Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session T2: Defects and Strain in Graphene |
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Sponsoring Units: DCMP Chair: Brian LeRoy, University of Arizona Room: Ballroom A2 |
Wednesday, March 23, 2011 2:30PM - 3:06PM |
T2.00001: Electron Interactions in Graphene Invited Speaker: Electrons confined in two dimensions (2D) can exhibit strongly correlated states. Recent experimental discovery of integer and fractional quantum Hall effect in graphene amplified interest in correlated 2D electronic systems, owning to presence of the unusual topological phase associated with zero effective mass of charge carriers. In this talk, we will discuss the role of the many-body effects due to the electron-electron interaction in graphene manifested in electron transport phenomena. In particular, we will discuss the nature unusual spontaneous symmetry breaking Landau levels graphene under the extreme quantum condition, the appearance of unique low density insulating states and fractional quantum Hall states. Employing extremely high quality samples obtained by suspending graphene and graphene on atomically flat defect free insulating substrate such as hexa-bron nitride, we now investigate various broken symmetry states under high magnetic field. The nature of these broken symmetry state can be explained generally considering underlying SU(4) symmetry in the single particle level of the Landau levels. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:42PM |
T2.00002: Spatially Resolving Spin-split Edge States of Chiral Graphene Nanoribbons Invited Speaker: A central question in the field of graphene-related research is how graphene behaves when it is patterned at the nanometer scale with different edge geometries. The most fundamental shape in this regard is the graphene nanoribbon (GNR), a narrow strip of graphene that is characterized by its width and chirality. GNRs have been predicted to exhibit a wide range of behavior that includes tunable energy gaps and unique 1D edge states with unusual magnetic structure. I will discuss a scanning tunneling microscopy and spectroscopy (STS) study of GNRs that allows us to examine how GNR electronic structure depends on the chirality of atomically well-defined GNR edges. Our STS measurements reveal the presence of 1D GNR edge states that closely match theoretical expectations for GNRs of similar width and chirality. We additionally observe width-dependent energy splitting in GNR edge states, providing compelling evidence of their magnetic nature. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 4:18PM |
T2.00003: Graphene under a tip Invited Speaker: The strictly two dimensional structure of graphene results in 2D charge carriers that are readily accessible by surface probes such as scanning tunneling microscopy (STM) and spectroscopy (STS), and in electronic properties that can be controlled through doping, strain and external potentials. At the same time the 2D structure causes graphene to be extremely sensitive to environmental disturbances. I will describe STM, STS and magneto-transport experiments showing that when graphene is decoupled from substrate-induced potential fluctuations the intrinsic properties of the carriers become apparent. This is clearly seen in suspended graphene devices where, in the absence of substrate induced potential fluctuations, electron-electron interactions lead to a fractional quantum Hall effect and to an insulating phase at the Dirac point [1]. We find that even for non-suspended graphene it is possible to find non-invasive substrates on which one can directly observe the sequence of quantized Landau levels [2] and to track their evolution with field and doping down to the Dirac point where interaction effects kick in [3]. When the ``substrate'' is another graphene layer with relative orientation other than that of the standard Bernal stacking, it can profoundly affect the electronic density of states transforming it from the linear massless Dirac spectrum to one containing prominent Van Hove singularities which are controlled by the degree of twist between the layers [4].\\[4pt] [1] X. Du, I. Skachko, F. Duerr, A. Luican, EYAndrei, Nature \textbf{462}, 192 (2009)\\[0pt] [2] G. Li, A. Luican and E. Y. Andrei, Phys. Rev. Lett. 102, 176804 (2009).\\[0pt] [3] A. Luican, G. Li, and E. Y. Andrei, Phys Rev. B (R) (2011)\\[0pt] [4] G. Li, A. Luican and E. Y. Andrei, Nature Physics 6, 109 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:54PM |
