Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session B29: Focus Session: Carbon Nanotubes and Related Materials II: Graphene Transport |
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Sponsoring Units: DMP Chair: Shan-Wen Tsai, University of California, Riverside Room: Morial Convention Center 221 |
Monday, March 10, 2008 11:15AM - 11:51AM |
B29.00001: Intrinsic and Extrinsic Limits of Mobility in Graphene Invited Speaker: Graphene is an exciting new condensed matter system, both for the opportunity to observe the physics associated with massless Dirac Fermions in the laboratory, and because of materials parameters which make it attractive for technological applications. However, in the few years since the experimental realization of graphene, progress toward cleaner (higher mobility) samples has largely stalled. I will discuss experiments performed on atomically-clean graphene on SiO$_{2}$[1] in ultra-high vacuum to determine the intrinsic and extrinsic limits of mobility in graphene[2,3], which point out both the promise of the material as well as the technological challenges that lie ahead in realizing better graphene samples. Intrinsic scattering by the acoustic phonons of graphene[3] limits the room-temperature mobility to 2 x 10$^{5}$ cm$^{2}$/Vs at a carrier density of 10$^{12}$ cm$^{-2}$, higher than any known material. However, extrinsic scattering due to charges in the substrate[2] and substrate polar optical phonons[3] currently impose much more severe limits on the mobility, pointing out the importance of substrate choice for graphene devices[4]. \newline [1] M. Ishigami, et al., \textit{Nano Letters} \textbf{7}, 1643 (2007). \newline [2] J. H. Chen, et al., arXiv:0708.2408. \newline [3] J. H. Chen, et al., arXiv:0711.3646. \newline [4] J.-H. Chen, et al., \textit{Advanced Materials} \textbf{1}9, 3623 (2007). [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B29.00002: Intrinsic and Extrinsic performance limits of graphene device on SiO$_{2}$ Jianhao Chen, Chaun Jang, Shudong Xiao, Masa Ishigami, Michael Fuhrer We have measured the temperature-dependent resistivity of clean graphene devices on SiO$_{2}$ from 16K to 485K in ultra high vacuum[1]. Longitudinal acoustic phonons, intrinsic to graphene, give rise to the measured resistivity linearly dependent on temperature from 16 to $\sim $200K. Above 200 K, a sharp upturn in resistivity is observed due to remote interfacial phonon (RIP) scattering by the polar optical phonons of the SiO$_{2}$ substrate. Combining the contributions from intrinsic and extrinsic phonons, we are able to explain the complete temperature and carrier density dependence of the graphene resistivity on SiO$_{2}$. For a technologically relevant carrier density of n = 10$^{12}$ cm$^{-2}$ at room temperature, the intrinsic phonon scattering will only limit the mobility to $\sim $2x10$^{5}$ cm$^{2}$/Vs, while the extrinsic RIP scattering from SiO$_{2}$ will limit the mobility to $\sim $4x10$^{4}$cm$^{2}$/Vs. [1] J. H. Chen, et al., http://arxiv.org/abs/0711.3646 [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B29.00003: Single-particle relaxation time versus scattering time in 2D graphene layers Euyheon Hwang, Sankar Das Sarma We calculate the transport scattering time ($\tau_t$) and the single particle relaxation time ($\tau_s$) for disordered graphene in the lowest order of the electron-impurity interaction (Born approximation). We find that the ratio of $\tau_t$ to $\tau_s$ is always greater (less) than two for charged Coulomb (short-ranged neutral) scatterers. Thus, the calculated scattering time ratio can be a good criterion of directly selecting the dominant scattering mechanism in graphene. As a direct consequence of scattering times we calculate graphene mobility, damping rate, and density of states of single particle state. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B29.00004: The Coulomb Impurity Problem in Graphene Vitor Pereira, Johan Nilsson, Antonio Casto Neto We address the problem of an unscreened Coulomb charge in graphene and calculate the local density of states and displaced charge as a function of energy and distance from the impurity. This is done nonperturbatively in two different ways: (1) solving the problem exactly by studying numerically the tight-binding model on the lattice and (2) using the continuum description in terms of the 2D Dirac equation. We show that the Dirac equation, when properly regularized, provides a qualitative and quantitative low energy description of the problem. The lattice solution shows extra features that cannot be described by the Dirac equation: namely, bound state formation and strong renormalization of the van Hove singularities. \newline [Reference: Phys. Rev. Lett. 99, 166802 (2007)]. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B29.00005: Coulomb Impurity Screening in Graphene Valeri Kotov I will discuss the vacuum polarization charge density around a Coulomb impurity with charge $Z|e|$. Perturbation theory in powers of $Z\alpha$ (where $\alpha = e^{2}/v_{F}$ is the effective coupling constant in graphene), shows that the polarization charge is localized at the impurity site. An exact calculation, based on the Green's function in a Coulomb field, leads to a non-perturbative result, valid to all orders in $Z\alpha$ [1]. Taking into account also electron-electron interactions in the Hartree approximation, we solve the problem self-consistently in the subcritical regime, where the impurity has an effective charge $Z_{\mbox{eff}}$, determined by the localized induced charge. We find that an impurity with bare charge $Z=1$ remains subcritical, $Z_{\mbox{eff}} \alpha < 1/2$, for any $\alpha$, while impurities with $Z=2,3$ and higher can become supercritical at certain values of $\alpha$. \newline [1] I.S. Terekhov, A.I. Milstein, V.N. Kotov, and O.P. Sushkov, arXiv:0708.4263. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B29.00006: MagnetoResistance of Graphene-based spin valves. Luis Brey, Herbert Fertig In this work we present a detailed study of the conduction properties of wide graphene strips, with two different models for the source and drain leads. We reconfirmed that for undoped graphene, the system can be described by a {\it conductivity} in the $L \rightarrow \infty$ limit even when defects are absent from the system, and examined this behavior with respect to a broad range of lead parameters. Our results indicate that the conductance is relatively insensitive to the electronic structure of the leads. We then compute the conductivity of a simple three stripe spin-valve device with graphene acting as the non-magnetic material between the ferromagnetic leads. Two types of ferromagnetic lead systems were considered: one with a single ($s$) orbital for each spin state, with band centers separated in energy to induce spin polarization, and another with a narrow $d$ band which was taken to be spin-polarized. We find that the conductivity depends only weakly on the relative spin orientations of the leads, and therefore the magnetoresistance is rather small for most circumstances, largely due to the insensitivity of the conductivity with respect to conditions in the leads. Our results indicate that, although graphene has properties that make it attractive for spintronic devices, the performance of a graphene-based spin-valve is likely to be poor. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B29.00007: Spin transport in graphene strongly coupled to ferromagnetic leads Jelena Trbovic, Hagen Aurich, Gunnar Gunnarsson, Christian Schoenenberger We study low temperature spin transport in graphene layers by using NiPd alloy as ferromagnetic contacts. This type of contacts has been successfully used in realizing carbon nanotube-based spin devices. The measurements are done in the temperature range between 240 mK and 1.6 K with average electrode separation of 0.7 $\mu $m. We find a clear two-terminal spin-valve signal while sweeping the magnetic field in plane of the device, with about 3{\%} effect. However, the signal rapidly decays with increasing temperature and vanishes above 1.6 K. We believe that the observed rapid dephasing is due to the strong coupling of PdNi contacts to the graphene layer. In addition, three-terminal measurements (quasi non-local) have been done in the same temperature range in order to study the influence of a single NiPd electrode on the observed spin transport. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B29.00008: Interference of Electron Waves in a Ballistic Graphene Transistor. Sungjae Cho, Michael Fuhrer We have prepared single- and few-layer graphene samples by mechanical exfoliation of Kish graphite on SiO$_{2}$/Si substrates. We have fabricated graphene field-effect transistors by electron beam lithography followed by thermal evaporation of Cr/Au or Permalloy source and drain electrodes; the conducting silicon underneath 300 nm silicon dioxide serves as a back gate electrode. We find that at low temperatures that the two-dimensional plot of conductance as a function of gate voltage and drain voltage shows an interference pattern of maxima and minima which occur along diagonal lines. We analyze the pattern in terms of interference of electron waves reflected between source and drain electrodes. The slope of the lines measures the compressibility of the two-dimensional electron system, and has strikingly different dependence on carrier density (gate voltage) for single- and few-layer graphene samples, as expected theoretically. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B29.00009: Local Gating of Graphene Devices via Contactless Top Gates Jairo Velasco Jr., Gang Liu, Chun Ning Lau Graphene devices with local electrostatic gates are promising candidates for investigation of novel phenomena such as Klein tunneling and the veselago lensing effect. However, it is experimentally challenging to fabricate local gates without inadvertent introduction of dopants or defects. We have developed a novel lithography process that enables fabrication of contactless, suspended top gates above single and bi-layer graphene devices. Using this technique, we have demonstrated graphene p-n junctions. We will discuss latest progress towards electrical transport of such devices in the zero-magnetic field regime, as well as in the quantum Hall regime. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B29.00010: Current-Voltage Characteristics of Electrolyte-Gated Graphene Field-Effect Transistors Inanc Meric, Sebastian Sorgenfrei, Melinda Han, Barbaros Oezyilmaz, Philip Kim, Kenneth Shepard We investigate the current-voltage characteristics of graphene field-effect transistors (FET) with ionic-solution gating. Single-layer graphene FETs are fabricated with different device dimensions and electrolytically gated with a potentiostat in which a Pt counter electrode and an Ag/AgCl reference electrode in a feedback configuration hold the solution at a desired potential. This setup enables the gating of graphene with high efficiency due to the short Debye length and high dielectric constant in ionic solutions, leading to enhanced measured transconductances. Electrolytic gating has direct applicability to field-effect sensor applications of graphene devices. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B29.00011: Fabrication of gated suspended graphene devices Kirill Bolotin, Martin Klima, Kenneth Sikes, Geoff Fudenberg, James Hone, Philip Kim, Horst Stormer We find that graphene acts as a catalyst for the vapor-phase etch of silicon dioxide: silicon dioxide under graphene is etched much faster compared to the bare surface. This is consistent with the presence of a trapped water layer between graphene and the silicon dioxide substrate which accelerates etching of the substrate. This unusual property allows us to fabricate devices where a large-area graphene flake is suspended over a micron-sized trench with the unetched silicon substrate serving as a gate electrode. Electronic transport in the resulting devices suggests enhanced sample mobilities near the Dirac point. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B29.00012: High resolution, temperature dependent Raman spectroscopy of graphene Sebastian R\'emi, Constanze Metzger, Billy Hubbard, Claire Thomas, Bennett B. Goldberg, Anna Swan Single and bi-layer graphene are studied with high resolution, temperature dependent Raman scattering. The electron-phonon coupling in graphene depends sensitively on both the concentration of charge carriers and the temperature. Raman spectroscopy directly probes electron-phonon coupling, and has been used to examine the stiffening of the G-band, phonon damping [1] and spatial inhomogeneities in the carrier density [2]. Our measurements are performed between room temperature and 4K in a confocal scanning Raman system. The samples are back-gated, allowing us to tune the carrier density and spectroscopically map the Raman response. We will discuss our recent measurements. [1] J. Yan, Y. Zhang, P. Kim, and A. Pinczuk, \textit{Phys. Rev. Lett, }\textbf{98}, 166802 (2007) [2] C. Stampfer, et al. Arxiv, cond-mat 0709.4156v1 [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B29.00013: Studies of limitations on the mobility and mean free paths in graphene devices. Xu Du, Ivan Skachko, Eva Y. Andrei The Dirac Fermion nature of the quasiparticles in graphene has led to many predictions for novel phenomena such as specular Andreev reflections at graphene-superconductor interfaces and a negative index of refraction for transmission of charge across graphene p-n junctions. These predictions presuppose ballistic transport, which requires long mean free paths compared to the distance between leads. However, within current fabrication techniques, the mean free paths of charge carriers in graphene devices are often too short for ballistic transport. The reduced mean free path is primarily due to excess scattering introduced by extrinsic factors such as material imperfections, substrate contamination, e-beam resist residue, chemical doping, contact potential and contact geometry. We will discuss the results of systematic studies of extrinsic factors, highlighting the case of graphene SNS weak links, and will propose strategies to increase the mean free path. [Preview Abstract] |
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