Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C40: Graphene Transport |
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Sponsoring Units: DCMP Chair: Ruoyu Chen, The Ohio State University Room: LACC 501C |
Monday, March 5, 2018 2:30PM - 2:42PM |
C40.00001: BEC-BCS crossover of exciton condensation in graphene double-layer Xiaomeng Liu, Jia Li, Kenji Watanabe, Takashi Taniguchi, Bertrand Halperin, Cory Dean, Philip Kim In an electronic double-layer system under strong magnetic fields, electron-like and hole-like carriers of different layers can bind by Coulomb interaction to form excitons, which condense into superfluid phases at low temperatures. When the densities of the excitons are low (electron separation lB >> interlayer distance d), the electrons and holes are spatially paired (BEC condensate), while at high densities (lB ∼ d), the pairing is in momentum space and caused by the Fermi surface instability (BCS condensate). In our study, we observed exciton condensation in dual-graphite-gated graphene double-layer devices for a large range of lB parameter. For d/lB << 1, the counter-flow resistance shows an activation behavior. On the contrary, at high densities (d/lB ≈ 0.75), the counter-flow resistance exhibits sharp transitions in temperature. In this BCS regime, we observed characters of Berezinskii–Kosterlitz–Thouless (BKT) transition from current (I)-voltage (V) relation: V ∝ Iα, where α grows rapidly as temperature decreases. In the low-density regime (d/lB << 1), I-V curves exhibit much smoother transitions (α < 3), deviating from the BKT expectation. Our experimental observations thus suggest the exciton condensate in graphene undergoes a BCS-BEC crossover around d/lB ≈ 0.5. |
Monday, March 5, 2018 2:42PM - 2:54PM |
C40.00002: Magnetotransport in superconductor-graphene-superconductor Josephson junctions of various aspect ratios Seong Jang, Tae Jong Hwang, Dong Ho Kim, Eunseong Kim Superconducting proximity effect in superconductor(S)-Graphene(G)-superconductor(S) junctions has been widely investigated since it provides a unique platform to study the fascinating physics of superconductivity carried by Dirac fermions. We investigated the superconducting proximity effect in ballistic SGS Josephson junction by measuring the magnetotransport in devices of different width/length ratios. BN encapsulated graphene devices with various aspect ratios are fabricated with edge-contacted superconducting electrodes. In order to obtain highly transparent electrical contact and high critical field, we deposited niobium nitride (70nm)/niobium (3nm)/ tantalum (3nm) trilayer for the superconducting electrodes by DC magnetron sputtering. We will discuss the dependence of superconducting critical current magneto-oscillation on device geometry. |
Monday, March 5, 2018 2:54PM - 3:06PM |
C40.00003: Graphene-based Planar Josephson Junction in Short and Ballistic Regime Jinho Park, Jae-Hyeong Lee, Gil-Ho Lee, Yositake Takane, Ken-Ichiro Imura, Takashi Taniguchi, Kenji Watanabe, Hu-Jong Lee Planar graphene-based Josephson junctions are prepared by the atomic edge contact of two closely arranged Al superconducting electrodes to h-BN encapuslated graphene. We first confirm the ballisticity of our junctions by the Fabry-Perot interference in both the normal and Josephson-coupled states. The IcRN product, a quality factor of Josephson coupling, almost reaches the theoretical limit (~2.4Δ0/e) of the short-and-ballistic Josephson coupling. But the T dependence of the junction Ic does not fit the conventional Kulik-Omel’yanchuk (KO) prediction in the short-and-ballistic regime. No exponentially decaying Ic(T) is observed with increasing temperature near Tc, which excludes the long-junction characteristics. Since the KO model is valid for a one-dimensional point-contact junction it is not relevant to our planar junctions. We confirm that Ic(T) fits well the short-and-ballistic Josephson coupling proposed by a recent theoretical model [1], which takes account of the spatial carrier inhomogeneity in graphene induced by the electronic doping near the superconducting electrodes. |
Monday, March 5, 2018 3:06PM - 3:18PM |
C40.00004: Josephson Junction Based on Multi-Layer Graphene Yulu Liu, Ruoyu Chen, Takashi Taniguchi, Kenji Watanabe, Chun Ning Lau Graphene based Josephson effect has been attractive for scientists since the discovery of graphene. The quality of the devices is always related to the channel and electrode properties. Here, by fabricating a graphene based Josephson junction, we study the supercurrent in graphene with different conditions, such as the channel length, electrode materials and mobility, as well as measure its dependence on both electric field and magnetic field. |
