Unraveling Quantum Hall Breakdown in Bilayer Graphene with Scanning Gate Microscopy (original) (raw)
Nature Communications
The quantum Hall (QH) effect, a topologically non-trivial quantum phase, expanded the concept of topological order in physics bringing into focus the intimate relation between the “bulk” topology and the edge states. The QH effect in graphene is distinguished by its four-fold degenerate zero energy Landau level (zLL), where the symmetry is broken by electron interactions on top of lattice-scale potentials. However, the broken-symmetry edge states have eluded spatial measurements. In this article, we spatially map the quantum Hall broken-symmetry edge states comprising the graphene zLL at integer filling factors of {{\nu }}={{0}},\pm {{1}}$$ ν = 0 , ± 1 across the quantum Hall edge boundary using high-resolution atomic force microscopy (AFM) and show a gapped ground state proceeding from the bulk through to the QH edge boundary. Measurements of the chemical potential resolve the energies of the four-fold degenerate zLL as a function of magnetic field and show the interplay of the m...
The breakdown of the quantum Hall effect, which is observed as an abrupt escalation in the longitudinal resistance with an associated loss in the quantization of Hall voltage is the major obstacle against improving the resistance standard which is currently based on this effect. Graphene is inherently a 2D material and has an unusual band structure that allows the quantization of the Hall resistance even at room temperature. These unique properties of graphene make it a good candidate as a high precision metrological characterization tool for the quantum Hall resistance. The uncertainty in the quantum Hall resistance in graphene has been rapidly improving recently and graphene samples have already been shown to reach the precision of the current best 2DEG samples. In this talk, experimental results on the breakdown of the quantum Hall effect in graphene on SiOx is presented. In narrow graphene samples of 1 micrometer width, the charge inhomogeneity is quite prominent and strongly af...
Scanning gate microscopy on graphene: charge inhomogeneity and extrinsic doping
Nanotechnology, 2011
We have performed scanning gate microscopy (SGM) on graphene field effect transistors (GFET), using a biased metallic nanowire coated with a dielectric layer as a contact mode tip and local top gate. Electrical transport through graphene at various back gate voltages is monitored as a function of tip voltage and tip position. Near the Dirac point, the dependence of graphene resistance on tip voltage shows a significant variation with tip position. SGM imaging reveals mesoscopic domains of electron-doped and hole-doped regions. Our measurements indicate a substantial spatial fluctuation (on the order of 10 12 /cm 2 ) in the carrier density in graphene due to extrinsic local doping. Important sources for such doping found in our samples include metal contacts, edges of graphene, structural defects, and resist residues.
Breakdown of the N=0 quantum Hall state in graphene: Two insulating regimes
Physical Review B, 2009
We studied the unusual Quantum Hall Effect (QHE) near the charge neutrality point (CNP) in high-mobility graphene sample for magnetic fields up to 18 T. We observe breakdown of the delocalized QHE transport and strong increase in resistivities ρxx, |ρxy| with decreasing Landau level filling for ν < 2, where we identify two insulating regimes. For 1 |ν| 1/2 we find an exponential increase of ρxx,xy ∼ e a(H−Hc) within the range up to several resistance quanta RK, while the Hall effect gradually disappears, consistent with the Hall insulator (HI) with local transport. Then, at ν ≈ 1/2 a cusp in ρxx(H) followed by an onset of even faster growth indicates transition to a collective insulator (CI) state. The likely candidate for this state is a pinned Wigner crystal.
Scanning gate microscopy of current-annealed single layer graphene
Applied Physics Letters, 2010
We have used scanning gate microscopy to explore the local conductivity of a current-annealed graphene flake. A map of the local neutrality point (NP) after annealing at low current density exhibits micron-sized inhomogeneities. Broadening of the local e-h transition is also correlated with the inhomogeneity of the NP. Annealing at higher current density reduces the NP inhomogeneity, but we still observe some asymmetry in the e-h conduction. We attribute this to a hole-doped domain close to one of the metal contacts combined with underlying striations in the local NP.
Numerical simulation of scanning gate spectroscopy in bilayer graphene in the Quantum Hall regime
2012 15th International Workshop on Computational Electronics, 2012
We propose a model for the numerical simulation of a two-terminal scanning gate spectroscopy experiment on bilayer graphene in the Quantum Hall regime. We start from the Chalker-Coddington random network model and link the model parameters with some of the relevant quantities in the experimental setup. The comparison between the simulation and the measurement results show a good qualitative and in several ways, quantitative agreement.
Quantum Hall Effect across Graphene Grain Boundary
Materials
Charge carrier scattering at grain boundaries (GBs) in a chemical vapor deposition (CVD) graphene reduces the carrier mobility and degrades the performance of the graphene device, which is expected to affect the quantum Hall effect (QHE). This study investigated the influence of individual GBs on the QH state at different stitching angles of the GB in a monolayer CVD graphene. The measured voltage probes of the equipotential line in the QH state showed that the longitudinal resistance (Rxx) was affected by the scattering of the GB only in the low carrier concentration region, and the standard QHE of a monolayer graphene was observed regardless of the stitching angle of the GB. In addition, a controlled device with an added metal bar placed in the middle of the Hall bar configuration was introduced. Despite the fact that the equipotential lines in the controlled device were broken by the additional metal bar, only the Rxx was affected by nonzero resistance, whereas the Hall resistanc...
Visualizing broken symmetry states in the zeroth Landau level of the graphene quantum Hall system
Bulletin of the American Physical Society, 2020
The quantum Hall (QH) effect, a topologically non-trivial quantum phase, expanded the concept of topological order in physics bringing into focus the intimate relation between the "bulk" topology and the edge states. The QH effect in graphene is distinguished by its fourfold degenerate zero energy Landau level (zLL), where the symmetry is broken by electron interactions on top of lattice-scale potentials. However, the broken-symmetry edge states have eluded spatial measurements. In this article, we spatially map the quantum Hall brokensymmetry edge states comprising the graphene zLL at integer filling factors of ν ¼ 0; ± 1 across the quantum Hall edge boundary using high-resolution atomic force microscopy (AFM) and show a gapped ground state proceeding from the bulk through to the QH edge boundary. Measurements of the chemical potential resolve the energies of the four-fold degenerate zLL as a function of magnetic field and show the interplay of the moiré superlattice potential of the graphene/boron nitride system and spin/valley symmetry-breaking effects in large magnetic fields.
Gate-Defined Graphene Quantum Point Contact in the Quantum Hall Regime
Physical Review Letters, 2011
We investigate transport in a gate-defined graphene quantum point contact in the quantum Hall regime. Edge states confined to the interface of p and n regions in the graphene sheet are controllably brought together from opposite sides of the sample and allowed to mix in this split-gate geometry. Among the expected quantum Hall features, an unexpected additional plateau at 0.5 h/e 2 is observed. We propose that chaotic mixing of edge channels gives rise to the extra plateau.