Insulator-quantum Hall transition in monolayer epitaxial graphene

Abstract

We report on magneto-transport measurements on low-density, large-area monolayer epitaxial graphene devices grown on SiC. We observe temperature (T)-independent crossing points in the longitudinal resistivity ρxx, which are signatures of the insulator-quantum Hall (I-QH) transition, in all three devices. Upon converting the raw data into longitudinal and Hall conductivities σxx and σxy, in the most disordered device, we observed T-driven flow diagram approximated by the semi-circle law as well as the T-independent point in σxy near e2/h. We discuss our experimental results in the context of the evolution of the zero-energy Landau level at low magnetic fields B. We also compare the observed strongly insulating behaviour with metallic behaviour and the absence of the I-QH transition in graphene on SiO2 prepared by mechanical exfoliation.

Temperature dependence of electron density and electron–electron interactions in monolayer epitaxial graphene grown on SiC

“…The red curve in Figure 5 is then plotted using Eq. (2) where σ xx (30 K, 0 T) and σ xy (30 K) were used to approximate the value of σ 0 and σ xy , respectively, and K ee = 0.71, which is determined by optimizing the elimination of the e - e interactions correction from the resistivity tensor [20, 30]. The good fit to our data demonstrates that the observed ln T dependence in R H can be ascribed to e - e interactions.…”

“…In contrast to our graphene samples with higher mobilities, SdH oscillations appear at lower fields, providing us an alternative and reliable method for the determination of the carrier density independently. In strongly disordered graphene, although a T -independent point in R xx over a wide range of temperature does suggest that the carrier density is temperature-independent [30, 34], the lack of SdH oscillations did not allow us to probe the carrier density [30]. …”

Unusual renormalization group (RG) flow and temperature-dependent phase transition in strongly-insulating monolayer epitaxial graphene

“…In contrast, monolayer graphene epitaxially grown on SiC, 9 which can be of wafer-size and most importantly does not require any transfer process, is now widely believed to be an ideal candidate for replacing existing GaAs-based QHR reference standards. [10][11][12][13][14][15][16] In addition, recently it has been shown that monolayer epitaxial graphene grown on SiC is an interesting platform for studying the insulatorrelativistic quantum Hall transition in graphene, 17,18 insulating behavior, [17][18][19] variable range hopping, 19 and so forth. Such measurements may well reveal the true ground-state of disordered graphene and shed light on possible band-gap opening in graphene on SiC.…”

“…RG ow lines in disordered two-dimensional (2D) systems are understood by referring to the semicircle law with a clear unstable point. Such results provide important information on the insulating behaviour in monolayer epitaxial graphene grown on SiC [17][18][19] and the transition 17,18 at low elds between an insulator and the n ¼ 2 QH state, which cannot be seen in many other graphene-like systems. For example, in our previous work, RG ow approximated by the semicircle law is only observed in the strongest disorder device even if clear crossing points in the longitudinal resistivity r xx are observed in all three monolayer graphene devices grown on SiC.…”

Formation of the n = 0 Landau level in hybrid graphene

“…where ω c =eB/m is the cyclotron frequency, τ is the mean free scattering time, and ò(n)=(n+1/2)ÿω c are the high-field Landau level energies for conventional 2DEGs. This theory, which has continued to be subject to debate in conventional 2DEGs [29][30][31] and has just begun to be studied in graphene [32], yields a cross-over into the quantum Hall regime at ω c τ=1, corresponding semiclassically to the region where the scattering length becomes comparable to the magnetic length and phase-coherent Landau level orbits can be established. It is notable that the ω c τ=1 condition is equivalent to a B=1/μ condition, which for our samples is just under 2 T, the point at which the half-filling conductance asymmetries are at a maximum (figures 4(a) and (b)).…”