Therefore, further studies are required for a better understanding of our results. Figure 5 Magnetoresistivity measurements ρ xx (B) at various driving currents

I. The lattice temperature is constantly fixed at T ≈ 2 K. Figure 6 The magnetoresistivity measurements ρ xx (B) at different T for sample 1. The inset shows the Hall measurements ρ xy (B) at different T for sample 1. Figure 7 The determined exponent α in the power law T DF ∝ I α versus magnetic field B. In studying multilayer epitaxial graphene, top gating is difficult since depositing a dielectric layer is difficult and the top layers would screen the electric fields. Back gating is impractical because it would require SiC substrate thinning. Therefore, in order to further study the observed direct I-QH transition, PLX4032 we choose to study various samples with different classical mobilities (see Additional file 1). In all cases, an approximately T-independent point in ρ xx is observed. The approximated T-independent Hall results suggest that Dirac

fermion-Dirac fermion interactions are not significant in all our devices [35–38]. The crossing point and some other physical quantities are listed in Table 1. We note that for the same numbers of layer, the crossing field B PI3K inhibitor c is lower when the mobility μ is higher, consistent with the results obtained in conventional GaAs-based 2D systems [39, 40]. Moreover, the spin degree of freedom does not play an important role in the observed direct I-QH transition [41–45]. The dependence of the crossing magnetic field on the number of layers and sample does not seem to show a trend and thus requires further studies. Table 1 Sample parameters   Sample 1 Sample 2 Sample 3 Sample 4 ρ (Ω) 583 520 443 367 n (1013 cm−2) 2.08 1.98 2.16 2.44 μ (cm2/V.S) 511 605 651 694 B c (T) 9.2 4.2 6.0 5.7 v c 94 194 148 178 ρ xx/ρ xy at B c 2.1 3.7 2.5 2.8 μB c 0.47 0.25 0.39 0.40 Samples 1 and 2 were from the same chip, processed at 1,850°C

for 45 min; the former is close to the edge, and the later is near the center. Samples 3 and 4 were also from the same chip, processed at 1,950°C for 30 min; the former is close to the center, and the latter is near the edge. Lower resistivity near the edge is expected in the FTG Reverse transcriptase process; near the center the graphene growth is suppressed because of the higher concentration of Si vapor. At the crossing fields, the corresponding Landau filling factors are much larger than 2. Therefore, we have observed direct I-QH transition in all our devices [17–20]. It was argued that for direct I-QH transition in conventional semiconductor-based 2D systems, near the crossing field, ρ xx is approximately ρ xy, and the product of μB c is close to 1 [46]. However, in all our devices, ρ xx/ρ xy is much greater than 1, and μB c is always smaller than 1.