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Register a new userCharge Puddles in Graphene Near the Dirac Point
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The charge carrier density in graphene on a dielectric substrate such as SiO$_2$ displays inhomogeneities, the so-called charge puddles. Because of the linear dispersion relation in monolayer graphene, the puddles are predicted to grow near charge neutrality, a markedly distinct property from conventional two-dimensional electron gases. By performing scanning tunneling microscopy/spectroscopy on a mesoscopic graphene device, we directly observe the puddles growth, both in spatial extent and in amplitude, as the Dirac point is approached. Self-consistent screening theory provides a unified description of both the macroscopic transport properties and the microscopically observed charge disorder.
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We investigate the quantum Hall (QH) states near the charge neutral Dirac point of a high mobility graphene sample in high magnetic fields. We find that the QH states at filling factors $ u=pm1$ depend only on the perpendicular component of the field with respect to the graphene plane, indicating them to be not spin-related. A non-linear magnetic field dependence of the activation energy gap at filling factor $ u=1$ suggests a many-body origin. We therefore propose that the $ u=0$ and $pm1$ states arise from the lifting of the spin and sub-lattice degeneracy of the $n=0$ LL, respectively.
We present an experimental study of nonlocal electrical signals near the Dirac point in graphene. The in-plane magnetic field dependence of the nonlocal signal confirms the role of spin in this effect, as expected from recent predictions of Zeeman spin Hall effect in graphene, but our experiments show that thermo-magneto-electric effects also contribute to nonlocality, and the effect is sometimes stronger than that due to spin. Thermal effects are seen to be very sensitive to sample details that do not influence other transport parameters.
Effects of disorder on the electronic transport properties of graphene are strongly affected by the Dirac nature of the charge carriers in graphene. This is particularly pronounced near the Dirac point, where relativistic charge carriers cannot efficiently screen the impurity potential. We have studied time-dependent conductance fluctuations and magnetoresistance in graphene in the close vicinity of the Dirac point. We show that the fluctuations are due to the quantum interference effects due to scattering on impurities, and find an unusually large reduction of the relative noise power in magnetic field, possibly indicating that an additional symmetry plays an important role in this regime.
The magnetic field-dependent longitudinal and Hall components of the resistivity rho_xx(H) and rho_xy(H) are measured in graphene on silicon dioxide substrates at temperatures from 1.6 K to room temperature. At charge densities near the charge-neutrality point rho_xx(H) is strongly enhanced and rho_xy(H) is suppressed, indicating nearly equal electron and hole contributions to the transport current. The data are inconsistent with uniformly distributed electron and hole concentrations (two-fluid model) but in excellent agreement with the recent theoretical prediction for inhomogeneously distributed electron and hole regions of equal mobility. At low temperatures and high magnetic fields rho_xx(H) saturates to a value ~h/e^2, with Hall conductivity << e^2/h, which may indicate a regime of localized v = 2 and v = -2 quantum Hall puddles.
Graphene on a dielectric substrate exhibits spatial doping inhomogeneities, forming electron-hole puddles. Understanding and controlling the latter is of crucial importance for unraveling many of graphenes fundamental properties at the Dirac point. Here we show the coexistence and correlation of charge puddles and topographic ripples in graphene decoupled from the metallic substrate it was grown on. The analysis of interferences of Dirac fermion-like electrons yields a linear dispersion relation, indicating that graphene on a metal can recover its intrinsic electronic properties.
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