Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session T28: Graphene: Optical and Transport Properties |
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Sponsoring Units: DCMP Chair: Tony Heinz, Columbia University Room: C156 |
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T28.00001: Selection rule for Raman spectroscopy at graphene edge Ken-ichi Sasaki, Katsunori Wakabayashi, Toshiaki Enoki The optical matrix element may depend on position in graphene since an electronic wave function is position dependent. In particular, the matrix element near the edges of graphene can differ greatly from that in the bulk. We are pursuing our studies on this point in relation to Raman spectroscopy. We found a selection rule for the G band near the edges of graphene: the intensity is enhanced when the polarization of incident laser is parallel (perpendicular) to the armchair (zigzag) edge [1]. This asymmetry between the armchair and zigzag edges is useful in identifying the orientation of the edge of graphene. Some application of the selection rule is mentioned. We have extended our study to the polarization dependences of the D and 2D (G') bands [2]. The D and 2D bands have different selection rules at bulk and edge. At bulk, the 2D band intensity is maximum when the polarization of the scattered light is parallel to that of incident light, whereas the D band intensity does not have a polarization dependence. At edge, the 2D and D bands exhibit a selection rule similar to that of the G band.\\[4pt] [1] Sasaki et al., J. Phys. Soc. Jpn. 79, 044603 (2010).\\[0pt] [2] Sasaki et al., Phys. Rev. B 82, 205407 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T28.00002: Photoconductive response study on a dual-gated bilayer graphene M.-H. Kim, J. Yan, G.S. Jenkins, A.B. Sushkov, D.C. Schmadel, M.S. Fuhrer, J. Melngailis, H.D. Drew A continuously tunable bandgap as high as 100 meV is produced in a gated bilayer graphene (BLG) by applying an electric field perpendicular to the layers (J. Yan, Nano Lett. 2010). The bandgap and the Fermi energy of BLG are tuned by top and bottom gate potentials. We measure the infrared photoconductive response from the dual-gated BLG from far infrared 30 cm$^{-1}$ to mid-infrared 5000 cm$^{-1}$ by broadband spectroscopy and with a CO$_2$ laser near 10.6 $\mu$m. We report the photoresponse and the measured band gap as a function of an applied electric field perpendicular to the BLG layers. This work is supported by IARPA grant \#W911NF1010443. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T28.00003: Ultrafast carrier dynamics in pristine and FeCl$_{3}$-intercalated bilayer graphene Xingquan Zou, Da Zhan, Xiaofeng Fan, Dongwook Lee, Saritha K. Nair, Li Sun, Zhenhua Ni, Zhiqiang Luo, Lei Liu, Ting Yu, Zexiang Shen, Elbert E.M. Chia Ultrafast carrier dynamics of pristine bilayer graphene (BLG) and bilayer graphene intercalated with FeCl$_{3}$ (FeCl$_{3}$-G), were studied using time-resolved transient differential reflection ($\Delta R/R)$. Compared to BLG, the FeCl$_{3}$-G data showed an opposite sign of $\Delta R/R$, a slower rise time, and a single (instead of double) exponential relaxation. We attribute these differences in dynamics to the down-shifting of the Fermi level in FeCl$_{3}$-G, as well as the formation of numerous horizontal bands arising from the $d$-orbitals of Fe. Our work shows that intercalation can dramatically change the electronic structure of graphene, and its associated carrier dynamics. Appl. Phys. Lett. \textbf{97, 141910 }(2010) [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T28.00004: Microwave microscopy of graphene and graphite Vladimir Talanov, Christopher Del Barga, Lee Wickey, Irakli Kalichava, Edward Gonzales, Eric Shaner, Aaron Gin, Nikolai Kalugin Graphene has emerged as a promising material for high speed nano-electronics applications due to the relatively high carrier mobility that can be achieved. To further investigate electronic transport in graphene and reveal its potential for microwave applications [1,2], a near-field scanning microwave microscope with the probe formed by an electrically open end of a 4 GHz half-lambda parallel-strip transmission line resonator has been employed [3]. We find that the microwave response of mono- and few-layer graphene flakes is determined by the local sheet impedance, which is found to be predominantly active. From fitting a quantitative electrodynamic model (relating the probe resonant frequency shift to 2D conductivity of single- and few-layer graphene) to the experimental data we evaluate graphene sheet resistance as a function of thickness. Near-field scanning microwave microscopy can simultaneously image location, geometry, thickness, and distribution of electrical properties of graphene without a need for device fabrication. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T28.00005: Laser Scanning Microscopy of Few-Layer Graphene: Optical Reflectivity Contrast Behnood Ghamsari, Alexander Zhuravel, Daniel Lenski, Michael Fuhrer, Steven Anlage We report laser scanning microscopy (LSM) of few-layer graphene, where a laser beam is raster scanned over the samples and the local reflectivity of the structure is directly measured through a silicon photodiode. The samples are grown by ambient-pressure chemical vapor deposition on copper foils, and transferred to SiO2/Si substrates, and consist of regions of single- and multi-layer graphene (D. R. Lenski, and M. S. Fuhrer, e-print arXiv: 1011.1683). While the local reflectivity of the structure depends on the thickness of the graphene layer, the LSM data is used to construct a two-dimensional reflectivity image of the sample which, in turn, enables identifying the local distribution of different graphene multilayers and local microscopic properties of the graphene sample. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T28.00006: Terahertz Imaging and Spectroscopy of Large-Area Single-Layer Graphene Ethan Minot, Joe Tomaino, Andrew Jameson, Joshua Kevek, Michael Paul, Arend van der Zande, Robert Barton, Paul McEuen, Yun-Shik Lee The high electron mobility of graphene points to potential for high-speed electronic and opto-electronic devices operating at terahertz (THz) switching rates. Therefore, there is great interest in probing the electronic properties of large-area graphene at ultrafast time scales. We have demonstrated THz imaging and spectroscopy of a 15x15-mm$^{2}$ single-layer graphene film using broadband THz pulses. The THz images clearly map out the THz carrier dynamics of the graphene-on-Si sample, allowing us to measure sheet conductivity with sub-mm resolution without fabricating electrodes. The THz carrier dynamics are dominated by intraband transitions and the THz-induced electron motion is characterized by a flat spectral response. A theoretical analysis based on the Fresnel coefficients for a metallic thin film shows that the local sheet conductivity varies across the sample from 1.7 -- 2.4 x 10$^{-3}$ Ohm$^{-1}$ (sheet resistance 420 - 590 Ohm/sq). [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T28.00007: Aromatic molecule-like fluorescence from Graphene Oxide Charudatta Galande, Aditya Mohite, Anton Naumov, Wei Gao, Lijie Ci, Anakha Ajayan, Hui Gao, Anchal Srivastava, R. Bruce Weisman, Pulickel M. Ajayan Graphene Oxide (GO) is a functionalized derivative of graphene, obtained by chemical exfoliation and chemical oxidation of graphite. Recent NMR studies on GO have revealed presence of hydroxyl, epoxy, carbonyl, carboxyl and lactols. Although there have been several studies on electronic and optical properties of GO, the role of functional groups in determining the electronic density of states is still unclear. Here we report pH dependent fluorescence and excitation spectra of GO, with spectroscopic signatures indicating the presence of molecule-like fluorophores in GO. In acidic medium, a single, broad emission peak is observed at ca. 660nm. In contrast, relatively sharp emission at lower wavelengths (480nm-515nm) appears in a short pH range between 7.6 and 8.0, while the broad peak is completely quenched in basic conditions. The fluorescence and excitation spectra have pH-dependence strikingly similar to several aromatic carboxylic acids. The observed spectral features are proposed to arise from quasi-molecular fluorophores, similar to polycyclic aromatic compounds that are formed by the electronic coupling of carboxylic acid groups with nearby carbon atoms of the graphene. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T28.00008: Broadband spatial self-phase modulation of few-layer graphene sheet in solution Rui Wu, Yingli Zhang, Fei Bian, Shichao Yan, Rui Wang, Wenlong Wang, Xuedong Bai, Xinghua Lu, Jimin Zhao Spatial self-phase modulation (SPM) was found for a suspension of few-layer graphene flakes. Multiple concentric conical diffraction rings was observed as a 532nm cw laser beam passes through the nearly transparent suspension, of which self-focusing occurred. The dependence of ring numbers and ring diameters on the laser intensity was recorded, from which we obtained the third order optical nonlinearity $n_{2 }$of the sample. In our case $n_{2}$=10$^{-9}$ m$^{2}$/W, which is the one of the largest among the reported carbon materials including carbon nanotubes and C$_{60}$ samples. We also found that the intensity threshold for observing the diffraction rings is as low as about 0.6W/cm$^{2}$, which is the smallest compared with most of the reported sample having spatial SPM, including nematic liquid crystals. Furthermore we found that both 267nm and 800nm ultrashort laser pulses can also easily generate spatial SPM. This large and broadband optical nonlinearity is a manifestation of the few-layer graphene's conical-shaped band structure, which is true for a relatively large energy scale. