The two-dimensional phase of boron nitride: Few-atomic-layer sheets and suspended membranes

We describe the synthesis of very thin sheets (between a few and ten atomic layers) of hexagonal boron nitride (h-BN), prepared either on a SiO2 substrate or freely suspended. Optical microscopy, atomic force microscopy, and transmission electron microscopy have been used to characterize the morphology of the samples and to distinguish between regions of different thicknesses. Comparison is made to previous studies on single- and few-layer graphene. This synthesis opens the door to experimentally accessing the two-dimensional phase of boron nitride

Drude Conductivity of Dirac Fermions in Graphene

Electrons moving in graphene behave as massless Dirac fermions, and they exhibit fascinating low-frequency electrical transport phenomena. Their dynamic response, however, is little known at frequencies above one terahertz (THz). Such knowledge is important not only for a deeper understanding of the Dirac electron quantum transport, but also for graphene applications in ultrahigh speed THz electronics and IR optoelectronics. In this paper, we report the first measurement of high-frequency conductivity of graphene from THz to mid-IR at different carrier concentrations. The conductivity exhibits Drude-like frequency dependence and increases dramatically at THz frequencies, but its absolute strength is substantially lower than theoretical predictions. This anomalous reduction of free electron oscillator strength is corroborated by corresponding changes in graphene interband transitions, as required by the sum rule. Our surprising observation indicates that many-body effects and Dirac fermion-impurity interactions beyond current transport theories are important for Dirac fermion electrical response in graphene

Intraband Optical Transitions in Graphene

Abstract: We measured tunable interband and intraband transitions in graphene using infrared spectroscopy. Graphene electrons have strong intraband absorption at terahertz frequency range. The absorption spectra are described by a Drude-like frequency dependence

Giant Phonon-induced Conductance in Scanning Tunneling Spectroscopy of Gate-tunable Graphene

The honeycomb lattice of graphene is a unique two-dimensional (2D) system where the quantum mechanics of electrons is equivalent to that of relativistic Dirac fermions. Novel nanometer-scale behavior in this material, including electronic scattering, spin-based phenomena, and collective excitations, is predicted to be sensitive to charge carrier density. In order to probe local, carrier-density dependent properties in graphene we have performed atomically-resolved scanning tunneling spectroscopy measurements on mechanically cleaved graphene flake devices equipped with tunable back-gate electrodes. We observe an unexpected gap-like feature in the graphene tunneling spectrum which remains pinned to the Fermi level (E_F) regardless of graphene electron density. This gap is found to arise from a suppression of electronic tunneling to graphene states near E_F and a simultaneous giant enhancement of electronic tunneling at higher energies due to a phonon-mediated inelastic channel. Phonons thus act as a "floodgate" that controls the flow of tunneling electrons in graphene. This work reveals important new tunneling processes in gate-tunable graphitic layers

A Tunable Phonon-Exciton Fano System in Bilayer Graphene

Interference between different possible paths lies at the heart of quantum physics. Such interference between coupled discrete and continuum states of a system can profoundly change its interaction with light as seen in Fano resonance. Here we present a unique many-body Fano system composed of a discrete phonon vibration and continuous electron-hole pair transitions in bilayer graphene. Mediated by the electron-phonon interactions, the excited state is described by new quanta of elementary excitations of hybrid phonon-exciton nature. Infrared absorption of the hybrid states exhibit characteristic Fano lineshapes with parameters renormalized by many-body interactions. Remarkably, the Fano resonance in bilayer graphene is continuously tunable through electrical gating. Further control of the phonon-exciton coupling may be achieved with an optical field exploiting the excited state infrared activity. This tunable phonon-exciton system also offers the intriguing possibility of a 'phonon laser' with stimulated phonon amplification generated by population inversion of band-edge electrons.Comment: 21 pages, 3 figure

Conductance Quantization of Massless Dirac Fermions and the Synthesis, Characterization, and Manipulation of Graphene

