Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session M6: Graphene: Multilayer and Tunneling |
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Sponsoring Units: DCMP Chair: Chun Ning (Jeanie) Lau, University of California, Riverside Room: 302 |
Wednesday, March 20, 2013 8:00AM - 8:12AM |
M6.00001: Quantumn Hall Effect in single-, bi- and tri-layer graphene Zeng Zhao, Kevin Myhro, David Tran, Hang Zhang, Jhao-wun Huang, Jairo Velasco, Yanmeng Shi, Fenglin Wang, Yongjin Lee, Chun Ning Lau Quantum Hall Effect has been extensively studied in single layer, bilayer and trilayer graphene. Our recent studies showed intrinsic gapped state at the charge neutrality point in bilayer and trilayer graphene. Here we describe the fabrication of high-quality single-bilayer and bi-trilayer hybrid graphene devices, and present results from magneto-transport measurements. [Preview Abstract] |
Wednesday, March 20, 2013 8:12AM - 8:24AM |
M6.00002: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 8:24AM - 8:36AM |
M6.00003: Velocity renormalization in multilayer graphene Hongki Min Multilayer graphene has recently attracted considerable attention because of its chiral electronic structure which is sensitive to stacking sequences, and its possible use as the basis of new electronic devices. Furthermore, as sample quality improves, it is expected that electron-electron interactions play a significant role which was hidden by disorder. In this talk, we study velocity renormalization in multilayer graphene due to electron-electron interactions. After analyzing velocity renormalization in the chiral two-dimensional electron gas which is a low-energy effective model of graphene systems, we discuss its implication for multilayer graphene. [Preview Abstract] |
Wednesday, March 20, 2013 8:36AM - 8:48AM |
M6.00004: Emergent Electromagnetism in Bilayer Graphene Roland Winkler, Ulrich Z\"ulicke Recently atomically flat layers of carbon known as graphene have become the rising star in spintronics as their electrons carry not only the ordinary spin degree of freedom, but they also have a pseudospin degree of freedom tied to the electrons' orbital motion which could enable new routes for spintronics. Here we focus on bilayer graphene (BLG). Using group theory we have established a complete description of how electrons in BLG interact with electric and magnetic fields. We show that electrons in BLG experience an unusual type of matter-field interactions where magnetic and electric fields are virtually equivalent: every coupling of an electron's degrees of freedom to a magnetic field is matched by an analogous coupling of the same degrees of freedom to an electric field. This counter-intuitive duality of matter-field interactions allows novel ways to create and manipulate spin and pseudo-spin polarizations via external fields that are not available in other materials. See arXiv:1206.4761. [Preview Abstract] |
Wednesday, March 20, 2013 8:48AM - 9:00AM |
M6.00005: Coexisting massive and massless Dirac fermions in quasi-freestanding bilayer graphene Keun Su Kim, Andrew L. Walter, Luca Moreschini, Thomas Seyller, Karsten Horn, Eli Rotenberg, Aaron Bostwick The most widely accepted theoretical model to describe charge carriers in bilayer graphene is ``massive Dirac fermions'', characterized by a nearly parabolic band pair touching each other at the Dirac energy. This electronic structure of bilayer graphene is widely believed to be unstable towards symmetry breaking either by structural distortions, such as twist and strain, or electronic interactions. In this work, we investigate quasi-freestanding bilayer graphene by angle-resolved photoemission spectroscopy, which shows an unexpected electronic spectrum, consisting of both massive and massless Dirac fermions. The latter has a unique band topology with a chiral pseudospin texture, and its origin will be discussed in terms of symmetry breaking induced by a native imperfection of bilayer graphene. [Preview Abstract] |
Wednesday, March 20, 2013 9:00AM - 9:12AM |
M6.00006: Quasiparticle Energy and Excitonic Effects of Gated Bilayer Graphene Li Yang By employing the first-principles GW-Bethe-Salpeter Equation simulation, we obtain the accurate quasiparticle (QP) band gap and optical absorption spectra of gated bilayer graphene (GBLG). Many-electron effects are shown to be extremely important for understanding these excited-state properties; enhanced electron-electron interactions dramatically enlarge the QP band gap; infrared optical absorption spectra are dictated by bright bound excitons. In particular, these QP band gaps, exciton binding energies, and even the exciton spectra can be tuned in a wide range by the gate field. Our results satisfactorily explain recent experiments. Moreover, our calculation predicts exotic excitonic effects that have not been observed yet, which can be of interest for optoelectronics applications based on GBLG. [Preview Abstract] |
Wednesday, March 20, 2013 9:12AM - 9:24AM |