Monday, March 5, 2018 3:18PM - 3:30PM |
C40.00005: Imaging Electron Flow through Graphene Point Contacts Mary Keenan, Sagar Bhandari, Robert Westervelt Cooled scanning gate microscopy is used to image the flow of electrons through graphene point contacts. Point contacts in 2-dimensional materials are known to be useful to understanding mesoscopic physics, and generally exhibit conductance quantization of 2πe2/h. However, observation of conductance quantization in graphene often proves challenging. Using a charged scanning probe tip to manipulate the motion of electrons in graphene, we will map conductance as a function of back gate voltage in order to better understand the causes behind this challenge. |
Monday, March 5, 2018 3:30PM - 3:42PM |
C40.00006: Magnetic imaging of electronic transport in graphene Brian Schaefer, Lei Wang, George Ferguson, Kenji Watanabe, Takashi Taniguchi, Erich Mueller, Paul McEuen, Katja Nowack We study electronic transport in graphene devices using scanning superconducting quantum interference device (SQUID) microscopy. Specifically, we image the magnetic fields originating from current flow inside a device. This allows us to visualize current patterns from different transport regimes in graphene by reconstructing the current density from the magnetic field images. A secondary measurement involves studying the response of a device to a small, localized magnetic field applied using an integrated current loop on the SQUID. This allows us to assess the performance of these devices as scanning Hall probes, which will enable magnetic imaging up to room temperature and at high magnetic fields. We report on our progress towards these measurements and present simulations of electronic transport in different regimes. The simulations inform our choice of device geometry and help us evaluate the feasibility of these measurements. |
Monday, March 5, 2018 3:42PM - 3:54PM |
C40.00007: Nano-infrared imaging of the localized plasmon resonance modes in graphene nanoribbons Yilong Luan, Fengrui Hu, Zhe Fei, Michael Fogler, Dimitri Basov We performed a nano-infrared imaging study of the localized surface plasmon resonance (SPR) modes of graphene nanoribbons by using the scattering-type scanning near-field optical microscope (s-SNOM). From the real-space imaging data, we found symmetric plasmonic interference fringes when the excitation field is parallel to ribbons and asymmetric ones in the case of perpendicular field excitation. Based on the finite-element simulations, we conclude that the observed asymmetric fringes are formed due to the interference between the localized SPR mode excited by the ribbons and the propagative surface plasmon polariton mode launched by the s-SNOM tip. Furthermore, by varying the ribbon widths, we observed different orders of SPR modes that lead to distinct asymmetric plasmonic fringes. |
Monday, March 5, 2018 3:54PM - 4:06PM |
C40.00008: Abstract Withdrawn
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Monday, March 5, 2018 4:06PM - 4:18PM |
C40.00009: Ballistic transport in natural and lithographically patterned epigraphene sidewall ribbons Dogukan Deniz, Jean-Philippe Turmaud, Yiran Hu, Yue Hu, James Gigliotti, Vladimir Prudkovskiy, Lei Ma, Claire Berger, Walt de Heer
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Monday, March 5, 2018 4:18PM - 4:30PM |
C40.00010: Reconfigurable Nanoscale Control of the Charge Neutrality Point of graphene Qing Guo, Jianan Li, Lu Chen, Mengchen Huang, Jen-Feng Hsu, Jung-Woo Lee, Hyungwoo Lee, Chang-Beom Eom, Brian D'Urso, Patrick Irvin, Jeremy Levy
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Monday, March 5, 2018 4:30PM - 4:42PM |
C40.00011: Tuning the graphene band gap by thermodynamic control of molecular self-assembly on graphene Mariana Hildebrand, Faris Abualnaja, Nicholas Harrison Using functionalised graphene is motivated by the fact that graphene is a zero band gap semiconductor. Various approaches to open a band gap for electronic applications have been made, one method being chemical functionalisation of graphene. In this work, a generic physical model to predict the self-assembly of halogenated carbene layers on graphene depending on the curvature of the graphene sheet, local distortions, as introduced by molecular adsorption, and short-range intermolecular repulsion is suggested. The thermodynamics of bidental covalent molecular adsorption on graphene and the electronic structure are examined using DFT as implemented in the Quantum Espresso code. A direct band gap opening can be found. Controlling and predicting molecular patterns and to therefore functionalise graphene in a precise way opens the possibility to effectively tune the band gap which potentially makes graphene applicable for semiconductor technologies |