T2.00004: High Resolution Tunneling Spectroscopy of Graphene in Strong and Weak Disorder Potentials Invited Speaker: Using scanning tunneling spectroscopy (STS), the local density of states can be mapped in real space to give insight into the role the local disorder potential plays in determining the 2-dimensional electron gas (2DEG) properties. In this talk I describe studies using scanning tunneling spectroscopy to examine various graphene systems with varying degrees of disorder. The amount of disorder depends on how the graphene was made. In the growth of graphene on the Si-face termination of SiC in UHV, local defects are found which contribute to strong inter- and intra-valley scattering [1]. Medium disorder is found in exfoliated graphene on SiO$_{2}$. Using a back-gated exfoliated graphene device on SiO$_{2}$ we observe a Landau level spectrum and charging resonances [2] that are completely different from previous STS measurements on weak disorder graphene systems. Applying a gating potential allows us to obtain ``STS gate maps'' which show the graphene 2DEG breaking up into a network of interacting quantum dots formed at the potential hills and valleys of the SiO$_{2}$-induced disorder potential. Graphene grown on the C-face termination of SiC is shown to have weak disorder with Landau level line widths approaching thermal limits at liquid He temperatures [3]. Using a new STM system operating at 10 mK, we are able to resolve a graphene ``quartet'' of the N=1 Landau level [4]. The quartet structure shows the complete lifting of the valley and spin degeneracies, which we determine as a function of magnetic field. \\[4pt] [1] \textit{Scattering and Interference in Epitaxial Graphene}, G.~M.~Rutter, J.~N.~Crain, T.~Li, P.~N.~First, and J.~A.~Stroscio, \textit{Science} \textbf{317, }5835, 219 (2007). \\[0pt] [2] \textit{Evolution of Microscopic Localization in Graphene in a Magnetic Field: From Scattering Resonances to Quantum Dots}, S. Jung, G. M. Rutter, N. N. Klimov, D. B. Newell, I. Calizo, A. R. Hight-Walker, N. B. Zhitenev, and J. A. Stroscio, (Nature Physics in press DOI:10.1038). \\[0pt] [3] \textit{Observing the Quantization of Zero Mass Carriers in Graphene}, D. L. Miller, K. D. Kubista, G. M. Rutter, M. Ruan,W. A. de Heer, P. N. First, and J. A. Stroscio, Science \textbf{324}, 924 (2009). \\[0pt] [4] \textit{High Resolution Tunneling Spectroscopy of a Graphene Quartet}, Y. Jae Song, A. F. Otte, Y. Kuk, Y. Hu, D. B. Torrance, P. N. First, W. A. de Heer, H. Min, S. Adam, M. D. Stiles, A. H. MacDonald, and J. A. Stroscio, Nature \textbf{467}, 185 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:30PM |
T2.00005: Influence of edge structure, substrate structure and grain boundaries on the electronic properties of graphene quantum dots and transferred graphene Invited Speaker: We have used UHV STM to study the quantum size effect gap and the effects of edge electronic structure on graphene quantum dots (GQDs) and nanoribbons [1]. GQDs on H-Si(100) exhibit the expected size-dependent gap with the exception of those with predominantly zigzag edges, which are metallic. STM spectroscopy elucidates the predicted zigzag metallic edge state, which has a characteristic decay length of 1nm. Monolayer graphene deposited in UHV on cleaved GaAs(110) and InAs(110) substrates exhibits an electronic semitransparency effect in which the substrate electronic structure can be observed `through' the graphene [2]. This effect is observed when the equilibrium graphene-substrate spacing is reduced by about 0.06nm. We have also studied the grain boundaries in graphene monolayers that have been grown on copper and then transferred to silicon dioxide substrates. On the annealed copper foils, we find many crystallographic facets, grain boundaries, and annealing twins, all of which affect the carbon species nucleation. Graphene does not grow as readily on the foil annealing twins and non-primary crystal facets, leading to varying nucleation and graphene grain boundaries in the transferred film. STM images show graphene misorientation angles of approximately 7\r{ }, 23\r{ }, and 30\r{ } at the grain boundaries. Standing wave patterns with a decay length on the order of 1 nm were observed adjacent to the grain boundaries and depend on the structure of the boundary. Spectroscopy across the boundaries showed enhanced conduction in empty states on the grain boundaries. \\[4pt] [1] K.A. Ritter and J.W. Lyding, Nat. Mat. \textbf{8}, 235 (2009). \\[0pt] [2] K.T. He, J.C, Koepke, S. Barraza-Lopez and J.W. Lyding, Nano Lett. \textbf{10}, 3446 (2010). [Preview Abstract] |
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