Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session T21: Focus Session: Graphene: Bilayers II |
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Sponsoring Units: DMP Chair: Antonio Castro Neto, Boston University Room: Portland Ballroom 251 |
Wednesday, March 17, 2010 2:30PM - 2:42PM |
T21.00001: Transport and electronic structure of bilayer graphene Sankar Das Sarma, E. Rossi, E. H. Hwang We calculate the transport properties of bilayer graphene. Away from the neutrality point we find that the Boltzmann theory well agrees with current experimental results. Close to the neutrality point charge impurities break up the carrier density landscape in electron-hole puddles and the physics is dominated by the disorder induced strong charge density inhomogeneities. We quantitatively characterize the charge density fluctuations close to the neutrality point. Using the calculated carrier density probability distribution we develop a transport theory that takes into account the strong charge density inhomogeneities present in the vicinity of the neutrality point. [Preview Abstract] |
Wednesday, March 17, 2010 2:42PM - 2:54PM |
T21.00002: Electronic Transport in Dual Gated Bilayer Graphene Thiti Taychatanapat, Pablo Jarillo-Herrero The ability to control the band gap in bilayer graphene by applying a perpendicular electric field has attracted a lot of interest for its potential in nanoelectronic devices based on this material. Here, we examine the electronic properties of top-and-bottom gated bilayer graphene devices. The local top gate and global back gate enable us to control the size of the band gap and the Fermi energy separately and hence create an insulating state in bilayer graphene. We observe a transport gap of $\sim $4 meV at electric displacement of $\sim $2.5 V/nm. In addition, we use quantum point contact geometries to study transport in laterally confined bilayer graphene constrictions. We observe a non-monotonic resistance behavior as the transverse electric field is increased, which we attribute to the onset of conductance through the nanoconstriction. [Preview Abstract] |
Wednesday, March 17, 2010 2:54PM - 3:06PM |
T21.00003: Inversion symmetry breaking induced phonon-phonon anti-crossing in bilayer graphene Jun Yan, Theresa Villarson, Erik A. Henriksen, Philip Kim, Aron Pinczuk We use Raman scattering to study the breaking of inversion symmetry in bilayer graphene. The electron and hole doped states of the system reveal phonon band splitting with spectral intensity transfer that is tuned by a polymer electrolyte top gate. The observations suggest that the in-phase and out-of-phase long wavelength optical phonons (G bands) are coupled to each other, and are thus no longer energy eigenstates. The coupling results in an intriguing phonon-phonon anti-crossing phenomenon induced by the broken inversion symmetry. The Raman spectral transfer between the observed two normal modes offers quantitative measurements of the evolution of the phonon wavefunction and suggests that the Raman activity of the out-of-phase mode is negligibly small in the presence of the broken inversion symmetry. [Preview Abstract] |
Wednesday, March 17, 2010 3:06PM - 3:42PM |
T21.00004: Spectroscopic and Transport Properties of Bilayer Graphene Invited Speaker: We review our theoretical work on spectroscopic and transport properties of bilayer graphene [PRB 78, 045405 (2008)]. In particular we study the effects of short-range scattering centers on the electronic self-energy and the resulting experimental consequences for spectroscopy (ARPES) and conductivity measurements. This type of disorder is believed to be especially relevant for suspended samples where the amount of other types of scatterers have been minimized. We also consider effects of disorder in gapped graphene bilayers such as band gap smearing and creation of bound states whose binding energies and localization lengths depends on the externally applied bias (and hence the gap) between the two layers. [Preview Abstract] |
Wednesday, March 17, 2010 3:42PM - 3:54PM |
