Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q17: Graphene: Disorder and Defects |
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Sponsoring Units: DCMP Chair: Shaffique Adam, National University of Singapore Room: 102AB |
Wednesday, March 4, 2015 2:30PM - 2:42PM |
Q17.00001: Conductivity noise as a transport-based probe to study the charge-carrier transmission across grain boundaries in polycrystalline graphene Vidya Kochat, Chandra Sekhar Tiwary, Tathagata Biswas, Gopalakrishnan Ramalingam, Srinivasan Raghavan, Kamanio Chattopadhyay, Manish Jain, Arindam Ghosh Grain boundaries (GBs) form a class of topological defects, intrinsically present in polycrystalline graphene films and inevitably affect the electronic properties of the otherwise perfect honeycomb lattice. In this work, we have studied the charge carrier transmission across individual GBs in graphene having varying levels of disorder. We find that the defect density in the interface region of two adjoining graphene grains is directly related to their misorientation angle. Conductivity noise (low frequency 1/f noise) is a more sensitive probe to identify the microscopic mechanism of carrier scattering and can quantify the disorder levels of various types of GBs. The 1/f noise across wider interfaces were found to be higher by an order of magnitude when compared to the single-crystalline graphene regions, indicating huge amount of dynamic scattering processes at the interface region. We also obtain evidence for enhanced spin-flip scattering at the GBs from the measured conductivity noise at low temperatures suggestive of the fact that the GBs in graphene could sustain local magnetic moments. [Preview Abstract] |
Wednesday, March 4, 2015 2:42PM - 2:54PM |
Q17.00002: Intrinsic carrier mobility of Dirac cones: limitations of the deformation potential theory Zhenzhu Li, Jinying Wang, Zhirong Liu An analytic formula for the intrinsic carrier mobility of Dirac cones under acoustic phonon scattering mechanism was obtained for 2D systems such as graphene and graphyne. The influence of both transverse acoustic (TA) and longitudinal acoustic (LA) phonon modes as well as the anisotropy were considered. Some extraordinary characteristics different from the prediction of the deformation potential theory were revealed: the mobility at the neutrality point is proportional to 1/$T^{3}$ where $T$ is the temperature; carrier scattering by TA phonons dominates the mobility of graphene, which explains the overestimated measured deformation potential of graphene in experiments. The theory was combined with first-principles calculation to determine the mobility of graphene and five graphynes with Dirac cones. It was predicted that most graphynes possess much higher mobility than graphene due to the suppression of the scattering by TA phonons. [Preview Abstract] |
Wednesday, March 4, 2015 2:54PM - 3:06PM |
Q17.00003: Dephasing time in graphene due to interaction with flexural phonons Wei L.Z Zhao, Konstantin Tikhonov, Alexander Finkel'stein We investigate decoherence of an electron in graphene caused by electron-flexural phonon interaction. We find out that the flexural phonons can produce dephasing rate comparable to the electron-electron one. The problem appears to be quite special because there is a large interval of temperatures where dephasing rate cannot be obtained using the golden rule. We evaluate this rate for a wide range of density ($n$) and temperature ($T$) and determine several asymptotic regions with temperature dependence crossing over from $\tau_{\phi}^{-1}\sim T^{2}$ to $\tau_{\phi}^{-1}\sim T$ when temperature increases. We also find $\tau_{\phi}^{-1}$ to be a non-monotonous function of $n$. These distinctive features of the new contribution can provide an effective way to identify flexural phonons in graphene through the electronic transport by measuring the weak localization corrections in magnetoresistance. [Preview Abstract] |
Wednesday, March 4, 2015 3:06PM - 3:18PM |
Q17.00004: ABSTRACT WITHDRAWN |
Wednesday, March 4, 2015 3:18PM - 3:30PM |
