determination of the transverse and the longitudinal resistance, as well as the observation of a pronounced quantum Hall plateau at magnetic field B as low as 30 mT.…”

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High-Quality Multiterminal Suspended Graphene Devices

Morpurgo

2013

Nano Lett.

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We introduce a new scheme to realize suspended, multi-terminal graphene structures that can be current annealed successfully to obtain uniform, very high quality devices. A key aspect is that the bulky metallic contacts are not connected directly to the part of graphene probed by transport measurements, but only through etched constriction, which prevents the contacts from acting invasively. The device high quality and uniformity is demonstrated by a reproducibly narrow (n ~ 10 9 cm -2 ) resistance peak around charge neutrality, by carrier mobility values exceeding 10 6 cm 2 /V/s, by the observation of integer quantum Hall plateaus starting at 30 mT and of symmetry broken states at about 200 mT, and by the occurrence of a negative multi-terminal resistance directly proving the occurrence of ballistic transport. As these multi-terminal devices enable measurements that cannot be done in a simpler two-terminal configuration, we anticipate that their use in future studies of graphene-based systems will be particularly relevant.KEYWORDS: Multi-terminal suspended graphene, Ballistic transport, Negative resistance, Graphene bilayer, Quantum Hall effect. 2The investigation of the intrinsic electronic properties of graphene relies heavily on the study of transport through nano-electronic devices in which extrinsic disorder is minimized. 1-3 When produced through mechanical exfoliation of graphite, graphene and its multilayers usually exhibit an extremely high level of structural perfection and chemical purity, sufficient for the realization of very high-quality devices. However, eliminating the influence of external perturbations is very challenging because graphene-based materials have atomic scale thickness, so that nearby materials -the substrate or adsorbed molecules-can easily affect the carrier mobility and induce pronounced inhomogeneity in carrier density. Two main routes have been pursued to minimize these sources of extrinsic disorder. The first consists in realizing suspended devices, 1, 2 in which a graphene layer is hanging between source and drain electrodes without being in a direct contact with a substrate material. The second relies on the use of hexagonal boron-nitride (hBN) substrates, 3 which has been shown to enable the realization of high-quality graphene structures.Key experiments have been performed by using either one of these techniques, with each having its own advantages and limitations. The realization of the graphene devices on hBN, for instance, allows the use of large area flakes, which enables the fabrication of more complex device structures. [4][5][6][7][8] It also appears that graphene on hBN can be cleaned from unwanted adsorbates rather reproducibly, once the device fabrication technique is under control. 9 However, it has been shown in a series of beautiful experiments 10-13 that, when placed onto hBN, charge carriers in graphene are influenced by the electrostatic potentials generated by the B and N atoms. As a result, owing to the lattice mismatch between graphene and hBN, the su...

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Homogeneity of bilayer graphene

Cited by 18 publications

References 26 publications

d-wave superconductivity on the honeycomb bilayer

d-wave superconductivity on the honeycomb bilayer

Spin symmetry of the bilayer graphene ground state

High-Quality Multiterminal Suspended Graphene Devices

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