Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session U8: Focus Session: Scanning Tunneling Microscopy of Graphene |
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Sponsoring Units: DMP Chair: Eva Andrei, Rutgers University Room: 307 |
Thursday, March 21, 2013 11:15AM - 11:51AM |
U8.00001: Defect engineering of graphene Invited Speaker: Lin He One of the most fascinating aspects of graphene is that its topological features of the electronic states can be fundamentally changed by modifying its local lattice structure. In this talk, I will show how to tune the electronic structures of graphene by defect engineering: (1) we observed superlattice Dirac points and space-dependent Fermi velocity in a corrugated graphene monolayer; (2) we reported angle dependent van Hove singularities (VHSs) of slightly twisted graphene bilayer; (3) we studied the evolution of local electronic properties of twisted graphene bilayer induced by a strain; The strain results in pseudo-Landau levels, which mimic the quantization of massive Dirac fermions in a magnetic field of about 100 T, and valley polarization along a strained graphene wrinkle. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U8.00002: Probing Interactions between Graphene and Cu(111) Surface State Liuyan Zhao, Scott Goncher, George Flynn, Abhay Pasupathy Monolayer graphene and the surface state of Cu(111) are both examples of two-dimensional electronic states. The quasiparticles in monolayer graphene behave as massless Dirac fermions, whereas the ones in the Cu(111) surface state obey the non-relativistic Schrodinger equation. What is the nature of the interactions when these two states are coupled electronically? We probe these interactions using Scanning Tunneling Microscopy/Spectroscopy (STM/S) to investigate how the Cu(111) surface state has been modified with monolayer graphene overlaid on it. In this presentation, we will show that graphene decreases the band width of the Cu(111) surface state and renormalizes the effective mass of the quasiparticles in the Cu(111) surface state. Further, we will show that the modification of the Cu(111) surface state is independent of the registry between Cu(111) and the graphene crystalline orientations. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U8.00003: Visualizing the influence of an isolated Coulomb impurity on the Landau level spectrum in graphene using scanning tunneling microscopy Adina Luican-Mayer, Maxim Kharitonov, Guohong Li, ChihPin Lu, Ivan Skachko, Alem-Mar Goncalves, Eva Y. Andrei Charged impurities play a crucial role in determining the electronic properties of graphene. We report on experiments that elucidate the effect of an isolated charged impurity on the electronic spectrum of graphene in a magnetic field. Using scanning tunneling microscopy and gated graphene devices, we follow the evolution of quantized Landau levels with carrier density and find that the apparent strength of the impurity is controlled by the partial filling of the Landau levels. At low filling the impurity is cloaked and becomes essentially invisible. The cloaking effect diminishes with filling until, for fully occupied Landau levels, the impurity reaches its maximum strength causing a significant perturbation in the local density of states. In this regime we report the first observation of Landau level splitting due to lifting of the orbital degeneracy. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U8.00004: STM/STS study of graphene directly grown on h-BN films on Cu foils Won-Jun Jang, Min Wang, Seong-Gyu Jang, Minwoo Kim, Seong-Yong Park, Sang-Woo Kim, Se-Jong Kahng, Jae-Young Choi, Young Jae Song, Sungjoo Lee Graphene-based devices on standard SiO2 substrate commonly exhibit inferior characteristics relative to the expected intrinsic properties of graphene, due to the disorder existing at graphene-SiO2 interface. Recently, it has been shown that exfoliated and chemical vapor deposition (CVD) graphene transferred onto hexagonal boron nitride (h-BN) possesses significantly reduced charge inhomogeneity, and yields improved device performance. Here we report the scanning tunneling microscopy (STM) and spectroscopy (STS) results obtained from a graphene layer directly grown on h-BN insulating films on Cu foils. STS measurements illustrate that graphene/h-BN film is charge neutral without electronic perturbation from h-BN/Cu substrate. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U8.00005: Kondo quantum criticality in graphene Jinhai Mao, Ivan Skachko, Guohong Li, Eva Andrei The Kondo effect, observed in the presence of coupling between a local magnetic moment and spin degenerate conduction electrons, is a hallmark of the electronic transport in conventional metallic systems. Screening of the local moment gives rise at low temperatures to characteristic signatures in the density of states and electronic spectral function such as the Kondo resonance. Graphene a strictly two dimensional system with carriers whose electronic properties mimic massless Dirac fermions provides a new paradigm for studying interactions in a system where the density of states is linear and can be made vanishingly small by gating, rather than being constant as is the case in standard metallic systems. We study the effect of interactions between the ultra-relativistic electrons in graphene and local magnetic moments introduced by point vacancies in the honeycomb lattice of graphene. Using scanning tunneling spectroscopy and transport measurements we measure the Kondo quantum critical transition and its dependence on carrier density. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U8.00006: Structure and magnetism of cobalt intercalated graphene/Ir(111) via spin-polarized STM Regis Decker, Jens Brede, Nicolae Atodiresei, Vasile Caciuc, Stefan Bluegel, Roland Wiesendanger The presence of intercalation compounds in graphite, i.e. impurities or layer(s) trapped between carbon sheets, can lead to changes in the transport, optical and catalytic properties compared to bulk graphite, or even superconductivity. Here, we present the local structure and magnetic properties of graphene on a magnetic substrate, resolved by spin-polarized STM. The magnetic substrate is obtained by the intercalation of a cobalt layer between graphene and an Ir(111) surface. The atomic structure of the graphene layer is dominated by a highly corrugated Moir\'{e} pattern, which arises due to the incommensurability and/or twisting angle of the graphene lattice and the Co/Ir(111) surface. Within the Moir\'{e} unit cell three different regions, i.e. top, fcc, and hcp regions are identified. Interestingly, these regions show very different electronic and magnetic signatures in the experiments, defining an atomic-scale magnetic Moir\'{e} pattern. The observed spin polarization is compared to density functional theory calculations. The calculations reveal that the bonding between the graphene layer and intercalated Co layer varies from weak to strong within the Moir\'{e} unit cell. Moreover, the interaction between the graphene and the intercalated cobalt layer leads to a spin dependent charge rearrangement, which induces magnetism in graphene as observed in experiment. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U8.00007: STM/STS studies of Ca-intercalated bilayer graphene Ryota Shimizu, Katsuaki Sugawara, Kohei Kanetani, Katsuya Iwaya, Takafumi Sato, Takashi Takahashi, Taro Hitosugi We have performed low temperature scanning tunneling microscopy/spectroscopy (STM/STS) measurements on a two-dimensional Ca-intercalated bilayer graphene epitaxially grown on a 6H-SiC(0001) substrate. The STM topographic images clearly resolve each intercalated Ca atom with graphene-based honeycomb lattice. Furthermore, we found a clear $\times$2.5 modulation in the topography, implying charge density wave or Moir\'{e} pattern originated from the interaction with the SiC substrate. Comparison with ARPES measurements provided us of further insight into the Fermi surface deduced from STS. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U8.00008: Observing Atomic Collapse in Graphene Yang Wang, Dillon Wong, Andrey Shytov, Victor Brar, Sangkook Choi, Qiong Wu, Hsin-zon Tsai, William Regan, Alex Zettl, Roland Kawakami, Steven Louie, Leonid Levitov, Michael Crommie Relativistic quantum mechanics predicts that super-heavy atoms possess unique properties not shared by ordinary atoms. In particular, a very strong electric field around the nucleus should result in ``atomic collapse,'' with an electron component falling onto the nucleus and a positron component escaping to infinity. Predicted by Dirac 80 years ago, atomic collapse has thus far remained experimentally out of reach using accelerator-based techniques. Here we report the observation of atomic collapse on gated graphene devices. The energy and spatial dependence of the atomic collapse state was measured using scanning tunneling microscopy (STM). [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U8.00009: Molecular adsorbates on HOPG: Toward modulation of graphene density of states Michelle Groce, Theodore Einstein, William Cullen Ordered molecular superlattices, particularly those made of planar aromatics with their attendant pi orbitals, have the potential to break the graphene sublattice degeneracy and create a band gap. Trimesic acid (TMA) is a promising candidate due to its self-assembly into symmetry-breaking superlattices nearly commensurate with that of graphene. We have used the graphite (0001) surface as a model system to explore the impact of TMA thin films on band structure. By examining correlations between STM topography and STS maps of corresponding regions, we are able to investigate the effects of TMA on the local density of states. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U8.00010: ABSTRACT WITHDRAWN |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U8.00011: Current and voltage dependent interactions between a scanning tunneling microscopy tip and a freestanding graphene sample Kevin Schoelz, Peng Xu, Steven Barber, Matt Ackerman, Paul Thibado The two dimensional nature of graphene gives rise to a number of unique properties. Chief among them are the ability to manipulate the electronic properties using mechanical deformations, opening a new field of ``straintronics.'' Previous work from our group demonstrated the ability to manipulate a freestanding graphene sample with atomic precision using electromagnetic manipulation scanning tunneling microscopy (EM-STM). In the EM-STM technique, the tip bias is ramped over a predetermined range while maintaining a constant tunneling current. The resulting change in height of the tip is then recorded. Typical EM-STM measurements show quick movement of the sample between 0.1-1.0 V, and then slower movement after this point. The height of this final plateau is dependent on the tunneling current. To look for the cause of this current dependence $z(I)$ curves taken at a constant tip bias were examined. It was found that at low tip bias (0.1-0.5 V) the sample drops between 10-20 nm, while at high tip bias (1.0-3.0 V) the sample only drops 2-3 nm. This current dependence is attributed to a drop in the electrostatic force as the tip approaches the sample and holes in the benzene rings become more important. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U8.00012: Gate-controlled modification of molecular electronic structure at the surface of graphene Alexander Riss, Sebastian Wickenburg, Hsin-Zon Tsai, Liang Tan, Miguel Moreno Ugeda, Aaron Bradley, Alex Zettl, Steven G. Louie, Felix R. Fischer, Michael F. Crommie Understanding the behavior of adsorbed molecules on graphene is important for a variety of reasons, including the fact that they can potentially be used to modify the optical, electronic, catalytic, and magnetic properties of graphene devices. Here we show how gate-induced shifting of the Fermi level of a single graphene layer can be used to induce electronic changes in adsorbed molecules. We have used scanning tunneling microscopy and spectroscopy to characterize the structure and electronic properties of 3,3',3''-(Benzene-1,3,5-triyl)tris(2-cyanoacrylonitrile) (BTC) molecules adsorbed onto the surface of a back-gated graphene device. We observe that the energy (with respect to the Fermi level) of the lowest unoccupied molecular orbital (LUMO) of individual BTC molecules can be tuned by application of a gate voltage. These results show the potential to control the physical and chemical properties of adsorbates via electrostatic gating. [Preview Abstract] |
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