Q17.00005: Multi-Barrier Electron Tunneling Effects in Bilayer and Gapped Graphene Dipendra Dahal, Godfrey Gumbs, Andrii Iurov A detailed study has been made of electron tunneling through a square potential barrier in bilayer, gapped graphene and the relatively new material silicone. The investigation has covered dressed electron states by circularly polarized light of electrons in bilayer graphene for various angles of incidence and through a periodic sequence of potential barriers as well as a quasiperiodic arrangement. The spin degenerate states of silicone may be lifted by a perpendicular electric field. We report spin-dependent tunneling in silicene. [Preview Abstract] |
Wednesday, March 4, 2015 3:30PM - 3:42PM |
Q17.00006: Violation of the Wiedemann-Franz law in clean graphene layers Alessandro Principi, Giovanni Vignale The Wiedemann-Franz law, connecting the electronic thermal conductivity to the electrical conductivity of a disordered metal, is generally found to be well satisfied even when electron-electron (e-e) interactions are strong. In ultra-clean conductors, however, large deviations from the standard form of the law are expected, due to the fact that e-e interactions affect the two conductivities in radically different ways. Thus, the standard Wiedemann-Franz ratio between the thermal and the electric conductivity is reduced by a factor $1+\tau/\tau_{\rm th}^{\rm ee}$, where $1/\tau$ is the momentum relaxation rate, and $1/\tau_{\rm th}^{\rm ee}$ is the relaxation time of the thermal current due to e-e collisions. Here we study the density and temperature dependence of $1/\tau_{\rm th}^{\rm ee}$ in the important case of doped, clean single layers of graphene, which exhibit record-high thermal conductivities. We show that at low temperature $1/\tau_{\rm th}^{\rm ee}$ is $8/5$ of the quasiparticle decay rate. We also show that the many-body renormalization of the thermal Drude weight coincides with that of the Fermi velocity. [Preview Abstract] |
Wednesday, March 4, 2015 3:42PM - 3:54PM |
Q17.00007: Dilute fluorinated graphene and bilayer graphene: resonant impurity scattering, anomalous magneto-transport and local spin-orbit coupling J. Zhu, X. Hong, A. Stabile, C. Herding, S.-H. Cheng, K. Zou, B. Wang, J. Li Graphene is a high-mobility semi-metal with weak spin-orbit coupling (SOC). I will discuss the striking effects of a dilute coverage of chemisorbed fluorine adatoms (F:C $<0.1\%$) on charge transport and magneto-transport of graphene and bilayer graphene. We show that electron scattering with the F-adatoms can be quantitatively described by resonant impurity scattering. The T-dependence of conductivity reveals strong quantum corrections not yet understood, which differs qualitatively between F-monolayer and F-bilayer. Both F-monolayer and F-bilayer exhibit weak localization in a magnetic field. The dephasing rate $\tau_{\phi}^{-1}$ is dramatically enhanced in fluorined samples, compared to pristine and defluorinated control samples. It is further tunable by a perpendicular electric field in dual-gated F-bilayer devices. Strikingly, the ratio of $\tau_{\phi}^{-1}$ over the transport relaxation rate $\tau_p^{-1}$ is independent of $n_F$ and scales with the carrier density n as $n^{-1}$ in both F-monolayer and F-bilayer. Strong local SOC induced by the F-adatoms, combined with the unusual effect of SOC on the magneto-resistance of WL, is likely to play a key role. Fluorine induced SOC has important implications on spin relaxation and spin Hall current in these engineered materials. [Preview Abstract] |
Wednesday, March 4, 2015 3:54PM - 4:06PM |
Q17.00008: Scattering of two-dimensional massless Dirac electrons by a circular potential barrier Jhih-Sheng Wu, Michael Fogler We calculate the differential, total, and transport cross-sections for scattering of two-dimensional massless Dirac electrons by a circular barrier. For scatterer of a small radius, the cross-sections are dominated by quantum effects such as resonant scattering that can be computed using the partial-wave series. Scattering by larger size barriers is better described within the classical picture of reflection and refraction of rays, which leads to phenomena of caustics, rainbow, and critical scattering. Refraction can be negative if the potential of the scatterer is repulsive, so that a $p$-$n$ junction forms at its boundary. Qualitative differences of this case from the $n$-$N$ doping case are examined. Quantum interference effects beyond the classical ray picture are also considered, such as normal and anomalous diffraction, and also whispering-gallery resonances. Implications of these results for transport and scanned-probe experiments in graphene and topological insulators are discussed. [Preview Abstract] |
