Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Q37: Focus Session: Graphene Structure, Dopants, and Defects: Transport II |
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Sponsoring Units: DMP Chair: Shaffique Adam, National Institute of Standards and Technology, Gaithersburg Room: C146 |
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q37.00001: Quantum motion of electrons and holes in the random puddle landscape of graphene Invited Speaker: The transport properties of graphene, especially close to the Dirac point, have puzzled physicists since its discovery in 2004. Only recently a fairly complete understanding of transport in graphene has emerged [1]. The interplay of disorder, gapless nature of the dispersion, and chirality of the quasiparticles induces the anomalous transport properties of graphene close to the Dirac point. In particular, in presence of long-range disorder the carrier density landscape close to the Dirac point breaks up in electron-hole puddles. In this highly inhomogeneous density landscape the standard theoretical approaches to transport are not valid. I will present a transport theory for graphene, and bilayer graphene, that is able to properly take into account the strong disorder-induced density inhomogeneities. The theory has three main features: {\em 1)} it treats disorder microscopically and can therefore take into account its long-range nature, {\em 2)} it provides a fully quantum mechanical analysis of transport, {\em 3)} it is able to model experimentally relevant sizes. In particular the theory presented can be used to calculate the transport properties in the crossover regime, particularly relevant for graphene, between the ballistic and the diffusive regime. I will present results for single layer graphene and bilayer graphene. In addition I will discuss the transport properties of disordered graphene p-n-p junctions for which the semiclassical approaches are inadequate and the full quantum transport analysis is necessary. \\[4pt] [1] S. Das Sarma, S. Adam, E. H. Hwang, E. Rossi, {\it Electronic transport in two dimensional graphene}, arXiv:1003.4731 (2010), to be published in Rev. Mod. Phys. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q37.00002: Interplay between density inhomogeneity and temperature in graphene transport Qiuzi Li, Euyheon Hwang, Sankar Das Sarma Motivated by recent experimental measurements of the temperature-dependent resistivity in graphene, we study the transport properties in monolayer graphene in the presence of electron-hole puddles induced by charged impurities in the environment. We explain the apparent insulating behavior of temperature-dependent conductivity observed in low mobility samples using an analytic statistical theory, which takes into account the non-mean-field nature of transport in the highly inhomogeneous density and potential landscape. In particular, the existence of puddles allows local activation of carriers in low density samples, mimicking an insulating temperature dependence in graphene conductivity. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q37.00003: Electron scattering in graphene by a correlated charged impurities Michael Fuhrer, Jun Yan, Jianhao Chen, Shudong Xiao We study charge transport in graphene with correlated charged impurities. Potassium is deposited on graphene in ultra-high vacuum at temperatures below 20 K, and the conductivity of graphene is measured as a function of carrier density in situ. Upon heating, the potassium ions order due to repulsive interactions, resulting in significant improvement of device mobility due to decrease of long range scattering. The charge density dependence of the conductivity becomes increasingly non-linear with increase of annealing temperature of the potassium/graphene. We find the experimental carrier-density-dependent conductivity in good agreement with a model of correlated charged impurities including a Gaussian-broadened structure factor at a finite wavevector corresponding to the potassium lattice. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q37.00004: Tunneling between two independently contacted graphene layers Christopher Corbet, Seyoung Kim, David C. Dillen, Babak Fallah, Michael Ramon, Emanuel Tutuc, Sanjay Banerjee We study the tunneling between two overlapped, independently contacted graphene monolayers. We use micromechanical exfoliation to deposit graphene monolayers on separate substrates. Using electron beam lithography (EBL) patterning and etching we isolate the two monolayers and remove the multilayer graphene in their close proximity. Once patterned, one monolayer was removed from the substrate and manually aligned to the other monolayer with an overlap region of a few square micrometers. EBL and metal deposition were used to define hall bars on the two separate monolayers. This design allows the extraction of each sheet's mobility and density using standard four-point resistance measurements. Using a finite element model, we calculate the current flow in each layer, as well as in between the two layers. The tunneling resistance is modeled as a contact resistance between the two graphene layers in this overlap region. We extract an upper limit for the specific tunneling resistance between the two graphene layers of 1.4E-4 Ohms*cm$^{2}$. We discuss the current density and potential dependence on the shape of the overlap region. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q37.00005: Electron Hole Asymmetry in Graphene Coupled to an SiO2 Substrate Robert Higginbotham, Nan Sun, Gerald Arnold, Steven Ruggiero The conductance of graphene generally exhibits an asymmetry in the electron and hole branches. We propose a contribution to this asymmetry that is based upon the coupling between the graphene and an SiO2 substrate. Treating the coupling in the tight-binding approximation, we calculate an exact Green's function for the coupled graphene/SiO2 system. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q37.00006: Electron-hole interference in graphene Atikur Rahman, Janice Wynn Guikema, Soo Hyung Lee, Nina Markovic The crystal symmetry of graphene gives rise to massless Dirac low-energy quasiparticles which are described by a two-component spinor that has contributions from two interpenetrating sublattices. As a result, the electron and hole states are interconnected, in sharp contrast to conventional semiconductors. Through the Aharonov-Bohm effect, we demonstrate that the electrons and holes in graphene exhibit quantum interference with each other. Our device is made of a graphene ring in contact with gold leads. A top gate on one arm of the ring independently controls the carrier type and concentration in that arm, while the back gate acts on both arms. We observe clear Aharonov-Bohm oscillations (overall visibility $\sim$10\%) in the magnetoresistance when the charge carriers are holes in one arm and electrons in the other arm. This indicates phase coherence between the electrons and holes in the two arms of the interferometer. Phase coherence is further substantiated by our observations of $T^{-1/2}$ temperature dependence of the oscillation amplitude. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q37.00007: Measurements of the energy gap in biased bilayer graphene Conor Puls, Ying Liu The application of bilayer graphene in logic-based electronics necessitates the demonstration of reliable bandgap opening, a matter complicated by charge inhomogeneity and midgap states due to local impurities and other disorder. We use dual-gated field effect transistor (FET) and planar tunnel junction devices prepared on mechanically exfoliated bilayer graphene flakes to probe the temperature dependent resistivity and density of states near the charge neutrality point. In both devices, the Fermi level and theoretical bandgap width are simultaneously controlled with a perpendicular displacement field. We report that at high displacement fields and with the Fermi level at the charge neutrality point, the temperature dependence of the resistivity follows a simple thermal activation across a gap width of up to 110 meV at high temperatures. Low temperature transport is dominated by hopping channels whose presence also increase conductivity at high temperatures, reducing the achievable $\sigma _{ON}$/$\sigma _{OFF}$ ratio, a value of great interest for FET devices. We explore the role of charged impurities found in the deposited dielectric in limiting FET performance in this respect. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q37.00008: A Unified Description of the DC Conductivity of Monolayer and Bilayer Graphene Based on Resonant Scatterers Aires Ferreira, J. Viana-Gomes, Johan Nilsson, Eduardo R. Mucciolo, Nuno M.R. Peres, Antonio H. Castro Neto We show that a coherent picture for the dc conductivity of monolayer and bilayer graphene emerges from considering that strong short-range potentials are the main source of scattering in these two systems. The origin of the strong short range potentials may lie in adsorbed hydrocarbons at the surface of graphene. The equivalence between results based on the partial wave description of scattering, the Lippmann-Schwinger equation, and the T-matrix approach is established. Scattering due to resonant impurities close to the neutrality point is investigated via a numerical computation of the Kubo formula using a kernel polynomial method. We find that realistic adsorbates originate impurity bands in monolayer and bilayer graphene close to the Dirac point. In the midgap region, a plateau of minimum conductivity of about $e^2/h$ (per layer) is induced by the resonant disorder. In bilayer graphene, a large adsorbate concentration can develop an energy gap between midgap states and high energy states. As a consequence, the conductivity plateau is supressed near the edges and a ``conductivity gap'' takes place. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q37.00009: Dynamic Screening and Spectral Functions in Bilayer Graphene Rajdeep Sensarma, Euyheon Hwang, Sankar Das Sarma We study the dynamic screening of Coulomb interactions in a bilayer graphene system within Random phase approximation. We derive an analytic expression for the dielectric function of the system and study the dispersion and damping of low energy plasmon modes. The quadratic dispersion and chirality of bilayer graphene systems lead to a plasmon dispersion which is distinct both from 2D electron gas and monolayer graphene plasmons. We also look at the effects of dynamic screening on the single particle spectral function of the system. We determine the quasiparticle weight, the effective mass and the damping of quasiparticles, which give a complete description of the low energy spectral function of the system.The compressibility of the system is also obtained from the self-energy renormalization of the chemical potential. We find that the presence of the second band leads to a well screened effective interaction, leading to much smaller renormalization of the Fermi liquid parameters in comparison to a 2D electron gas. However, the dynamic nature of the screening is very important in obtaining the single particle properties of this system. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q37.00010: Electron transport properties of bilayer graphene Kostyantyn Borysenko$^1$, Jeffrey Mullen$^2$, Xiaodong Li$^1$, Yuriy Semenov$^1$, John Zavada$^1$, Marco Buongiorno Nardelli$^{2,3}$, Ki Wook Kim$^1$ We investigate the role of different phonon scattering mechanisms in determining the electron transport properties of bilayer graphene (BLG). The ever-present electron-phonon interaction imposes the limitations on transport characteristics of any device and thus, must be always taken into account. However, in a realistic device setup, when BLG is laid (or epitaxially grown) on the top of a substrate, extrinsic scattering mechanisms (due to charged impurities, surface polar phonons, etc.) will dominate. The electron coupling with surface polar phonons of the substrate is always present and this scattering mechanism can be dominant. Using first principles approach (density functional perturbation theory) we calculate the electron-phonon matrix elements of BLG and estimate the intrinsic electron scattering rates. We show that the transport properties of the free-standing BLG resemble those of the bulk graphite. Using the Monte Carlo simulation we estimate the low-field mobility and saturation velocity of the free-standing BLG, as well as BLG on various substrates (SiC, SiO$_2$, HfO$_2$). [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q37.00011: Dual Gating of Suspended Graphene Devices via Contactless Gates Jairo Velasco Jr., Lei Jing, Gang Liu, Philip Kratz, Yongjin Lee, Wenzhong Bao, Jeanie Lau Monolayer and Bilayer graphene devices with local electrostatic gates present a rich platform for both academic and application driven inquiry. Realization of the veselago lensing effect and band gap engineering are a few of the most dazzling and promising physical phenomena that these systems are predicted to host. However, a major roadblock in this quest is the strict requirement of exceedingly clean samples. We have developed a method to fabricate suspended top gates above a freestanding graphene flake to address this challenge. Using this technique we demonstrate dual gating of a suspended graphene flake. We will discuss the latest experimental progress towards the electrical transport of such a device in the zero-magnetic field regime, as well as in the quantum Hall regime. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q37.00012: Current Annealing~of Suspended Graphene Atomic Membranes Fenglin Wang, Jairo Velasco Jr., Zeng Zhao, Hang Zhang, Philip Kratz, Lei Jing, Wenzhong Bao, Chunning Lau Using a multi-level lithographical technique, we are able to suspend graphene membranes coupled to vast majority of electrode materials. The device's mobility is significantly improved upon current annealing. By combining transport measurement with in-situ SEM imaging, we are able to monitor morphological changes in graphene and correlate with its current-voltage characteristics, thus optimizing the current annealing process. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q37.00013: Pulsed current-voltage measurements of GFETs Inanc Meric, Cory Dean, Andrea Young, Philip Kim, Kenneth Shepard Pulsed current-voltage measurements are used to measure high-bias characteristics of graphene field-effect transistors (GFET). In contrast to standard DC measurements, current saturation for channel lengths as small as 100 nm is observed when measured by this method. Our results indicate that hot carrier injection into traps in the gate oxide masks saturating characteristics in standard DC measurements. Devices exhibit constant transconductance and output conductance with scaling channel length, despite a variation in low field mobility, supporting a velocity saturation model due to optical phonon scattering. [Preview Abstract] |
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