Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session P22: Bilayer Graphene |
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Sponsoring Units: DCMP Chair: Xia Hong, Pennsylvannia State University Room: Portland Ballroom 252 |
Wednesday, March 17, 2010 8:00AM - 8:12AM |
P22.00001: Raman spectroscopy of suspended mono and bilayer graphene Alexander Kitt, Benjamin Feldman, Sebastian Remi, Jens Martin, Anna Swan, Amir Yacoby, Bennett Goldberg Suspended mono and bilayer graphene flakes have been shown to have higher mobility and lower disorder than their supported counterparts$^{1}$. The geometry which decouples the flake from the substrate also causes an as yet uncharacterized backgate specific strain due to an electrostatic attraction between the graphene and the back gated substrate. We study this strain using spatially resolved Raman spectroscopy with a diffraction limited spot size. Upon application of uni-axial strain the unit cell is stretched reducing the symmetry of the system and breaking the double degeneracy of the G band causing a split in the peak. Additionally the Raman modes show a linear softening as a function of strain in the case of supported graphene. Suspended flakes provide an ideal system to study back gate tunable strain while avoiding complications due to substrates including the determination of the Poisson ratio and sample slippage$^{2}$. Here we present preliminary results of our observations. 1: B Feldman, J Martin, A Yacoby, ``Broken-symmetry states and divergent resistance in suspended bilayer graphene'', Nature Physics, doi:10.1038/nphys1406 2: C Metzger et al, ``Biaxial strain in graphene adhered to shallow depressions'', Accepted for publication in Nano Letters [Preview Abstract] |
Wednesday, March 17, 2010 8:12AM - 8:24AM |
P22.00002: Tunable energy gap in suspended bilayer graphene Monica Allen, Thomas Weitz, Jens Martin, Benjamin Feldman, Amir Yacoby Bilayer graphene introduces a layer degree of freedom and provides a rich platform on which to study interaction driven effects that break its eightfold degeneracy (spin, valley, and orbital) [1]. Furthermore, it is possible to introduce a tunable bandgap in the density of states by applying a perpendicular electric field that breaks the inversion symmetry of the layers. The observation of correlated effects, increased mobilities, and this gap depends critically on high sample quality. For example, the electrostatic band gap has been observed optically, but its full observation in transport has been hindered due to disorder. Here we report a first realization of higher sample quality in suspended bilayer graphene devices with suspended top gates. This double-gated geometry allows for independent control of carrier density and electric field.~ Fabrication of suspended top-gated bilayers as well as our data on the tunable field-induced gap will be discussed. We report a significantly larger increase in peak resistance with electric field than in previous transport experiments.\\[4pt] [1] B.E. Feldman et al, Nature Physics [Preview Abstract] |
Wednesday, March 17, 2010 8:24AM - 8:36AM |
P22.00003: Formation and Characterization of Correlated Electron States at Room Temperature in Graphene Bilayers John Shumway, Matthew Gilbert The small dimensions of graphene bilayers suggest that exotic quantum states may be sustainable at room temperatures. In this work, we use quantum many-body computer simulations techniques with experimentally verified inputs to establish the transition temperature of an excitonic condensate in bilayer graphene and explore its transport properties [1]. To make robust predictions of the thermodynamic and transport properties of bilayers. we perform path integral Monte Carlo (PIMC) simulations of electrons and holes in two graphene layers separated by a one-nanometer thick oxide layer, which suppresses tunneling between layers while allowing for strong Coulomb correlations between layers. Top and bottom gates induce electron and hole densities of $5\times10^{12}$~cm$^{-2}$ in the two layers. As we vary the temperature, we see excitonic formation and Bose condensation. We calculate excitonic superfluid density from the winding statistics and estimate T$_c\sim 800$~K, well above room temperature. [1] M. J. Gilbert and J. Shumway, J. Comput.~Electron., {\bf 8}, 51-59 (2009). [Preview Abstract] |
Wednesday, March 17, 2010 8:36AM - 8:48AM |
P22.00004: ABSTRACT WITHDRAWN |
Wednesday, March 17, 2010 8:48AM - 9:00AM |
