The potential of graphene-based materials consisting of one or a few layers of graphite for integrated electronics originates from the large room-temperature carrier mobility in these systems (approximately 10,000 cm2 V(-1) s(-1)). However, the realization of electronic devices such as field-effect transistors will require controlling and even switching off the electrical conductivity by means of gate electrodes, which is made difficult by the absence of a bandgap in the intrinsic material. Here, we demonstrate the controlled induction of an insulating state--with large suppression of the conductivity--in bilayer graphene, by using a double-gate device configuration that enables an electric field to be applied perpendicular to the plane. The dependence of the resistance on temperature and electric field, and the absence of any effect in a single-layer device, strongly suggest that the gate-induced insulating state originates from the recently predicted opening of a bandgap between valence and conduction bands.

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Graphene-based materials are promising candidates for nanoelectronic devices because very high carrier mobilities can be achieved without the use of sophisticated material preparation techniques. However, the carrier mobilities reported for single-layer and bilayer graphene are still less than those reported for graphite crystals at low temperatures, and the optimum number of graphene layers for any given application is currently unclear, because the charge transport properties of samples containing three or more graphene layers have not yet been investigated systematically. Here, we study charge transport through trilayer graphene as a function of carrier density, temperature, and perpendicular electric field. We find that trilayer graphene is a semimetal with a resistivity that decreases with increasing electric field, a behaviour that is markedly different from that of single-layer and bilayer graphene. We show that the phenomenon originates from an overlap between the conduction and valence bands that can be controlled by an electric field, a property that had never previously been observed in any other semimetal. We also determine the effective mass of the charge carriers, and show that it accounts for a large part of the variation in the carrier mobility as the number of layers in the sample is varied.

We investigate experimentally transport through ring-shaped devices etched in graphene and observe clear Aharonov-Bohm conductance oscillations. The temperature dependence of the oscillation amplitude indicates that below 1 K, the phase coherence length is comparable to or larger than the size of the ring. An increase in the amplitude is observed at high magnetic field, when the cyclotron diameter becomes comparable to the width of the arms of the ring. By measuring the dependence on gate voltage, we find that the Aharonov-Bohm effect vanishes at the charge neutrality point, and we observe an unexpected linear dependence of the oscillation amplitude on the ring conductance. DOI: 10.1103/PhysRevB.77.085413 PACS number͑s͒: 73.23.Ϫb, 73.20.Ϫr, 73.22.Ϫf, 73.61.Wp The investigation of transport phenomena originating from quantum interference of electronic waves has proven to be a very effective probe of the electronic properties of conducting materials. Recent work has shown that this is also the case for graphene, a novel material consisting of an individual layer of carbon atoms, in which the electron dynamics is governed by the Dirac equation.1 The anomalous behavior of the weak-localization correction to the conductivity that is observed in the experiments, 2 for instance, is directly related to the presence of two independent valleys in the band structure of graphene 3,4 and can be used to extract the intervalley scattering time.5 Another example is provided by the observation of a Josephson supercurrent in graphene superconducting junctions, which permits to conclude that transport through graphene is phase coherent even when the material is biased at the charge neutrality point ͑i.e., where nominally no charge carriers are present͒. 6 Possibly, the phenomenon that most directly illustrates electronic interference in solid-state devices is the occurrence of periodic oscillations in the conductance of ringshaped devices, measured as a function of magnetic field. 7 This phenomenon, which is a direct consequence of the Aharonov-Bohm ͑AB͒ effect, has been investigated extensively in the past in rings made with metallic films or with semiconducting heterostructures, and its study has contributed significantly to our understanding of mesoscopic physics. For example, the analysis of h/e and h/2e AB conductance oscillations has clarified the difference between sample-specific and ensemble-averaged phenomena.7 The investigation of the temperature and magnetic field dependences of the oscillation amplitudes has been used to investigate processes leading to decoherence of electron waves, such as electron-electron interaction, 7,8 or the interaction with magnetic impurities. 9 In graphene, however, no experimental observation of AB conductance oscillations has been reported so far, although there is an emerging interest in the problem from the theoretical side. 10,11 In the course of recent experiments, we have observed AB conductance oscillations experimentally in several rings fabricated on few-layer graphene. In thi...