Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session W25: Focus Session: Graphene XV: Scanning Probes II and Hall Effect |
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Sponsoring Units: DMP Chair: Pablo Jarillo-Herrero, Massachusetts Institute of Technology Room: 327 |
Thursday, March 19, 2009 11:15AM - 11:27AM |
W25.00001: Atomically Flat Graphene on Mica Substrates Li Liu, Chun Hung Lui, Kin Fai Mak, George Flynn, Tony Heinz Much recent interest has focused on the question of the intrinsic flatness of monolayers of exfoliated graphene. In studies of both suspended graphene [\textit{Meyer et al, Nature 446 (2007)}] and graphene deposited on SiO$_{2}$ substrates [\textit{Stolyarova et al, PNAS 104 (2007)}], graphene monolayers exhibited clear variations in height. For suspended films, this variation was attributed to an intrinsic rippling instability [\textit{Meyer et al, Nature 446 (2007)}]. In the case of graphene on SiO$_{2}$ substrates, the role of intrinsic and substrate- induced effects remained unclear because of the corrugation of the substrate. In this paper we present results of a detailed study of the morphology of exfoliated graphene monolayers deposited on the atomically flat terraces of cleaved mica surfaces. Using high-resolution atomic force microscopy (AFM), we demonstrate that graphene monolayers on mica, when measured with lateral spatial resolution of $\sim $ 6nm, are flat to over micron lateral length scales to within the instrumental sensitivity of 50 pm. These results stand in sharp contrast to the behavior reported for both suspended graphene and graphene on SiO$_{2}$ substrates. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W25.00002: Scattering in monolayer graphene on SiO$_{2 }$observed by STS Aparna Deshpande, Brian LeRoy, Wenzhong Bao, Feng Miao, Chun Ning Lau The intrinsic ripples in graphene and its distinctive band structure make graphene a novel two dimensional system with intriguing structural and electronic properties. To probe the influence of graphene structure on its electronic properties we have carried out scanning tunneling spectroscopy (STS) measurements on exfoliated graphene on SiO$_{2}$ with an ultra high vacuum scanning tunneling microscope (UHV STM) at 4.2 K. Atomically resolved local density of states (LDOS) images show an interference pattern due to scattering. 2D Fourier transforms of the LDOS maps reveal two types of scattering wave vectors corresponding to long range intravalley scattering and short range intervalley scattering. Intervalley scattering due to short range potential variations leads to a $\sqrt 3 \times \sqrt 3 $ R30$^{o}$ interference pattern in the LDOS while intravalley scattering causes long range disorder in the LDOS images. Our measurements present a comprehensive picture of scattering mechanisms in exfoliated graphene and underline the contribution of random impurities, defects and SiO$_{2}$ morphology to the electronic properties of graphene. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 12:15PM |
W25.00003: STM on Gate-Tunable Graphene Invited Speaker: We have successfully performed atomically-resolved scanning tunneling microscopy and spectroscopy (STS) on mechanically exfoliated graphene samples having tunable back-gates. We have discovered that the tunneling spectra of graphene flakes display an unexpected gap-like feature that is pinned to the Fermi level for different gate voltages, and which coexists with another depression in density-of-states that moves with gate voltage. Extensive tests and careful analysis show that the gap-feature is due to phonon-assisted inelastic tunneling, and the depression directly marks the location of the graphene Dirac point. Using tunneling spectroscopy as a new tool, we further probe the local energetic variations of the graphene charge neutral point (Dirac point) to map out spatial electron density inhomogeneities in graphene. Such measurements are two orders of magnitude higher in resolution than previous experiments, and they can be directly correlated with nanometer scale topographic features. Based on our observation of energy-dependent periodic electronic interference patterns, our measurements also reveal the nature of impurity scattering of Dirac fermions in graphene. These results are significant for understanding the sources of electron density inhomogeneity and electron scattering in graphene, and the microscopic causes of graphene electron mobility. [Preview Abstract] |
Thursday, March 19, 2009 12:15PM - 12:27PM |
W25.00004: Effect of disorder on electron tunneling in graphene layers through potential barriers Vrinda Thareja, Manish Sharma, Sankalpa Ghosh Electrons at the fermi level in Graphene monolayer behave like massless Dirac fermions. Using a coherent potential approximation, we study the tunneling of such electrons through a double barrier potential in presence of disorder. We subsequently extend this study in the case of periodic lattice potentials. Our approach involves using the Green's function calculation and is particularly amenable to studying the effect of disorder, impurities and defects on electron propagation through Graphene. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W25.00005: Scanning Gate Microscopy on Patterned Graphene Nanoribbons Andrei Garcia, Markus Koenig, Kathryn Todd, David Goldhaber-Gordon Graphene-based electronic devices are of interest due to the unique nature of the graphene band structure. Bulk graphene exhibits a gapless linear dispersion near the Fermi level. When graphene is etched to form a narrow ribbon a transport gap opens at the Dirac point. The origin of this transport gap in patterned graphene nanoribbons remains an unresolved problem. Two possible explanations for the origins of the gap are confinement in the direction perpendicular to the length of the ribbon and localization due to disorder along imperfectly formed ribbon edges. We explore the local properties of this gap in nanoribbons using a scanning gate microscope. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W25.00006: Scanning tunneling microscopic (STM) studies of strain-induced local density of states modulations in single-layer graphene on SiO$_{2}$ A.P. Lai, M.L. Teague, C.R. Hughes, A.D. Beyer, N.-C. Yeh, M.W. Bockrath, J. Velasco, C.N. Lau We report strain-induced spatial modulations in the electronic density of states (DOS) of single-layer graphene on SiO$_{2}$. Spatially resolved topographic and spectroscopic measurements were performed simultaneously at 77 K and at pressures $<$ 10$^{-7}$ torr. Fourier transformation of local topography shows a distorted hexagon with lattice vectors ranging from a$_{0}$=3.0 $\pm $0.2{\AA} to 2.1$\pm $0.2 {\AA} as the result of surface corrugation from the roughness of the underlying substrate. A spatially varying strain map derived from local distortions of the lattice constants correlates well with the surface topography. Strained graphene, due to three dimensional surface corrugations of $\pm $ 5 {\AA} over 10 nm lateral distance, show parabolic ``U-shaped'' conductance vs. biased voltage spectra rather than the Dirac-like ``V-shaped'' spectra. In contrast, for regions of relaxed graphene, Dirac-like spectra are recovered. The Dirac voltage, V$_{D}$, determined from the biased voltage of conductance minimum, appears to be position independent at V$_{D}$=36$\pm $5 meV, while the minimum conductance and the degree of derivation from the Dirac-like spectra at low energies appear to correlate directly with the topography. This work was supported by NSF/NRI under Caltech/CSEM. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W25.00007: SPM measurements of graphene corrugation and spatial correlation William Cullen, Jianhao Chen, Masa Ishigami, Ellen Williams, Michael Fuhrer In order to determine the effect of graphene corrugation on electronic transport, it is most important to know the spatial correlation properties of the corrugated graphene structure. In spite of much experimental effort, there is still contentious debate about the structure of graphene, both in supported and suspended geometries. It has frequently been asserted that a graphene monolayer exfoliated onto a SiO2 substrate may display ``intrinsic'' corrugation -- rippled structure which is not derived from the topography of the underlying substrate. Here, we report recent UHV NC-AFM and STM results which show that anomalous corrugation may be observed due to local interaction between the tip and the graphene monolayer. Our results show that non-perturbative NC-AFM measurement reveals a graphene topography which is as smooth as the underlying SiO2, with height-height correlation exponent 2H = 1. STM measurement of graphene, due to uncontrolled tip-sample forces, may exhibit anomalous corrugation depending on tip condition. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W25.00008: Probing localized states in graphene by Scanning Gate Microscopy Markus Koenig, Andrei Garcia, Kathryn Todd, David Goldhaber-Gordon In various experiments, patterned graphene devices like nano-ribbons exhibit a complete suppression of transport for the Fermi level in the vicinity of the Dirac point. However, the experimental results cannot be explained purely by confinement based on the geometry of the devices. Usually, the transport gap is wider than would be expected for an energy gap based only on the confinement. Among other mechanisms, localization of charge carriers at imperfect sample edges has been proposed to contribute to the increased transport gap. We use Scanning Gate Microscopy to explore localized states at etched edges and within constrictions, and discuss their impact on the transport in patterned graphene devices. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W25.00009: Phonon mediated tunneling into graphene Tim Wehling, Ilya Grigorenko, Alexander Lichtenstein, Alexander Balatsky Recent scanning tunneling spectroscopy experiments [V. W. Brar et al., Appl. Phys. Lett. 91, 122102 (2007); Y. Zhang et al., Nature Phys. 4, 627 (2008)] on graphene reported an unexpected gap of about $\pm 60$\,meV around the Fermi level. Here, we give a theoretical investigation explaining the experimentally observed spectra and confirming the phonon mediated tunneling as the reason for the gap: We study the real space properties of the wave functions involved in the tunneling process by means of ab-initio theory and present a model for the electron-phonon interaction, which couples the graphene's Dirac electrons with quasi free electron states at the Brillouin zone center. The self-energy associated with this electron-phonon interaction is calculated and its effects on tunneling into graphene are discussed. In particular, good agreement of the tunneling density of states within our model and the experimental d$I$/d$U$ spectra is found. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W25.00010: Infrared Hall Conductivity in Graphene C.T. Ellis, M.-H. Kim, T. Wu, G. Sambandamurthy, J. Cerne, V. Lee, S. Banerjee Among the many different techniques which have revealed graphene's remarkable properties, infrared conductivity ($\sigma_{\mathrm{xx}}$) (Jiang, PRL 2007) and the DC Hall effect (Novoselov, Nature 2005; Zhang, Nature 2005; Zhang, PRL 2006) have provided new insights into this material. In our study we determine the infrared Hall conductivity ($\sigma_{\mathrm{xy}}$) for graphene in the 120-1000 meV range at temperatures down to 7K and magnetic fields up to 7T using Faraday measurements. Unlike $\sigma_{\mathrm{xx}}$, which measures the sum of the optical responses for left and right circularly polarized light, $\sigma_{\mathrm{xy}}$ measures the difference and therefore is sensitive to small changes in symmetry. We compare graphene samples that are prepared using several methods, including cleaving from parent materials such as highly ordered pyrolytic graphite, as well as sonication-assisted solution-phase exfoliation of natural flake graphite powder. The films are then deposited onto $Si/SiO_2$ substrates for infrared measurements. This work is supported by the NSF-CAREER-DMR0449899, also GS and SB thank the UB-IRDF for financial support. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W25.00011: Optical Hall conductivity in QHE systems Takahiro Morimoto, Yasuhiro Hatsugai, Hideo Aoki While the quantum Hall effect is among the most remarkable static properties of two-dimensional electron systems at low temperatures in magnetic fields, recent advances in optics in the THz region make spectroscopic measurements of the Hall angle possible in magnetic field of a few tesla. So a natural question we pose here is: can the quantum Hall effect, a topological phenomenon, evolve into an ``optical Hall conductivity'' in the ac regime, especially in the THz region which is the cyclotron energy scale. Motivated by this, we have theoretically calculated the optical Hall conductivity $\sigma_{xy}(\omega)$ for the ordinary quantum Hall system with Kubo formula, where the effect of disorder is taken into account with the self- consistent Born approximation. The result shows that the Hall plateaus do remain in the optical (THz) region when the disorder is not too large. Next we have extended the calculation to the graphene QHE system, and found that the optical Hall conductivity $\sigma_{xy}(\omega)$ reflects the massless Dirac dispersion and the associated Landau level structure. While the Hall palteaus are again retained in the ac region against disorder, the structure at the central ($N=0$) Landau level is particularly robust. We predict such phenomena should be measurable through an accurate detection of the Hall angle in the THz regime. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W25.00012: Magnetically induced low density phases near the Dirac point Xu Du, Ivan Skachko , Eva Y. Andrei We demonstrated techniques for suspending graphene that isolate the charge carriers from substrate-induced potential fluctuations. The suspended samples provide access to the intrinsic properties of pristine graphene close to the Dirac point. Magneto-transport measurements on these samples show low temperature mobilities exceeding 200,000 cm$^{2}$/Vs for carrier densities below 5x10$^{9}$ cm$^{-2}$, values not attainable in semiconductors or non-suspended graphene$^{\ast }$. At sub-Kelvin temperatures and in magnetic fields up to 7T, we observed unconventional quantum Hall plateaus indicative of interaction effects. Near the Dirac point, coulomb blockade-like transmission was observed. $^{\ast }$\underline { }Xu Du, Anthony Barker, Ivan Skachko, and Eva Y. Andrei, \textit{Nature Nanotechnology}, Vol.3, 491, 2008 [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W25.00013: Enhancement of nearest neighbor spin-singlet correlations in d-wave SNS graphene Josephson junctions Annica Black-Schaffer, Sebastian Doniach Using the self-consistent tight-binding Bogoliubov-de Gennes (BdG) formalism we investigate the effect of nearest neighbor spin-singlet bond (SB) correlations in a graphene SNS Josephson junction with d-wave superconducting contacts. All p$\pi$-bonded planar organic molecules, of which graphene is the infinite extension, show a preference for SB over polar configurations, as originally captured by Pauling's idea of resonating valence bonds. At strong enough coupling and/or high doping levels, these correlations will give rise to a d-wave superconducting state. However, the estimated coupling strength in graphene would require a doping level not currently experimentally achievable by a gating bias. We demonstrate that by creating a graphene SNS Josephson junction with d-wave contacts, for example by depositing a high-Tc cuprate on top of the graphene, it should be possible to enhance the effect of the SB correlations and see clear signatures of d-wave pairing in proximity effect, superconducting decay length, and supercurrent. [Preview Abstract] |
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