Magnetic field driven ambipolar quantum Hall effect in epitaxial graphene close to the charge neutrality point

Abstract

We have investigated the disorder of epitaxial graphene close to the charge neutrality point (CNP) by various methods: (i) at room temperature, by analyzing the dependence of the resistivity on the Hall coefficient; (ii) by fitting the temperature dependence of the Hall coefficient down to liquid helium temperature; (iii) by fitting the magnetoresistances at low temperature. All methods converge to give a disorder amplitude of (20 ± 10) meV. Because of this relatively low disorder, close to the CNP, at low temperature, the sample resistivity does not exhibit the standard value h/4e 2 but diverges. Moreover, the magnetoresistance curves have a unique ambipolar behavior, which has been systematically observed for all studied samples. This is a signature of both asymmetry in the density of states and in-plane charge transfer. The microscopic origin of this behavior cannot be unambiguously determined. However, we propose a model in which the SiC substrate steps qualitatively explain the ambipolar behavior.

Large nonlocality in macroscopic Hall bars made of epitaxial graphene

“…This remarkable behavior was recently reported by our group [22] in similar samples. The evolution of the magnetoresistances cannot be explained by a standard two-fluid Drude model.…”

“…The samples were then encapsulated in a bilayer of resists as described in Ref. [22]. The resists can be used to lower the Fermi level closer to the Dirac point using either UV illumination [23] or the corona discharge method [24].…”

“…Each sample was prepared close to the charge neutrality point at room temperature using the corona method as described in our previous work [22]. Afterwards, each sample was cooled down to low temperature T = 1.7 K several times (up to 13 times for G14) in order to assess the reproducibility of the nonlocality and to vary the residual doping.…”

The effect of quasi-free graphene layer on the electrical transport properties of sandwich-like graphene / Co nanoparticles / graphene structure

“…Generally, in experiments on the Hall effect without an additional gate electrode, a polycrystalline CVD graphene should be considered as an ambipolar conductor with two types of charge carriers: electrons and holes [39][40][41]. As it was shown earlier, the grain boundaries tend to be doped mainly with n-type charge carriers, while the graphene grains are usually p-type doped [40,42].…”

Ambipolar charge transport in quasi-free-standing monolayer graphene on SiC obtained by gold intercalation

“…The nonlinear R XY (B) allows us to quantify densities and mobilities of electrons and holes using a two-band model. In the presence of both electrons and holes, the conventional Hall resistivity ρ XY (B) = −( 1 e )B (pμ h 2 −nμ e 2 )+μ h 2 μ e 2 (p−n)B 2 (pμ h +nμ e ) 2 +μ h 2 μ e 2 (p−n) 2 B 2 , where n (p) is electron (hole) density and μ e(h) is the electron (hole) mobility [27,28]. Figures 4(b) and 4(c) show n, p, μ e , and μ h extracted by fitting R XY (B) using the constraint that ρ XX (0) = 1/[e(pμ h + nμ e )].…”