Graphene, a two-dimensional carbon allotrope, has interesting electronic properties resulting from its unique hexagonal mono-atomic lattice. Electronic quasiparticles in graphene, called massless Dirac fermions, are described by the Weyl equation in which the effective speed of light is the Fermi velocity, approximately c/300. Thus graphene provides a solid state system in which to study the physics of high-energy electrons or neutrinos, including interesting relativistic quantum phenomena such as Zitterbewugung, atomic collapse, and Klein tunneling. The comparison of graphene's quasiparticles to neutrinos is limited by the fact that scattering in the solid reduces the mean free path to about a micron. However, by fabricating clean graphene devices with closely spaced electrodes, one can probe the intrinsic properties of massless Dirac fermions in the ballistic regime where these quasiparticles do not undergo scattering.The theory of the Dirac equation, the band structure of graphene, and the Landauer formalism for electronic transport is explained. Techniques are presented for the extraction of graphene, synthesis of chemical-vapor deposited graphene, and fabrication of graphene devices for both characterization and electronic transport measurements. The various characterization methods include Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. Experiments approaching the ballistic transport limit in graphene devices such as point contacts, Josephson junctions, and short-and-wide junctions are described

Conductance Quantization of Massless Dirac Fermions and the Synthesis, Characterization, and Manipulation of Graphene

Graphene, a two-dimensional carbon allotrope, has interesting electronic properties resulting from its unique hexagonal mono-atomic lattice. Electronic quasiparticles in graphene, called massless Dirac fermions, are described by the Weyl equation in which the effective speed of light is the Fermi velocity, approximately c/300. Thus graphene provides a solid state system in which to study the physics of high-energy electrons or neutrinos, including interesting relativistic quantum phenomena such as Zitterbewugung, atomic collapse, and Klein tunneling. The comparison of graphene's quasiparticles to neutrinos is limited by the fact that scattering in the solid reduces the mean free path to about a micron. However, by fabricating clean graphene devices with closely spaced electrodes, one can probe the intrinsic properties of massless Dirac fermions in the ballistic regime where these quasiparticles do not undergo scattering.The theory of the Dirac equation, the band structure of graphene, and the Landauer formalism for electronic transport is explained. Techniques are presented for the extraction of graphene, synthesis of chemical-vapor deposited graphene, and fabrication of graphene devices for both characterization and electronic transport measurements. The various characterization methods include Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. Experiments approaching the ballistic transport limit in graphene devices such as point contacts, Josephson junctions, and short-and-wide junctions are described

Origin of spatial charge inhomogeneity in graphene.

In an ideal graphene sheet, charge carriers behave as two-dimensional Dirac fermions Topographic corrugations and charge puddles in graphene are two of the most significant types of disorder in this new material. Topographic corrugations 2-4 , for example, have been suggested as a cause for the suppression of anticipated antilocalization 5 . Electron and hole puddles 6 have similarly been blamed for obscuring universal conductivity in graphene 7 . These issues are part of a puzzle regarding the factors that limit graphene's mobility We explored the inhomogeneous graphene charge density by spatially mapping the Dirac point (that is, the charge neutral point in the density of states of undoped graphene). The graphene local density of states at the Dirac point shows a local minimum, which is reflected by a dip in the tunnelling spectra of graphene Charge puddles can also be probed by spatially mapping the tunnelling differential conductance, dI /dV , for a fixed sample-tip bias held slightly below V D . This technique reduces data acquisition time by an order of magnitude and is particularly suited for measuring large graphene areas containing multiple charge puddles. The basis for using this second technique to measure charge puddles is illustrated i

Controlling inelastic light scattering quantum pathways in graphene,”

Inelastic light scattering spectroscopy has, since its first discovery Graphene, a two-dimensional carbon sheet The excitation pathways in graphene samples are controlled through electrostatic doping using a high-capacitance ion-gel gate dielectric To determine the gate-induced Fermi energy shift in graphene samples, we use infrared transmission spectroscop