P22.00005: Infrared observation of gate-tunable bandgap and a giant Fano electron-phonon resonance in bilayer graphene Alexey Kuzmenko, Iris Crassee, Dirk van der Marel, Peter Blake, Konstantin Novoselov, Andre Geim, Lara Benfatto, Emmanuele Cappelluti We studied infrared spectra of bottom gated bilayer graphene on SiO$_{2}$/Si substrate. The two major results of our study are: (i) a determination of the gate-voltage dependent bandgap, and (ii) an observation of a new giant phonon resonance at 0.2 eV. In addition, the Slonczewski-Weiss-McClure tight binding model parameters were extracted by a simultaneous fitting of infrared data at all gate voltages. The gate-voltage dependence of the bandgap supports the calculations, which take electrostatic self-screening effects into account. The phonon peak shows several remarkable anomalies: (i) a giant enhancement with the applied gate voltage, which we ascribe to the so-called ``charged-phonon'' effect and (ii) a pronounced Fano lineshape, which is a manifestation of a coupling of this phonon to a continuum of electron-hole excitations. The obtained results show an outstanding potential of bilayer graphene for applications in electronics and opto-electronics. [Preview Abstract] |
Wednesday, March 17, 2010 9:00AM - 9:12AM |
P22.00006: Gap Opening by Asymmetric Doping in Graphene Bilayers Rodrigo Capaz, Marcos Menezes, Jorge Faria Graphene bilayers are very promising materials for nanoelectronic applications because they are metallic systems which can be made semiconducting by the application of an external electrical field. More importantly, the gap can be tuned by that field, which allows tailoring the electronic structure for specific applications. In this work, we explore theoretically another route for tuning the gap of graphene bilayers. We show that by controlling the doping with donor and acceptor species in separate sheets of the graphene bilayer, a gap can be opened and tuned in a similar way to an external electric field. Our calculations are based on the density functional theory and pseudopotentials , with a plane-wave basis.We explore specific realizations with potassium and nitrogen as donors and boron as acceptors, with similar results. We also investigate the dependence of the magnitude of the gap with respect to dopant concentration. [Preview Abstract] |
Wednesday, March 17, 2010 9:12AM - 9:24AM |
P22.00007: Temperature dependence of conductance in bilayer graphene with electric-field-induced band gap Hisao Miyazaki, Li Songlin, Takeo Minari, Akinobu Kanda, Kazuhito Tsukagoshi We experimentally investigated the transport properties of bilayer graphene gated by top and back gate. The conductance around the charge neutral point was strongly suppressed in the high electric field applied between the two gate electrodes, implying an existence of the band gap. To support the existence of the band gap, we examined temperature dependence of the conductance at the charge neutrality. Two thermal activation type conductions were needed to explain the observed temperature result. Low- temperature part ($T<\sim $100 K) of the temperature dependence was reasonably explained by the variable range hopping conduction. At higher temperatures ($T>\sim $100 K), an additional conductance component was necessary to explain the temperature dependence. The additional component has thermal activation energy $>$0.1 eV. This suggests the existence of the intrinsic band gap in the order of hundreds of meV, which agrees well with theoretical predictions and experimental results with optical method. [Preview Abstract] |
Wednesday, March 17, 2010 9:24AM - 9:36AM |
P22.00008: Laser Induced Structural Modification of Single Layer and Bilayer Graphene Pubudu Galwaduge, Joseph Lambert, Roberto Ramos Graphene is a two-dimensional crystal experimentally observed in its free standing form a few years ago. Electronic devices such as chemical sensors, superconducting transistors and room temperature single electron transistors have been fabricated using graphene. There is also evidence to suggest that graphene layers undergo physical transformation under laser irradiation. We report on our experimental progress in modifying single and bi-layer graphene under varying conditions of laser excitation energy, power and exposure time. [Preview Abstract] |
Wednesday, March 17, 2010 9:36AM - 9:48AM |
P22.00009: Electronic Structure and Optical Response of Electrically Gated Bilayer Graphene Li Yang We investigate electronic structure and optical response of electrically gated bilayer graphene using the density functional theory with pseudopotentials and plane waves. The atomic configurations of electrically gated bilayer graphene are fully relaxed according to the calculated forces and stress. The electric-field induced band gap and the corresponding infrared optical absorbance of bilayer graphene are obtained. Through these first-principles calculations, we suggest novel ways to efficiently tune the optical properties of bilayer graphene. Finally, our calculated results are in good agreement with recent experimental measurements. [Preview Abstract] |
Wednesday, March 17, 2010 9:48AM - 10:00AM |