Wednesday, March 4, 2015 4:06PM - 4:18PM |
Q17.00009: Phonon coupling and disorder induce infrared optical transparency in graphene Bruno Rousseau, Fran\c{c}ois Lapointe, Michel C\^ot\'e, Richard Martel Recent infrared spectroscopy measurements of doped graphene grafted with iodophenyl moieties have revealed fairly narrow transmission windows which vary as a function of the chemical potential, in contrast to the featureless, Drude-like spectrum of pristine graphene in this frequency range. These asymmetric windows appear at energies corresponding to phonon modes near the $\Gamma$ and K points. We propose a model which involves coherent intraband scattering with defects and phonons, thus relaxing the optical selection rule forbidding access to $\bf{q} \neq \Gamma$ phonons. Numerical simulations based on the model reproduce the features of the experimental observations (number of bands, energies, variation in energy and intensity with respect to chemical potential). [Preview Abstract] |
Wednesday, March 4, 2015 4:18PM - 4:30PM |
Q17.00010: Electron transmission through the stacking domain boundary on multilayer graphene Nam Nguyen, Mikito Koshino We present a theoretical study on the electron transmission through the AB-BA stacking boundary in bilayer trilayer and tetralayer graphene. Using the Green function method, we calculate the electron transmission probability through the stacking faults as the electron Fermi energy. In AB-BA bilayer boundary, the system is almost insulating at the low energy, while the transmission sharply rises as the Fermi energy increases to higher energy. This suggests that the stacking fault crucially suppresses the electron transmission in the intrinsic graphene bilayer at the charge neutral. We also study the effect of the perpendicular electric field which opens an energy gap, and find that the gap-opening and the Mexican-hat band deformation significantly enhance the electron transmission at the low-electron density. For the ABA-ABC domain boundary in trilayer graphene, we notice a similar behavior of electron transmission to the bilayer case, but in the tetralayer case (ABAB-ABAC boundary), the low-energy transmission is not much suppressed unlike in bilayer and trilayer cases. [Preview Abstract] |
Wednesday, March 4, 2015 4:30PM - 4:42PM |
Q17.00011: Inelastic carrier lifetime in a coupled graphene electron-phonon system: Role of plasmon-phonon coupling Hongki Min, Seongjin Ahn, E.H. Hwang We calculate the inelastic scattering rates and the hot electron inelastic mean free paths for both monolayer and bilayer graphene on a polar substrate. We study the quasiparticle self-energy by taking into account both electron-electron and electron-surface optical (SO) phonon interactions. In this calculation the leading order dynamic screening approximation (G$_0$W approximation) is used to obtain the quasiparticle self-energy by treating electrons and phonons on an equal footing. We find that the strong coupling between the SO phonon and plasmon leads to a new decay channel for the quasiparticle through the emission of the coupled mode, and gives rise to an abrupt increase in the scattering rate, which is absent in the uncoupled system. In monolayer graphene a single jump in the scattering rate occurs, arising from the emission of the low energy branch of the coupled plasmon-phonon modes. In bilayer graphene the emission of both low and high energy branches of the coupled modes contributes to the scattering rate and gives rise to two abrupt changes in the scattering rate. The jumps in the scattering rate can be potentially used in the hot electron device such as switching devices and oscillators. Ref) Seongjin Ahn, E. H. Hwang, and Hongki Min, arXiv:1409.8394. [Preview Abstract] |
Wednesday, March 4, 2015 4:42PM - 4:54PM |