T21.00005: The effect of screening on excitonic condensation in double-layer graphene Christopher Jamell, Yogesh Joglekar A double-layer graphene system - where two layers of graphene are embedded in a dielectric and are separated by a distance d - is expected to support ground states with interlayer coherence. In particular, when one layer has electrons as carriers and the other has holes, the system supports a uniform excitonic condensate ground state. We investigate the effect of screened electron-electron interaction on the formation and the strength of excitonic condensation using complementary momentum-space and real-space mean-field analyses. We obtain the dependence of the excitonic order parameter $\Delta_k$, quasiparticle energy dispersion $E_k$, and the quasiparticle density-of-states on the screening-length and the interlayer distance. By focusing on the momentum-dependence of the excitonic order parameter $\Delta_k$, we point out the differences between our results and those obtained in the literature. [Preview Abstract] |
Wednesday, March 17, 2010 3:54PM - 4:06PM |
T21.00006: One Dimensional Physics in Bilayer Graphene Matthew Killi, Tzu-Chieh Wei, Ian Affleck, Arun Paramekanti Bilayer graphene is an interesting material with many novel features. In the absence of a gate voltage, it exhibits quadratic band touching that is unstable to various forms of symmetry breaking. In the presence of a bias induced by a gate voltage, it behaves as a gapped semiconductor with a tunable gap. We discuss one dimensional edge modes and interaction effects in bilayer graphene. [Preview Abstract] |
Wednesday, March 17, 2010 4:06PM - 4:18PM |
T21.00007: Ultrafast carrier dynamics in bilayer graphene studied by broadband infrared pump-probe spectroscopy Thomas Limmer, Enrico Da Como, Alexander Niggebaum, Jochen Feldmann Recently, bilayer graphene gained a large interest because of its electrically tunable gap appearing in the middle infrared part of the electromagnetic spectrum. This feature is expected to open a number of applications of bilayer graphene in optoelectronics. In this communication we report on the first pump-probe experiment on a single bilayer flake with an unprecedented probe photon energy interval (0.25 -- 1.3 eV). Single flakes were prepared by mechanical exfoliation of graphite and transferred to calcium fluoride substrates. When illuminated with 800 nm (1.5 eV) pump pulses the induced change in transmission shows an ultrafast saturation of the interband transitions from 1.3 to 0.5 eV. In this energy range the saturation recovery occurs within 3 ps and is consistent with an ultrafast relaxation of hot carriers. Interestingly, we report on the observation of a resonance at 0.4 eV characterized by a longer dynamics. The results are discussed considering many-body interactions. [Preview Abstract] |
Wednesday, March 17, 2010 4:18PM - 4:30PM |
T21.00008: Many-body instability of Coulomb interacting bilayer graphene: RG approach Oskar Vafek, Kum Yang Low-energy electronic structure of (unbiased and undoped) bilayer graphene consists of two Fermi points with {\em quadratic} dispersions if trigonal-warping is ignored. We show that a short-range (or screened Coulomb) interactions are marginally {\em relevant} and use renormalization group to study their effects on low-energy properties of the system. We find that the two quadratic Fermi points spontaneously split into four Dirac points. This results in a nematic state that spontaneously breaks the six-fold lattice rotation symmetry (combined with layer permutation) down to a two-fold one, with a finite transition temperature. Critical properties of the transition and effects of trigonal warping are also discussed. [Preview Abstract] |
Wednesday, March 17, 2010 4:30PM - 4:42PM |
T21.00009: Electronic structure of bilayer graphene out of Bernal stacking Marcelo Kuroda, Razvan Nistor, Glenn Martyna The electronic properties of bilayer graphene have thus far been studied in the Bernal (AB) stacking, in which the A-carbon of one sheet lies on top of the B-carbon of the other. However, other configurations have been observed experimentally. In this work, we study the electronic properties of bilayer graphene using the density functional theory for the case when the two graphene layers are not aligned. We compare our results as a function of the angle between the two graphene lattices. We find that in contrast to the Bernal stacking, which exhibits parabolic band dispersion at the K-point, arbitrary alignments of the lattices recover the linear band dispersion of graphene, and the system behaves as two independent layers. In addition, the rotated graphene bilayers show weak energy dependence under translations. [Preview Abstract] |
Wednesday, March 17, 2010 4:42PM - 4:54PM |