P22.00010: Transport properties of HfO$_{2}$ top-gated bilayer graphene field effect transistors K. Zou, J. Zhu We present the fabrication and electrical transport studies of SiO$_{2}$/HfO$_{2}$ double-gated bilayer graphene field effect transistors (FETs). The top gate dielectric layer is formed by depositing 30nm HfO$_{2}$ onto graphene FETs fabricated on conventional SiO$_{2}$/doped Si substrates, using low temperature atomic layer deposition without the use of an adhesion layer. The top gate has an excellent gating efficiency of $\sim$2.8x10$^{12}$/cm$^{2}$V, which is 40 times larger than that of the Si backgate and can reach carrier density 1.4x10$^ {13}$/cm$^{2}$. We observe electron mobility up to 6,000 cm$^{2} $/Vs in double-gated bilayers. Pristine bilayer graphene on SiO$_{2}$, on the other hand, exhibits $\mu$ = 12,000 cm$^{2} $/Vs. We report and discuss the temperature-dependent conductivity in double and single-gated bilayer graphene at different densities and bias electric fields. [Preview Abstract] |
Wednesday, March 17, 2010 10:00AM - 10:12AM |
P22.00011: Superconducting Proximity Effect in Monolayer and Bilayer Graphene: Critical Current and Pair Amplitude Masahiko Hayashi, Hideo Yoshioka, Akinobu Kanda We study the superconducting proximity effect in monolayer and bilayer graphene, especially paying attention to the effects of band structure. The free energy of the superconductor-graphene-superconductor junction is calculated based on the tunnel Hamiltonian and the critical current through the junction and the pair amplitude in graphene are obtained. We numerically estimate the critical current for several forms of junctions and discuss the correspondence with experimental observations. The behavior of the proximity effect in monolayer system is rather close to that in normal metal, which shows monotonic dependence on temperature and junction separation. However, remarkably, the bilayer system shows a novel oscillating behavior as a function of temperature and junction separation. We discuss the origin of these behaviors by studying the pair amplitude in graphene. [Preview Abstract] |
Wednesday, March 17, 2010 10:12AM - 10:24AM |
P22.00012: Ballistic transport across domain walls in bilayer graphene Xiaoguang Li, Zhenyu Zhang, Di Xiao The transport properties of bilayer graphene across a junction connecting two regions with opposite electric gates are investigated.Using the recursive Green's function method, we study both infinite and nanoribbon graphene with zigzag or armchair edges. We find that the conductance of nanoribbon shows considerably different behavior compared to infinite graphene due to the appearance of edge states. The important role of electron tunneling between two layers is also revealed. [Preview Abstract] |
Wednesday, March 17, 2010 10:24AM - 10:36AM |
P22.00013: Optical phonons behavior in doped bilayer graphene Leandro Malard, D. L. Mafra, J. Leon, T. Campolina, D. C. Elias, F. Plentz, E. S. Alves, M. A. Pimenta The interaction of electrons and phonons is a fundamental issue for understanding the physics of graphene, resulting in the renormalization of phonon energy as a function of Fermi energy, which has been ascribed to the breakdown of the adiabatic approximation. In this work we study the behavior of the optical phonon modes in different bilayer graphene devices by applying bottom or top gate voltage, using Raman scattering. We observe the splitting of the Raman G band as we tune the Fermi level of the samples, which is explained in terms of distinct electron-phonon couplings involving the Raman (Eg) and infrared (Eu) phonon modes, the later one being Raman activated by inversion symmetry breaking in bilayer graphene due to different doping between the top and bottom layers [1]. We compare our data with recent theoretical calculations of the bilayer graphene phonon self-energy which includes non-homogeneous charge carrier doping between the graphene layers [2,3]. The comparison between the experiment and theoretical predictions can give information about the surrounding environment of the bilayer graphene. [1] L. M. Malard et al., Phys. Rev. Lett. \textbf{101}, 257401 (2008). [2] T. Ando and M. Koshino, J. Phys. Soc. Jpn. \textbf{78}, 034709 (2009). [3] P. Gava, M. Lazzeri, A. Marco Saitta, and F. Mauri, Phys. Rev. B \textbf{80}, 155422 (2009). [Preview Abstract] |
Wednesday, March 17, 2010 10:36AM - 10:48AM |
P22.00014: Berry-phase effect on reflection amplitude in bilayer graphene with potential step Sunghun Park, H.-S. Sim We theoretically study the phase of the reflection amplitude of an electron at a lateral potential step in Bernal-stacked bilayer graphene [1]. The phase shows an anomalous jump of $\pi $, as the electron incidence angle (relative to the normal direction to the step) varies to pass $\pm \pi $/4. The jump is attributed to the Berry phase associated with the pseudospin-1/2 of the electron. This Berry-phase effect is robust against the band-gap opening due to the external gates generating the step, and can be observed in an interferometry setup. \\[4pt] [1] Sunghun Park and H.-S. Sim, Phys. Rev. Lett. 103, 196802 (2009). [Preview Abstract] |
Wednesday, March 17, 2010 10:48AM - 11:00AM |
P22.00015: ABSTRACT WITHDRAWN |
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