Monday, March 5, 2018 4:42PM - 4:54PM |
C40.00012: Geometrically Enhanced Thermoelectric Effects in Graphene Nanoconstrictions Achim Harzheim, Jean Spiece, Charalambos Evangeli, Yuewen Sheng, Jamie Warner, Andrew Briggs, Jan Mol, Pascal Gehring, Oleg Kolosov Graphene’s thermal and electronic properties have been studied extensively over the last decade, however the influence of geometry on the thermoelectric properties has not been investigated to date. Here we will present for the first time spatially resolved measurements of Peltier, Joule and Seebeck effects on various graphene nanoconstriction geometries with a resolution down to tens of nms. Using Scanning Thermal Microscopy, we observe a strong enhancement of the local Peltier and Seebeck effect around the constriction. We attribute these observations to a combination of the geometric properties of the constriction and edge doping induced during fabrication. These findings offer a new understanding of the influence of geometry on the thermoelectric effects in graphene, opening a pathway for readily enhancing its thermoelectric properties but also those of other materials. This could possibly lead to the design of more efficient heating/cooling and thermoelectric energy conversion devices in the future. |
Monday, March 5, 2018 4:54PM - 5:06PM |
C40.00013: Transport Properties in Graphene with Antidot Lattice Lei Wang, Timir Datta, Ming Yin, Eun Choi, Jan Jaroszynski Magneto transport properties of CVD monolayer graphene with antidot lattice have been studied up to 32 Tesla with temperature from 200 mK to 50 K. Weak localization and prominent Shubnikov-de Haas oscillations are observed. The antidot lattice with different radii from 50 nm to 200 nm has great impact on effective mass of electron, carrier density and SdH effect. With increasing antidot radius, the amplitude of SdH oscillation becomes stronger. From the temperature dependent SdH oscillation and Dingle plot, the effective mass of electron is estimated and it decreases with increasing antidot radius. The lattice with 50 nm radius antidots opens a ~10 meV band gap on graphene. Geometric manipulation is an effective way to control electric properties of graphene. |
Monday, March 5, 2018 5:06PM - 5:18PM |
C40.00014: Many-Body Interactions in the Cyclotron Resonance of Encapsulated Graphene Billy Russell, Boyi Zhou, Jesse Balgley, Takashi Taniguchi, Kenji Watanabe, Erik Henriksen We present observations of interband Landau level transitions in high-mobility graphene encapsulated in hexagonal boron nitride by way of Fourier-transform infrared magneto-spectroscopy. By varying the carrier density, n, at constant magnetic field, B, we observe a non-monotonic dependence of the transition energies on the Landau level filling factor, $\nu \propto$ n/B. These findings support the idea that electron-electron interactions contribute to the Landau level transition energies in graphene, beyond the single-particle picture. A splitting of transitions involving the zeroth Landau level is interpreted as a Dirac mass arising from the coupling of the graphene and hBN lattices. Additionally, we report recent work on the intraband Landau level resonances in both encapsulated graphene and graphene on silicon dioxide. |
Monday, March 5, 2018 5:18PM - 5:30PM |
C40.00015: Dynamical THz Conductivity of Graphene for Ultrawide Band Antennas Mojtaba Dashti, David Carey Graphene is able to sustain electromagnetic wave propagation via plasmon formation and at GHz and THz frequencies and this opens up the possibility of graphene being employed for near field device-to-device communications. Here we show that at THz frequencies it is possible to adjust the inter- and intraband contributions to graphene’s dynamical conductivity to produce a variable surface impedance microstrip antenna with a several hundred GHz bandwidth. Specifically we report the optimization of a circular graphene microstrip patch antenna on silicon with an optimized return loss of -26 dB, a -10 dB bandwidth of 504 GHz and an antenna efficiency of -3.4 dB operating at a frequency of 2 THz. An improved antenna efficiency of -0.36 dB can be found at 3.5 THz but is accompanied by a lower bandwidth of about 200 GHz. Such large bandwidths and antenna efficiencies offers significant hope for a graphene based flexible directional antennas that can be employed for future THz local device-to-device communications not currently available. We also determine the real and imaginary components of the dielectric constant of graphene in the 1-10 THz regime. |
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