M6.00007: A theoretical study of symmetry-breaking organic overlayers on single- and bi-layer graphene Josue Morales-Cifuentes, T.L. Einstein An ``overlayer'' of molecules that breaks the AB symmetry of graphene can produce (modify) a band gap in single- (bi-) layer graphene.\footnote{M. Li et al., Phys. Rev. B 76, 155438 (2007)} Since the triangular shaped trimesic acid (TMA) molecule forms two familiar symmetry breaking configurations, we are motivated to model TMA physisorption on graphene surfaces in conjunction with experiments by Groce et al. at UMD. Using VASP, with ab initio van der Waals density functionals (vdW-DF), we simulate adsorption of TMA onto a graphene surface in several symmetry-breaking arrangements in order to predict/understand the effect of TMA adsorption on experimental observables. [Preview Abstract] |
Wednesday, March 20, 2013 9:24AM - 9:36AM |
M6.00008: Vortex zero mode and charge of mass skyrmion in graphene Chi-Ken Lu, Igor Herbut We investigate the skyrmion formed by the mass order parameters in graphene and bilayer graphene. The skyrmion out of the three quantum anomalous spin Hall order parameters carries charge of 2e and 4e, respectively, in graphene and BA-stacking bilayer graphene. The origin of the above is related to the counting of vortex zero-mode and the representation of Clifford algebra imposed on the mass order parameters. The doubling of charge in bilayer case is due to the Kramers's degeneracy implied by the pseudo time-reversal symmetry, which is a result of the quadratic band touching at low-energy.\\[4pt] [1] Chi-Ken Lu and Igor F. Herbut, Phys. Rev. Lett. {\bf 108}, 266402 (2012)\\[0pt] [2] Igor F. Herbut, Chi-Ken Lu, and Bitan Roy, Phys. Rev. B {\bf 86} 075101 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 9:36AM - 9:48AM |
M6.00009: Broken Symmetry Phases in ABC Trilayer Graphene Vladimir Cvetkovic, Oskar Vafek We study the effects of electron-electron interaction in ABC-stacked trilayer graphene (TLG) within the framework of weak coupling renormalization group (RG). We find that, when the interaction is mainly in the forward scattering channel, the system orders into a gapless phase characterized by breaking of the TLG lattice mirror symmetries. A presence of small but finite back scattering changes the nature of the leading instability and results in gapped phases. The repulsive back scattering favors layered anti-ferromagnetic order, while the attractive back scattering yields the quantum spin Hall phase (gapped in bulk only). By classifying order parameters in TLG according to irreducible representations of the TLG space group, we conclude that any orders that break the rotational symmetry (e.g., the nematic state) in TLG are disfavored compared to the orders that do not break the lattice trifold rotational symmetry. The results are discussed in the context of present experiments on TLG. [Preview Abstract] |
Wednesday, March 20, 2013 9:48AM - 10:00AM |
M6.00010: Unravelling the intrinsic and robust nature of van Hove singularities in twisted bilayer graphene Felix Yndurain, Ivan Brihuega, Pierre Mallet, Hector Gonzalez-Herrero, Guy Trambly de Laissardi\`ere, Miguel Ugeda, Jose Maria G\'omez-Rodr\'Iguez, Laurence Magaud, Jean Yves Veuillen Extensive scanning microscopy and spectroscopy experiments completed by first principles and parameterized tight binding calculations provide a clear answer to the existence, origin and robustness of van Hove singularities in twisted grapheme layers. Our results are conclusive: vHs due to interlayer coupling are present in abroad range of rotation angles. From the variation of the energy separation of the vHs with rotation angle we recover the Fermi velocity of the grapheme monolayer as well as the strength of the interlayer interaction. The robustness of the vHs is assessed both by experiments and calculations which test the role of the periodic modulation and absolute value of the interlayer distance. We clarify the origin of the moir\'{e} corrugation observed in the STM images. [Preview Abstract] |
Wednesday, March 20, 2013 10:00AM - 10:12AM |
M6.00011: Study on Metal/Metal oxide/Graphene Tunnel Junctions Ke Chen, Ying Feng, Raja Khalid Zahir Metal/metal-oxide/graphene (Metal $=$ Al, Ti, Hf, Zr) tunnel junctions were fabricated by transferring single-layer graphene grown by chemical vapor deposition on Cu onto metal strips by either a wet or dry approach. The metal strips were prepared by dc magnetron sputtering through a shadow mask and were exposed to air for about 10 minutes for native oxides to grow prior to the transfer. Good tunneling properties were observed for all the junctions fabricated by either means of graphene transfer. The zero-bias resistance of these junctions all increases with time to a final value, indicating continuing oxidation of the metals with a self-limited oxidation rate. Some junctions show the final area-normalized zero-bias resistances and self-limited oxidation time scales for Al, Ti, Hf, Zr are about 0.15, 0.2, 6000, 1000 k$\Omega $cm$^{2}$ and 25, 90, 6, 9 hour, respectively. The tunneling spectra were studied at various temperature down to 4.2 K and analyzed by the Brinkman-Dynes-Rowell model to get the height and width of the tunnel barriers, taking into account the electron structure of graphene. The junctions are good candidates for chemical sensing applications. [Preview Abstract] |
Wednesday, March 20, 2013 10:12AM - 10:24AM |
