Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session L26: Focus Session: Graphene VI: Phonons and Raman Spectroscopy |
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Sponsoring Units: DMP Chair: Kostya Novoselov, Manchester University Room: 328 |
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L26.00001: Photovoltaic Hall Effect in Dirac systems -- Application to Graphene Takashi Oka, Hideo Aoki We theoretically propose to irradiate electron systems with massless Dirac dispersion with circularly polarized light, for which we predict that the photo-irradiation can induce a dc Hall effect in the absence of static, uniform magnetic fields. The effect bears a geometric origin, traced back to the non-adiabatic phase (Aharonov-Anandan phase) which is acquired by the motion of $k$-points in the Brilliouin zone when they encircle the Dirac cones. The Kubo formula for linear responses is extended to the nonlinear effects via the Floquet formalism for strong ac fields, which is used to calculate the \textit{photo-induced Berry curvature}. The irradiation induces a dynamical gap at the Dirac point which gives rise to a universal ac Wannier-Stark ladder in Dirac systems observable in the density of states. We further use the Keldysh + Floquet method to analyze finite graphene systems, which confirms the existence of photovoltaic dc Hall effect. The required strength of the circularly polarized light to observe these effects is estimated to be O($10^7$eV/m), which is within an accessible range for present laser sources. (arXiv:0807.4767) [Preview Abstract] |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L26.00002: Thermal conductivity and thermal rectification in graphene nanoribbons: a molecular dynamics study Jiuning Hu, Xiulin Ruan, Zhigang Jiang, Yong Chen We have used molecular dynamics based on the Brenner potential to calculate the thermal conductivity of graphene nanoribbons. For symmetrical nanoribbons, the calculated thermal conductivity is the similar order of magnitude of the experimentally measured value for graphene. We have investigated the effects of edge chirality and found that nanoribbons with zigzag edges have considerable larger thermal conductivity than that of nanoribbons with armchair edges. For asymmetric nanoribbons, we have found considerable thermal rectification. For example, for a 6nm-long triangular shaped nanoribbon, the thermal conductivity from the wider to the narrower end is nearly 2.5 times that from the narrower to the wider end. Furthermore, the thermal rectification can be significantly enhanced by increasing the size of the asymmetrical nanoribbon. Such rectification effects can be useful in nanoscale thermal management. [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:30PM |
L26.00003: Raman scattering in bilayer graphene: probing phonons, electrons and electron-phonon interactions Invited Speaker: The application of resonance Raman spectroscopy to study the electronic, vibrational and electron-phonon interaction properties in bilayer graphene will be presented. From the dependence of the second-order Raman bands on the laser excitation energy, we obtain experimental values for the Slonczewski-Weiss-McClure band parameters of bilayer graphene. We will discuss in detail the effect of each one of the tight band parameters on the electronic structure, showing that bilayer graphene has a larger electron-hole asymmetry compared to graphite. We will also present experimental results for the phonon dispersion relations near the K point, showing a strong Kohn anomaly for one of the phonon branches. In a gating experiment, the change in Fermi level of bilayer graphene gives rise to a symmetry breaking, allowing the observation of both the symmetric (S) and anti-symmetric (AS) phonon modes. The dependence of the energy and damping of these phonons modes on the Fermi level energy is explained in terms of distinct couplings of the S and AS phonons with intra- and inter-band electron-hole transitions. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L26.00004: Phonon dispersion of graphene revisited Silvia Viola Kusminskiy, David Campbell, Antonio Castro Neto We calculate the phonon spectrum for a graphene sheet resulting from the model proposed by T. Lenosky {\it et al.} (Nature {\bf 355}, 333 (1992)) for the free energy of the lattice. This model takes into account not only the usual bond bending and stretching terms, but captures the possible misalignements of the $p_z$ orbitals. We compare our results with previous models used in the literature. We analyze the effect of anharmonic terms. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L26.00005: Giant nonadiabatic effects in layer metals: Raman spectra of intercalated graphite and doped graphene explained. A. Marco Saitta, Michele Lazzeri, Matteo Calandra, Francesco Mauri The occurrence of nonadiabatic effects in the vibrational properties of metals has been predicted since the 1960s [1], but hardly confirmed experimentally. We report the first fully ab initio calculations of nonadiabatic frequencies of a number of conventional (hcp Ti and Mg), layered metals (MgB2, CaC6, other intercalated graphites) [2] and doped graphene [3]. Nonadiabatic effects can be spectacularly large (up to 30\% of the phonon frequencies), but they can only be experimentally observed in the Raman spectra of layered compounds. In layered metals nonadiabatic effects are crucial to explaining the observed Raman shifts and linewidths. Moreover, we show that those quantities can be used to extract the electron momentum-relaxation time. [1] S. Engelsberg and J.R. Schrieffer, Phys. Rev. 131, 993 (1962). [2] A.M. Saitta et al., Phys. Rev. Lett. 100, 226401 (2008). [3] M. Lazzeri and F. Mauri, Phys. Rev. Lett. 97 , 266407 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L26.00006: Probing the Intrinsic Properties of Exfoliated Graphene: Raman Spectroscopy of Free-Standing Monolayers Stephane Berciaud, Sunmin Ryu, Louis Brus, Tony Heinz The properties of pristine, free-standing graphene monolayers prepared by mechanical exfoliation of graphite are investigated. The graphene monolayers, which were suspended over open trenches, are examined by means of spatially resolved Raman spectroscopy of the G, D, and 2D phonon modes. The G-mode phonons exhibit low energies (1580 cm$^{-1})$ and broad widths (14 cm$^{-1})$ compared to the response for samples supported on the SiO$_{2}$--covered substrate. From analysis of the G-mode Raman spectra, we deduce that the free-standing graphene monolayers are essentially undoped, with an upper bound of 2$\times $10$^{11 }$cm$^{-2}$ for the residual carrier concentration. On the supported regions, significantly higher and spatially inhomogeneous doping is observed. The free-standing graphene monolayers show very little local disorder, based on the very low Raman D mode intensity. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L26.00007: Micro Raman spectroscopy of graphene Hall Bars in the QHE regime Sebastian Remi, Constanze Metzger, Anna Swan, Bennett B. Goldberg One of the most intriguing aspects in the physics of graphene are new types of quantum Hall effects which differ significantly from observations on conventional 2DEG samples. Furthermore the Raman spectrum of graphene shows charge carrier dependent Kohn anomalies. So far transport and optical measurements on graphene haven't been combined to explore the behavior of the Raman features dependent on the Landau levels. We present our latest measurements of Raman scattering on graphene single and bilayers in the Quantum Hall regime. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L26.00008: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L26.00009: Raman Scattering from Pt Island-Decorated Graphene Awnish Gupta, Humberto Gutierrez, Peter Eklund We performed microRaman studies of decorated $n$-Graphene Layers ($n$GLs). Nano-islands (NI; dia$\sim $5-10 nm) of Pt were created by deposition on the $n$GL with gaps between the NI in the range of few nm. When the NI were present, we observed D and D' Raman bands as well as splitting of the G-band into G$^{+}$ and G$^{-}$ (most pronounced for 1GL). The observations may be related to graphene ``confined'' in the interstitial spaces between NIs. The D and D' bands show the following properties: (1) Intensity of D and D' relative to G band decreases with increasing number of layers $n $in the $n$GL. (2) Peak frequencies, $\omega _{D}$ decreases linearly with 1/$n $while $\omega _{D\mbox{'} }$remains constant. (3) Linewidth $\Gamma _{D}$ decreases linearly with 1/$n$ , while $\Gamma _{D\mbox{'}}$ increases linearly with 1/$n$. Our results will be discussed in terms of results theoretically predicted by zone folding (Jishi \textit{et al}). [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L26.00010: Raman Spectroscopy of the 2D Mode in Free-Standing Graphene Monolayers Janina Maultzsch, Stephane Berciaud, Louis Brus, Tony Heinz Raman spectroscopy of the 2D (or G') mode is a critical tool for the analysis of graphene mono- and multilayers. This symmetry-allowed overtone mode permits one to probe zone-edge phonons by Raman spectroscopy. Moreover, its doubly-resonant electronic character$^{1}$ renders it readily observable and encodes in it information about the electronic structure of the graphene sample. In particular, the 2D mode provides a clear signature of the thickness of mono- and multilayer graphene films$^{2}$. In this paper, we present a detailed study of