Q17.00012: Conductivity and thermoelectric power in graphene: Interplay of disorder, Coulomb interaction, and optical phonons Matthew Foster, Hong-Yi Xie We study the electric and thermoelectric transport of Dirac fermions in graphene using the Boltzmann-equation approach. We consider the effects of quenched disorder, Coulomb interactions, and optical-phonon scattering and analyze the electric conductivity and the thermoelectric power (TEP) as functions of temperature $T$ and chemical potential $\mu$ by unbiased numerical solutions to the Boltzmann equation. In the absence of optical phonons, for very clean graphene we observe the crossover from the interaction-limited hydrodynamic regime $\mu \ll T$ to the disorder-limited Fermi liquid regime $\mu \gg T$. In the hydrodynamic regime, the TEP significantly deviates from Mott's law and follows the result anticipated by the relativistic hydrodynamic theory. Moreover, we analyze the doping and screening effects upon the quantum minimal conductance, which indicates the dissipation induced by inelastic electron-hole scattering. On the other hand, we find that optical phonons start to contribute at relatively low temperatures, about one order of magnitude less than the phonon excitation energy. Especially, the TEP shows a non-monotonic temperature dependence and a peak appears at about $T \sim 200$-$300$ K for a large variety of doping. [Preview Abstract] |
Wednesday, March 4, 2015 4:54PM - 5:06PM |
Q17.00013: Temperature and bias dependence of barrier heights in graphene / semiconductor Schottky diodes under reverse bias Dushyant Tomer, Shivani Rajput, Lawrence Hudy, Lian Li Sensors based on graphene / semiconductor Schottky diodes have shown significant enhancement in sensitivity over field effect devices when operated under reverse bias, where the conductivity has an exponential dependence on the Schottky barrier height. In this work, chemical vapor deposited monolayer graphene is transferred onto Si- and C-face of hexagonal SiC, Si(111), and GaAs(001) substrates, as confirmed by scanning tunneling microscopy. Temperature and bias dependence of the barrier height are obtained by current-voltage measurements between 250 and 340 K. For all four junctions, the barrier increases linearly with temperature. However, as a function of reverse bias, it decreases linearly for graphene / SiC, but exhibits a non-linear dependence for graphene / (Si, GaAs) Schottky junctions. These findings and their implication on the performance of sensors based on graphene/semiconductor Schottky diodes will be discussed at the meeting. Supported by U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-FG02-07ER46228.. [Preview Abstract] |
Wednesday, March 4, 2015 5:06PM - 5:18PM |
Q17.00014: ABSTRACT WITHDRAWN |
Wednesday, March 4, 2015 5:18PM - 5:30PM |
Q17.00015: Screening of substrate charged impurities as mechanism of conductance change in graphene gas sensing Sang-Zi Liang, Gugang Chen, Avetik Harutyunyan, Jorge Sofo In graphene sensing of gaseous $NO$, $NO_2$, and $NH_3$, the measured conductance change after the sensor is exposed to the molecules has been traditionally attributed to carrier density change due to charge transfer between the sample and the adsorbed molecule. However, this explanation ignores the effect of the adsorbates on the electron mobility, and analysis of the electron affinity/ionization potential does not favor charge transfer. In this talk, we propose and explore an alternative mechanism. When adsorbed, charged and dipolar functional on the surface of graphene may counteract and screen charged impurities on the substrate. Because scattering of electrons with these charged impurities has been shown to be a limiting factor in graphene conductivity, the screening leads to significant changes in the transport behavior. A model for the conductivity is established using the random phase approximation dielectric function of graphene and the first-order Born approximation for scattering. The model predicts maximal screening magnitudes for the charge and dipole moment. The dipole screening is generally weaker than the charge screening although the former becomes more effective with higher gate voltage. [Preview Abstract] |
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