T21.00010: Misalignment and Disorder in Graphene Bilayers and Nanoribbons: Electronic-structure and Electric-field modulation Hassan Raza In Bernal stacked bilayer graphene, the coupling between the two layers is a function of the stacking distance [1] as well as the relative orientation of the two layers. We computationally study the effect of stacking misalignments on the electronic structure and electric-field modulation using the extended H\"{u}ckel theory. We report that certain stacking misalignments, either induced from adjacent dielectrics or stress, would lead to various characteristics of electronic-structure and out-of-plane electric-field modulation, which can have a significant effect on the band gap opening. Furthermore, we study the impact of edge disorder in armchair and zigzag graphene nanoribbons on their electronic-structure for semiconductor applications. Due to disorder, the quantized transverse momentum does not cross the Dirac point and hence a semiconducting system is observed. We further study the bandgap modulation [2] by a transverse electric-field for these disordered nanoribbons. \\[4pt] [1] H. Raza, E. C. Kan, J. Phys.: Condens. Matter 21, 102202 (2009). \\[0pt] [2] H. Raza, E. C. Kan, Phys. Rev. B 77, 245434 (2008). [Preview Abstract] |
Wednesday, March 17, 2010 4:54PM - 5:06PM |
T21.00011: Commensuration and Interlayer Coherence in Twisted Bilayer Graphene E.J. Mele The low energy electronic spectra of rotationally faulted graphene bilayers are studied using a long wavelength theory applicable to general commensurate fault angles. We find that Lattice commensuration requires low energy electronic coherence across a fault and preempts massless Dirac behavior near the neutrality point. Sublattice exchange symmetry distinguishes two families of commensurate faults that have distinct low energy spectra which can be interpreted as energy-renormalized forms of the spectra for the limiting Bernal and AA stacked structures. Sublattice-symmetric faults are generically fully gapped systems due to a pseudospin-orbit coupling appearing in their effective low energy Hamiltonians. We use the model to study the dependence of the interlayer coherence scale on fault angle and the electronic response of faulted bilayers to applied static fields. [Preview Abstract] |
Wednesday, March 17, 2010 5:06PM - 5:18PM |
T21.00012: Electron-electron Interactions in ABC-stacked Multilayer Graphene Fan Zhang, Allan MacDonald The electronic band structures of ABC-stacked multilayer graphene systems are obtained by the tight-binding calculation and the density function theory. We predict that the electron- electron interactions drive the neutral graphene multilayer systems to pseudospin magnets in which the charge density contribution spontaneously shifts to either the top or the bottom layers, based on the HF and PRG calculations. We show that the spin and valley degrees of freedom enhance the instabilities. We investigate the influence on the broken symmetry phase by the trigonal warping, the external electric field and the number of coupled graphene layers. [Preview Abstract] |
Wednesday, March 17, 2010 5:18PM - 5:30PM |
T21.00013: Parity and valley degeneracy in multilayer graphene Edward McCann, Mikito Koshino The lattice of Bernal (ABA-stacked) trilayer graphene does not satisfy spatial inversion symmetry. Instead, mirror reflection symmetry in the plane of the central graphene layer plays the role of parity, and, owing to this symmetry, the low-energy electronic band structure of Bernal trilayers consists of separate linear, monolayer-like and parabolic, bilayer-like bands. Mirror reflection does not transform between states at the two valleys and is, therefore, unable to guarantee valley degeneracy. We show that this leads to a peculiar Landau level spectrum in Bernal trilayers, with an unusual structure of broken valley degeneracy that is markedly different from monolayer and bilayer graphene. Finally, we explain how this picture is modified in Bernal graphene multilayers with an even or odd number of layers. [Preview Abstract] |
Wednesday, March 17, 2010 5:30PM - 5:42PM |
T21.00014: Electrical Transport in bilayer graphene PNP junctions Lei Jing, Jairo Velasco, Philip Kratz, Wenzhong Bao, Chun Ning Lau Graphene provides a unique platform to study quantum transport phenomena in mesoscopic physics system. We apply multilevel lithography method to fabricate contactless top gate above bilayer graphene flakes. To explore its electrical properties, transport spectroscopy measurements are carried out to investigate \textit{pnp} junction's conductance and magnetoresistance. Our latest experimental results will be discussed and compared with theoretical predictions. [Preview Abstract] |
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