Abstract
We report on the observation of strong backscattering of charge carriers in the quantum Hall regime of polycrystalline graphene, grown by chemical vapor deposition, which alters the accuracy of the Hall resistance quantization. The temperature and magnetic field dependence of the longitudinal conductance exhibits unexpectedly smooth power-law behaviors, which are incompatible with a description in terms of variable range hopping or thermal activation but rather suggest the existence of extended or poorly localized states at energies between Landau levels. Such states could be caused by the high density of line defects (grain boundaries and wrinkles) that cross the Hall bars, as revealed by structural characterizations. Numerical calculations confirm that quasi-one-dimensional extended nonchiral states can form along such line defects and short circuit the Hall bar chiral edge states.
Highlights
One manifestation of the Dirac physics in graphene is a quantum Hall effect (QHE) [1,2] with an energy spectrum √quantized in Landau levels (LLs) at energies En = ±vF 2 neB, with a 4eB/ h degeneracy [3] and a sequence of Hall resistance plateaus at RH = ±RK/[4(n + 1/2)], where n ≥ 0 and RK ≡ h/e2
We report on the observation of strong backscattering of charge carriers in the quantum Hall regime of polycrystalline graphene, grown by chemical vapor deposition, which alters the accuracy of the Hall resistance quantization
The quantization of RH was measured with an uncertainty of 9 × 10−11 in a large 35 × 160-μm2 sample made of graphene grown by sublimation of silicon from silicon carbide, at 14 T and 0.3 K [11], it was recently demonstrated, both experimentally [12] and theoretically [13], that bilayer stripes forming along the silicon-carbide edge steps during the growth and crossing the Hall bar can short circuit the edge states and strongly alter the Hall quantization
Summary
The QHE at LLs filling factor ν = ±2 (ν = nsh/eB, where ns is the carrier density) is very robust and can even survive at room tempera√ture [4]. The question arises whether some defects, specific to each source of graphene, can jeopardize the quantization accuracy It was thereby shown, using exfoliated graphene, that the presence of a high density of charged impurities in the substrate on which graphene lies can limit the robustness of the Hall resistance quantization by a reduction of the breakdown current of the QHE [8]. A GaAs-based quantum resistance standard satisfies Rxx < 100 μ This highlights the need for exploration of the precise electronic transport mechanisms at work in CVD graphene. With the support of numerical simulations we highlight their paramount role in limiting the Hall quantization
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