Camara et al. Nanoscale Research Letters 2011, 6:141 http://www.nanoscalereslett.com/content/6/1/141 Page 4 of 6 Figure 4 Typical magnetoresistance measurements for low doped and highly doped epitaxial graphene-based Hall Bars. (a) Longitudinal resistance of highly p-type doped epitaxial monolayer versus the magnetic field B, measured at 1.6 K. The resistance increases linearly with B with the superimposed SdH oscillations clearly resolved. Index of Landau levels (8-14) is also reported. Inset: the Landau plot indicates a phase equal to 0°, as expected for Dirac electrons. (b) Longitudinal and transverse resistance of low p-type doped epitaxial monolayer versus applied magnetic field B, at T = 1.6 K. The Hall resistance approaches the integer plateau Rxy ~12.9 kΩ at B ~13 T. The second plateau at 4 kΩ is hardly visible. For the low doped layers, the transverse resistance exhibits now quantized Hall plateaus, clearly governed by the sequence RK/4(N + 1/2) in which RK = h/e2 is the Von Klitzing constant [20] and N = 0, 1, 2... As already known, this peculiar sequence of resistance values is the well-known quantum transport signature of the mono- layer graphene Landau levels [14]. In Figure 4(b).we show the longitudinal and Hall resistance values for such a low-doped SLEG device with hole concentration ns = 1.2 × 1012 cm-2 and mobility μ ~5000 cm2/V s at T = 1.6 K. At B = 12 T, the longitudinal resistance cancels while the transverse resistance tends to 12.9 kΩ which is the expected value for the N = 0 plateau (RK/2). In Figure 5a, we present similar resistance measure- ments obtained with a lower doped sample with a hole concentration ns = 8 × 1011 cm-2 and a mobility μ ~11,000 cm2/V s. The mobility is high enough and the concentration low enough to make the N = 0 and N = 1 plateaus well resolved and stable up to 13.5 T. The experimental results of Figure 5a have been obtained in a three probes configuration with low resistance con- tacts (40 Ω). The Hall resistance corresponds to the symmetric part of the signal: rxy ~(V(B)+V(-B))/2I, where the voltage V is measured between a lateral probe and the current drain. At high magnetic fields, we iden- tify V(+B)/IG as rxx, where G~4 is the geometric factor and I is the current. The temperature dependence of rxx(B) is shown in Figure 5a, between 1.6 and 44 K. In this temperature range, an activated behaviour is found for the resistivity: rxx ~exp(-Ea/kBT) of the N = 0 plateau. This activation energy Ea is the energy separation between the Fermi energy EF and the delocalised states of the N = 1 Landau level. In Figure 5b we plot the resistivities values rxx taken at different magnetic fields in the vicinity of the RK/2 plateau. The activation energy Ea varies from 0.7 to 3.3 meV between B = 10 and 13 T, which remains much smaller than the distance between the first and the second Landau level (~120 meV at B = 10 T). This indicates that the Fermi energy is firmly pinned by localised states. Ea has been calculated by tak- ing into account only temperatures above 6 K. At lower temperatures, there is an additional contribution to the conductivity, which is visible in Figure 5b as a change in the slope. We attribute this additional contribution to hopping. In principle, from the activation energy, we can recon- struct the density of state r(E). The filling factor is cal- culated from B = 10 to 13 T, each filling factor change Δν at a given magnetic field corresponding to a density variation Δns = nsΔν/ν. The Fermi energy shifts by ΔEa to compensate for the density variation and the mean value for the density of states at energy ~Ea is given by r(E) = Δns/ΔEa. Following this procedure, already used in the early times after the discovery of the integer QHE [21], we find the density of states plotted in Figure 6. The forma- tion of the Landau level is evidenced as, when Ea decreases, the density of states r(E) increases and becomes one order of magnitude larger than the density of states r0(E) without magnetic field at a comparable energy EF ~100 meV: r0(E) ~15 × 109 cm-2 meV-1. The shape of r(E) gives a rough upper bound of the half- width at half-maximum (HWHM) of the N = 1 Landau Level. We find HWHM ≤ 3 meV. This value is in good agreement with results obtained recently on EG by STM