M6.00012: Tunneling Spectroscopy of Graphene using Planar Pb Probes Yanjing Li, Nadya Mason We show that evaporating lead directly on graphene can create high-quality tunnel probes. By monitoring and comparing the resistances of probes made from Pb, Al and Ti/Au, we have found unique and robust behavior of the Pb probes: the contact resistance between the Pb and graphene first increases and then saturates over a time period of approximately one week. Characterization via transport measurements at low temperature shows that after oxidation a well-formed tunnel barrier is created between the Pb and the graphene. Tunneling spectroscopy using the Pb probes manifests energy-dependent features such as scattering resonances and localization behavior, and can thus be used to probe the microscopic electronics of graphene. [Preview Abstract] |
Wednesday, March 20, 2013 10:24AM - 10:36AM |
M6.00013: Mg/MgO/Graphene Tunnel Junctions Made by Dry Transfer of Graphene in Vacuum Ying Feng, Ke Chen Mg/MgO/Graphene junctions were fabricated by dry transfer of single layer graphene film grown by chemical vapor deposition on Cu Mg strips were deposited onto Si/SiO$_{2}$ or glass substrates by thermal evaporation through a shadow mask. The tunnel barrier MgO was formed by exposing deposited Mg for about 10 minutes in air prior to the graphene transfer. To prevent degradation of MgO by liquids, a dry transfer technique is used. First a graphene film was transfer onto a free-standing 4$\mu $m-thick Cu film using the traditional wet method, then pressed onto a transparent and flexible PDMS stamp followed by etching away the Cu film in FeCl$_{3}$ solution, and finally stamped onto the Mg strips in vacuum to prevent any gas bubbles that may form between graphene and Mg strips. The dry-transferrd graphene has similar properties to traditional wet-transferred graphene, characterized by scanning electron microscopy, atomic force microscopy, Raman spectroscopy, and transport measurements. It has a sheet resistance of 1.6 $\sim$ 3.4 k$\Omega$/$\Box$, charge carrier density of 4.1 $\sim$ 5.3 $\times$ 10$^{12}$ /cm$^{2}$ and mobility of 460 $\sim$ 760 cm$^{2}$/Vs without doping at room temperature. Mg/MgO/graphene junctions show good tunneling characteristics at temperatures down to 4.2 K. The barrier height and width were obtained by fitting with the Brinkman-Dynes-Rowell trapezoid-shaped barrier model with consideration of graphene electron structure. [Preview Abstract] |
Wednesday, March 20, 2013 10:36AM - 10:48AM |
M6.00014: Intrinsic Dirac Point Energy Level and Band Offset of Graphene/SiO$_2$ interface Kun Xu, Caifu Zeng, Kang Wang, Qin Zhang, Peide Ye, Rusen Yan, Alan Seabaugh, Huili Xing, John Suehle, Curt Richter, David Gundlach, Nhan Nguyen Advancing toward the rational design, fabrication, and implementation of graphene(GR)-based electronic and optical devices, the intrinsic barrier height of undoped GR (the Dirac point of GR to the conduction band(CB) edge of an insulator), as well as the intrinsic work function(WF) of GR must be accurately determined. We present an internal photoemission (IPE) investigation of a unique semi-transparent metal/high-k/GR/SiO$_{2}$/Si structure, and focus our study on the photoemission phenomena at the GR/SiO$_{2}$ interface. By taking advantage of the optical interference of the SiO$_2$ cavity, the enhanced photoemission from GR was observed. As a result, a complete electronic band alignment at the GR/SiO$_{2}$/Si interfaces is established. The intrinsic positions of the undoped GR Dirac point with respect to the CB of SiO$_2$, 3.58 eV (Al$_2$O$_3$ TG) and 3.60 eV (HfO$_2$ TG), are obtained. The intrinsic WF of graphene is found to be 4.50 eV. The determination of the WF of GR is of significant importance to the engineering of GR-base devices and the IPE spectroscopy, combined with specific interference cavity structures, would be a valuable measurement technique for other GR-like2-D material systems. [Preview Abstract] |
Wednesday, March 20, 2013 10:48AM - 11:00AM |
M6.00015: Point Contacts to Graphene for Corbino Disk Geometry Devices Bin Cheng, Peng Wang, Lei Jing, Chun Ning Lau, Marc Bockrath A Corbino disk geometry raises new possibilities for observing novel phenomena for Dirac electrons in graphene [1]. For example, Recent theoretical work has suggested the possibility of observing a quantum relativistic Corbino effect in which the conductance of a graphene layer measured in a Corbino disk geometry shows magneto-oscillations related to to the number of flux quanta threading the area of the disk [2]. We will discuss a technique we have developed for making air-bridge contacts to graphene layers based on a multi-layer resist technique [3]. The air bridge enables a Corbino disk geometry in the absence of topside dielectric layers, potentially facilitating annealing techniques in conjunction with placement on, for example, BN substrates to enable high mobility devices. The latest transport results will be discussed.\\[4pt] [1] Zhao et al., Phys. Rev. Lett. 108, 106804 (2012). \newline [2] Rycerz, Phys. Rev. B 81, 121404?[U+0351]R (2010). \newline [3] Liu et al., Appl. Phys. Lett. 92, 203103 (2008). [Preview Abstract] |
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