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2009 CARBON ALLOTROPESEdge fingerprints and magneto-conductance in graphene nanoribbonsThe control of the current flow in gated graphene nanoribbons(GNR) constitutes a fascinating challenge for the futureof carbon-based electronic devices. The combinationof the extraordinary electronic properties of the Diracfermions in graphene, like their outstanding mobility atroom temperature, with a sizeable energy gap, opens a routeto outperform the ultimate scaling of silicon field effecttransistors [Wang et al, Phys. Rev. Lett. 100 206803(2008)]. However, the lateral confinement goes along witha problematic decay of the mobility. Low temperaturetransport measurements performed on nano-lithographedGNR also unveil a ubiquitous energy gap, irrespective ofthe edges orientation and exceeding the expected confinementgap.Drawing a parallel between GNR and carbon nanotubes(CNT) is certainly tempting. The presence of edges in GNRimposes Dirichlet boundary conditions and results in a lossof one conducting channel at the Charge Neutrality Point(CNP), compared to armchair CNT.obtained on high quality GNR is sorely lacking. Applyinga large transverse magnetic field is also a suitable tool toboth explore the expected 1D-electronic band structure andthe disorder induced localization.In this work, we present evidence of 1D-subbands signatureson the electronic transport on an 11 nm wide GNR.Comparison to band structures calculations of a set ofGNRs in a width window of 11±0.3 nm allows an assignationof the carbon atoms arrangement at the edges and thenumber of dimers (figure 7).The application of a 60 T perpendicular magnetic field inducesa drastic increase of the conductance, whose magnitudedepends on the doping level (figure 8(left). At zeroenergy, we demonstrate that the large positive magnetoconductanceoriginates from the onset of the first Landaustate accompanying the closing of the energy gap (figure8(right). Landauer-Buttiker simulations of the conductanceof GNR including bulk and edge disorder (and currentlyin progress) demonstrate that the magnetoconductanceresults from a subtle combination of magnetic bandformations and quantum interference effects in presence ofdisorder.Figure 7: Conductance versus back-gate voltage at 80 K measuredon the 11 nm wide GNR for two V bias , 50 mV and 1 mV(respectively red and blue curve). Superimposed is the calculateddensity of states (black curve) as a function of V g , for four distinctGNRs : (a,b,c) aGNR, N = 90,91,92 and (d) zGNR, N = 52.Non-perfect edges are also source of short range potentialdisorder responsible for inter-valley backscattering. Drasticconsequences on the electronic transport have been theoreticalanticipated with the formation of a mobility gap at zeroenergy, even in the presence of an ultra-smooth edge roughness.However, a direct comparison with experimental dataFigure 8: (Left) Magnetoconductance measured on the 11 nmwide GNR for various back gate voltages at 80 K. For comparison,the black dashed curve is the magnetoconductance measuredon a 90 nm GNR varying from 0.6 to 0.4G 0 under 55 T. (Right)Magnetoconductance at the CNP measured at 80,50 and 20 K.The black dashed curves are the simulated magnetoconductanceassuming a thermally activated regime and a magnetic field inducedclosing of the energy gap along with the onset of the firstLandau level at zero energy.J-M Poumirol, W. Escoffier, M. Goiran, J-M Broto, B. RaquetS. Roche, A. Cresti (Commissariat à l’Energie Atomique, INAC, SP2M, Grenoble), X. Wang, H. Dai (Physics departmentand Chemistry Laboratory, Stanford, US)9