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T28.00009: Optical transitions between Landau levels: AA-stacked bilayer graphene Yen-Hung Ho, Rong-Bin Chen, Jhao-Ying Wu, Yu-Huang Chiu, Ming-Fa Lin The magneto-absorption spectra are calculated for the AA- stacked bilayer graphene. Two groups of Landau levels with different symmetry in wave function are found to coexist in the low energy region. The optical transitions between the two groups give rise to two kinds of absorption peaks. The wave- function distribution can clearly characterize individual Landau levels, and further determine the optical selection rules and absorption rates. The AA bilayer has quite different spectral features compared to the AB bilayer and monolayer, as a result from the interlayer interactions and stacking symmetry. Only a single absorption survives below certain critical frequency, while other peaks are paired together and sequentially emerged above this critical energy. With a continuous change in field strength, the excitation channels are switched, associated with the abrupt changes in their frequency. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T28.00010: High temperature Graphene-based Quantum Hall Effect Infrared photodetector Nikolai G. Kalugin, Lei Jing, Wenzhong Bao, Lee Wickey, Christopher Del Barga, Mekan Ovezmyradov, Eric A. Shaner, Chun Ning Lau We demonstrate successful operation of quantum Hall effect (QHE) graphene-based detectors at 70K, a temperature achievable using simple pumped liquid Nitrogen cryostats, and in magnetic field of 7.35T. Because of graphene's unique band structure, the first few Landau levels are well-separated energetically, thus allowing observation and manipulation of QHE at unprecedentedly high temperatures [1]. Our results overcome the obstacle of low operating temperature of traditional semiconductor systems-based QHE photodetectors [2], and open the door for wide arrays of applications. \\[4pt] [1] K.S. Novoselov \textit{et al}. \textit{Science} \textbf{315}, 1379 (2007).\\[0pt] [2] N. G. Kalugin \textit{et.al.} \textit{Phys.Rev.B} 66, 085308 (2002). [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T28.00011: Phonon scattering in intrinsic graphene using tight-binding Bloch waves Nishant Sule, Irena Knezevic The overall interest in graphene as a material for devices has led to tremendous advances in the understanding of transport in graphene. However, there are still questions about the intrinsic limit to electron mobility. Recent experiments have demonstrated mobility greater than $10^{7}$ cm$^{2}/$Vs at temperatures close to $50$ K, exceeding previous theoretical predictions of the limit for intrinsic mobility. Here, we present a simple model for phonon scattering rates in intrinsic graphene using tight-binding Bloch wave functions for electrons. The tight binding approximation produces an accurate band structure near the Dirac points, as opposed to the nearly free electron model; thus, it is reasonable to assume that the electron wave functions are localized near the atomic centers. These tight-binding Bloch wave functions are calculated by linear combination of the carbon $p_{z}$ orbitals. We show that the scattering matrix is anisotropic and the small overlap of the Bloch functions results in scattering rates that are lower in comparison to those calculated by assuming plane-wave wave functions. Electron mobility calculated in the relaxation time approximation is compared for scattering rates with Bloch functions and as well as plane waves. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T28.00012: Chirality-dependent phonon-limited resistivity in graphenes Hongki Min, Euyheon Hwang, Sankar Das Sarma We develop a theory for the temperature and density dependence of phonon-limited resistivity $\rho(T)$ in bilayer and multilayer graphene, and compare with the corresponding monolayer result. For the unscreened case, we find $\rho \approx C T$ with $C \propto v_{\rm F}^{-2}$ in the high-temperature limit, and $\rho \approx A T^4$ with $A \propto v_{\rm F}^{-2} k_{\rm F}^{-3}$ in the low-temperature Bloch-Gr\"uneisen limit, where $v_{\rm F}$ and $k_{\rm F}$ are Fermi velocity and Fermi wavevector, respectively. If screening effects are taken into account, $\rho \approx C T$ in the high-temperature limit with a renormalized $C$ which is a function of the screening length, and $\rho \approx A T^6$ in the low-temperature limit with $A \propto k_{\rm F}^{-5}$ but independent of $v_{\rm F}$. These relations hold in general with $v_{\rm F}$ and a chiral factor in $C$ determined by the specific chiral band structure for a given density.\\[4pt] Reference: Hongki Min, E. H. Hwang, and S. Das Sarma, arXiv:1011.0741 (unpublished). [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T28.00013: Boundary properties between monolayer and bilayer graphene and valley filter Takeshi Nakanishi, Mikito Koshino, Tsuneya Ando Graphene consists of a two-dimensional hexagonal crystal of carbon atoms, in which electron dynamics is governed by the Dirac equation. The purpose of this paper is to study the boundary between monolayer and bilayer graphenes and show a valley polarization in transmission probability through the boundary [1]. We consider the boundary of monolayer and bilayer graphene, in which lower layer in bilayer graphene is continuously connected to the monolayer graphene and upper layer is terminated along a straight edge having zigzag or armchair structures. Boundary conditions between monolayer and bilayer graphene are derived in an effective-mass scheme. The transmission probability vanishes at the Dirac point and increases roughly in proportional to the electron density. The transmission probability varies strongly as a function of the incident angle and its maximum appears at an angle deviating from the vertical direction. This asymmetry is opposite between the K and K' points, showing that strong valley polarization can be induced across the interface of monolayer and bilayer graphenes.\\[4pt] [1] T. Nakanishi et al., PRB 82 (2010) 125428. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T28.00014: Anomalous tunneling of dressed Dirac electrons through potential barrier Andrii Iurov, Oleksiy Roslyak, Godfrey Gumbs It has been shown that when a potential barrier is placed on a layer of graphene, electrons incident on the barrier head-on can be transmitted without any reflection, regardless of how high the barrier is made to become. This anomalous scattering has also been investigated in the case of bilayer graphene. The energy gap between the valence and conduction bands for bilayer graphene leads to perfect reflection for head-on collisions for all barrier heights. We report on results for reflection and transmission coefficients for dressed Dirac electrons when circularly polarized light is applied to graphene and an energy gap in the energy bands is opened up. Since this gap depends on the frequency and intensity, we investigate how the electron and hole scattering off a fixed barrier is modified by varying the energy gap produced by light. We also present results for the transmission for the perpendicular incidentce. Both numerical and analytical results are obtained. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T28.00015: In-gap transport in random-gap grapheme: metallic and insulating phases Vagharsh Mkhitaryan, Mikhail Raikh 1D- like counter-propagating states at a gap center of graphene with random gap constitute two chiral networks. In the absence of intervalley scattering, transport over each network is either metallic or insulating, depending on the gap randomness. We demonstrate that properties of both phases as well as transitions between them are accurately captured within a simple real-space renormalization group approach. The most striking feature of this network transport is that it can be metallic even when the neighboring plaquettes are weakly coupled. We show that randomness in local gap signs reflected in randomness in signs of local transmission coefficients, gives rise to resonant transmission of the RG superblock. Delocalization occurs by proliferation of these resonances to larger scales. As the disorder exceeds a critical value, the RG flow towards insulator switches to a flow towards metallic fixed point. Evolution of the conductance distribution to metallic fixed point is synchronized with evolution of transmission coefficient signs, so that delocalization is accompanied with sign percolation. [Preview Abstract] |
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