Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Q25: Focus Session: Graphene IX: Structure and Strain |
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Sponsoring Units: DMP Chair: Antonio Castro Neto, Boston University Room: 327 |
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q25.00001: Interaction of magneto-excitons with phonons in graphene and graphite Jun Yan, Trevor David Rhone, Sarah Goler, Melinda Han, Vittorio Pellegrini, Philip Kim, Aron Pinczuk We study the Landau levels of graphite and single layer graphene by measurements of the magneto-phonon resonance effect in which there is coupling between the inter-Landau level magneto-exciton with the long wavelength optical phonon (G band). In graphite the G band displays a rich line shape evolution as the magnetic field is finely tuned between 5 and 7 Tesla. These observations indicate that the G band is resonantly coupled to the magneto-excitions at these fields. In the interpretation we postulate that the anticrossing of the phonon band with the inter-Landau level transitions results in a mode-splitting at around 6.2 T. The evolution of the energy and spectral weight of the two coupled modes indicates that the phonon is a probe of the unique structure of Landau levels in graphene-related materials. In an as-prepared single-layer graphene, much smaller changes are observed for fields reaching as high as 12 Tesla. [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q25.00002: Klein Backscattering and Fabry-Perot Interference in Graphene Heterojunctions Andrei Shytov, Mark Rudner, Leonid Levitov Fabry-Perot (FP) interference in a lateral p-n-p structure in graphene is proposed as a vehicle to probe Klein scattering phenomenon [Phys. Rev. Lett. 101, 156804 (2008)]. In ballistic regime, interference between waves scattered from p-n interfaces leads to oscillations in conductance as a function of electron density. Perfect transmission at zero incidence angle (Klein effect) implies a sign change of the backreflection amplitude. This change contributes a phase~$\pi$ to interference and shifts FP fringes by half a period. Alternatively, the $\pi$ phase can be understood as Berry's phase accrued by electron bouncing between p-n boundaries. This effect is revealed in the evolution of fringes when a relatively weak, non-quantizing magnetic field is applied. The behavior of the interference fringes recently observed by Young and Kim (arXiv:0808.0855) is consistent with this picture. The observed crossover to Shubnikov-de Haas oscillations can be also understood from quantization of perioidic orbits bouncing between the two p-n interfaces. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 12:15PM |
Q25.00003: Graphene electronics via strain engineering Invited Speaker: Recently, graphene has been confirmed as the strongest material ever measured, being able to sustain reversible deformations in excess of 20\%. These mechanical measurements arise at a time where graphene draws considerable attention on account of its unusual and rich electronic properties. Besides the great crystalline quality, high mobility and resilience to high current densities, they include a strong field effect, absence of backscattering and a minimum metallic conductivity. While many such properties might prove instrumental if graphene is to be used in future technological applications in the ever pressing demand for miniaturization in electronics, the latter is actually a strong deterrent: it hinders the pinching off of the charge flow and the creation of quantum point contacts. In addition, graphene has a gapless spectrum with linearly dispersing, Dirac-like, excitations. Although a gap can be induced by means of quantum confinement in the form of nanoribbons and quantum dots, these ``paper-cutting'' techniques are prone to edge roughness, which has detrimental effects on the electronic properties. We explore an alternative route for tailoring the electronic structure of graphene, based on a strain engineering. We will discuss how local and global strain profiles can be suitably tailored to impact the bandstructure of graphene and control its transport characteristics. Electron confinement, electron beam collimation, energy filtering, surface modes and bulk spectral gaps are some examples of what might be achieved. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q25.00004: Fabrication and transport measurement of suspended graphene devices. Wenzhong Bao, Feng Miao, Gang Liu, Chunning Lau We developed a lithography-free technique to fabricate suspended graphene devices. Graphene sheets are exfoliated over pre-defined trenches over the substrates, and electrodes are deposited via shadow mask evaporation. This technique eliminates resist residues which may affect electrical properties of graphene. We will discuss results from electrical transport measurement at different temperatures and magnetic fields. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q25.00005: Temperature dependent resistivity of suspended graphene Eros Mariani, Felix von Oppen In this talk we discuss the temperature dependence of the resistivity for suspended single layer graphene, due to electron-phonon scattering. All the temperature regimes are studied, as well as the contributions due to the different acoustic phonon branches in graphene. We show how tension in the membrane suppresses the otherwise dominant contribution due to flexural phonons [1], leaving a linear temperature scaling compatible with recent experiments. The eventual crossover to quadratic temperature dependence at very high temperatures could be used as an experimental tool to investigate the otherwise unknown strength of the tension. Finally, we discuss the transition to the quasi-nondegenerate regime for electrons in graphene. This is relevant for current experiments on the temperature dependent resistivity in most temperature regimes, and can shed light on the unexpected density dependence of the linear-T resistivity. \\[3pt] [1] Eros Mariani and Felix von Oppen, Phys. Rev. Lett. {\bf 100}, 076801 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q25.00006: Neutron Scattering Studies of Graphene Alice Acatrinei, Zhijun Lin, Luke Daemen We synthesized graphene by thermal exfoliation at 500 deg C. The material is nanosize, as confirmed by TEM/SEM, with flake transverse dimensions 50-100 nm. We present the first detailed measurement of the vibrational Spectrum of both graphene and graphite using INS, along with a neutron Scattering study of hydrogen adsorbed on graphene. Our measurements were collected at 10K using the Filter Difference Spectrometer (FDS) at Lujan Neutron Scattering Center, Los Alamos National Laboratory. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q25.00007: Effect of film corrugation on the optical phonon lifetime in graphene Peter Eklund, Awnish Gupta We present results of a microRaman study of n-layer graphene films supported on $\sim $atomically flat mica, Si/SiO$_{2}$ (or varying roughness) and suspended above a trench. Using the Raman G-band line width \textit{$\Gamma $}$_{G }$, we find that the optical phonon lifetime \textit{$\tau \sim $1/$\Gamma $} decreases linearly with increasing rms substrate roughness $\delta $, and independent of the chemical composition of the substrate. In agreement with this general observation, we find that \textit{$\Gamma $}$_{G}$ for unsupported graphene is significantly higher (i.e., \textit{the q=0} optical phonon lifetime is significantly lower) than observed when the film is supported on mica. Correlating \textit{$\Gamma $}$_{G}$ with values obtained from supported films, we infer an inherent rms roughness \textit{$\delta \quad \sim $ 2 nm} for unsupported graphene, in reasonable agreement with recent STM reports that first suggested that graphene might prefer to spontaneously convert to a corrugated system. Our observations may then relate to the effect of the local bending of the sp$^{2}$ sheet on the electron-phonon interaction. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q25.00008: Edge stress induced warping of graphene sheets and nanoribbons Vivek Shenoy We show that edge stresses introduce intrinsic ripples in free-standing graphene sheets even in the absence of any thermal effects. Compressive edge stresses along zigzag and armchair edges of the sheet cause out-of-plane warping to attain several degenerate mode shapes. Based on elastic plate theory, we identify scaling laws for the amplitude and penetration depth of edge ripples as a function of wavelength [1]. We also demonstrate that edge stresses can lead to twisting and scrolling of nanoribbons as seen in experiments. Our results underscore the importance of accounting for edge stresses in thermal theories and electronic structure calculations for free-standing graphene sheets. [1] V. B. Shenoy, C. D. Reddy, A. Ramasubramaniam and Y. W. Zhang, \textit{Phys. Rev. Lett} (in press) [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q25.00009: Infrared spectroscopy of electronic bands in bilayer graphene Alexey Kuzmenko, Erik van Heumen, Dirk van der Marel, Philippe Lerch, Peter Blake, Konstantin Novoselov, Andre Geim We present infrared spectra (0.1-1 eV) of electrostatically gated bilayer graphene as a function of doping and compare them with tight binding calculations. All major spectral features corresponding to the expected interband transitions are identified in the spectra: a strong peak due to transitions between parallel split-off bands and two onset-like features due to transitions between valence and conduction bands. A significant electron-hole asymmetry is observed. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q25.00010: Sharp Landau Levels in Scanning Tunneling Spectroscopy of Epitaxial Graphene on SiC(000 -1) David Miller, Kevin Kubista, Gregory Rutter, Ming Ruan, Walt de Heer, Phillip First, Joseph Stroscio Monolayer graphene has unique electronic properties stemming from a low-energy band structure that is linear, with chiral Dirac quasiparticles. In a magnetic field, the Landau level (LL) energies for graphene $E_n$ vary proportional to $\sqrt{nB}$, where $n$ is the LL index. Conversely, Bernal-stacked bilayer graphene and graphite have parabolic dispersion at low energies, resulting in $E_n \propto B$. In this talk we measure the LL spectrum of the top graphene layer directly via scanning tunneling spectroscopy (STS) at a 4.3 K. We show that for $\approx 10$-layer epitaxial graphene grown on SiC(000 -1), the spectrum exhibits very sharp peaks (including a strong n=0 peak) spaced as $E_n \propto \sqrt{nB}$. This spectrum indicates that the rotational stacking in multilayer epitaxial graphene effectively decouples the layers, producing single-layer graphene behavior. Work supported in part by NSF, NRI-INDEX, and the W. M. Keck Foundation. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q25.00011: Electronic structures of single- and multi-layer epitaxial graphene on SiC (0001) Seungchul Kim, Jisoon Ihm, Young-Woo Son The electronic structures of single- and multi-layered epitaxial graphene on silicon carbide (0001) surface are studied theoretically. To calculate energy bands of the systems, we construct the simple Hamiltonian with tight-binding approximations. We confirm that the present simple model do give identical electronic structure to the previous ab-initio study on the single layer case [1]. We extend the model up to four epitaxial graphene layers to explain various interesting experimental findings. The roles of the coupling between graphenes and the buffer layer, and their large scale reconstructions to the electronic structures are also investigated. [1] S. Kim, J. Ihm, H. J. Choi, Y.-W. Son, Phys. Rev. Lett. 100, 176802 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q25.00012: G-band Phonon Symmetry Breaking of Graphene Monolayers Lain-Jong Li, Yang Zhao, Xiaochen Dong, Peng Chen Aromatic molecules can effectively exfoliate graphite into graphene monolayers. And the resulting graphene monolayers sandwiched by the aromatic molecules exhibit pronounced Raman G-band splitting, similar to that observed in rolled-up graphene sheet (single-walled carbon nanotubes). Raman measurements and the theoretical calculation based on force-constant model demonstrate that aromatic molecules are able to induce G-band splitting via breaking the symmetry of two in-plane longitudinal and transverse optical phonons at Gamma-point. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q25.00013: Edge states in a honeycomb lattice: effects of anisotropic hopping and mixed edges. Zi-Xiang Hu, Hari Dahal, Nikolai Sinitsyn, Kun Yang, Alexander Balatsky We study the effects of anisotropic hopping and mixed edges on the edge states of graphene. The discussion of the edge states in graphene so far is focused on either zigzag or armchair edge with isotropic hopping. In this case the zigzag (arm chair) edge has enhanced (suppressed) local density of states at E=0 near the edge. In practice electrons in graphene can have anisotropic hopping. The lattice can have mixed (zigzag and arm chair) edges. Hence we study the effects of the anisotropic hopping and mixed edges on the edge states. We show that the mixed edges smear the enhanced local density of states at E=0 of the zigzag edge and, the anisotropic hoping enhanced the LDOS at E=0 in the armchair edge. Edge states in graphene can be studied using scanning tunneling microscopy (STM) experiments. We suggest that care must be taken while interpreting the STM data because the distinction between the zigzag and arm chair edge will be affected by the anisotropic electron hopping and the mixed edges. [Preview Abstract] |
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