the properties of the 2D for a pristine, free-standing graphene monolayer prepared by mechanical exfoliation over a trench structure. In contrast to the behavior of monolayers of graphene on substrates, for this pristine graphene sample, we observe a positively skewed line shape. The linewidth is also somewhat reduced compared to that observed for graphene supported on a substrate. Further, the 2D mode in the free-standing graphene films exhibits a slightly stronger dispersion with energy of the pump photons than for supported monolayers. We discuss our findings within the framework of double resonance theory, taking into account the intrinsically undoped nature of the free-standing graphene samples. $^{1}$J. Maultzsch \textit{et al.}, Phys. Rev. B \textbf{70}, 155403 (2004) $^{2}$A. Ferrari \textit{et al.} Phys. Rev. Lett. \textbf{97}, 187401 (2006) [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L26.00011: Seeing and counting graphene layers by elastic light scattering C. Casiraghi, A. Hartschuh, E. Lidorikis, H. Qian, H. Harutyunyan, T. Gokus, K. S. Novoselov, A. C. Ferrari Raman scattering has recently emerged as a viable and nondestructive technique for the identification of graphene, its doping, edges and amount of defects [1-3]. However, the Raman scattered photons are a minority compared to those elastically scattered. Here we show that large graphene layers can be mapped and identified in a few minutes by elastic scattering. We report an extensive investigation of graphene on silicon/silicon oxide substrate by monochromatic and white-light elastic scattering and the theoretical understanding of the experimental data [4]. Maps of the scattered light are obtained by raster scanning the sample with a piezoelectric stage. We show that the image contrast depends sensitively on the dielectric properties of the sample as well as the substrate geometry and can be described quantitatively using the complex refractive index of bulk graphite. [1] A. C. Ferrari et al., PRL 97, 187401 (2006) [2] S. Pisana et al., Nature Materials 6, 198 (2007) [3] C. Casiraghi et al., APL 91, 233108 (2007) [4] C. Casiraghi et al., Nano Letters 7, 2711 (2007) [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L26.00012: Polarized micro Raman spectroscopy of bilayer graphene Hyerim Moon, Duhee Yoon, Young-Woo Son, Hyeonsik Cheong The frequency of Raman 2$D$ band of the graphite depends on the excitation laser energy. This phenomenon is explained with double resonance Raman process. In polarized micro-Raman spectroscopy of single layer graphene, Raman $G$ band ($\sim $1586 cm$^{-1})$ is isotropic, and 2$D$ band ($\sim $2686 cm$^{-1})$ strongly depends on relative polarizations of the incident and scattered photons. This strong polarization dependence originates from inhomogeneous optical absorption and emission mediated by resonant electron-phonon interaction. In bi-layer graphene, Raman 2$D$ band can be decomposed into four Lorenztian peaks which can be interpreted in terms of the four transition paths in the double resonance Raman process. We investigated the polarization dependence of each Lorenztian peak in the Raman 2$D$ band of bi-layer graphene for different excitation laser energies. Strong polarization dependence of the Raman 2$D$ band, similar to the case of single layer graphene, is observed. The excitation energy dependence of the polarized Raman scattering is analyzed in terms of the band structure of bi-layer graphene. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L26.00013: Interference effect on Raman spectrum of single layer graphene on SiO$_{2}$/Si Duhee Yoon, Hyerim Moon, Young-Woo Son, Jin Sik Choi, Bae Ho Park, Young Hun Cha, Young Dong Kim, Hyeonsik Cheong We studied the dependence of the Raman spectrum of graphene on the thickness of the SiO$_{2}$ layer. We prepared the single layer graphene samples with the various SiO$_{2}$ layer thicknesses ($\sim $240 to $\sim $388 nm) using micro-mechanical cleavage from the natural graphite and observed a strong variation of the Raman spectrum as a function of the thickness of SiO$_{2}$ layer. It is found that the intensity of Raman signal is strongly influenced by the interference due to multiple reflections in the graphene and SiO$_{2}$ layers. The Raman enhancement factor was calculated by taking into account the interference effect. The model calculation fits well with the experimental data. The interference also affects the Raman intensity ratio of the 2$D$ band to the $G$ band due to the difference in the wavelength of these signals. Moreover, we calculated the Raman enhancement factor as a function of the SiO$_{2}$ thickness and the excitation laser wavelength. [Preview Abstract] |
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