Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session S17: Graphene: Transport Properties |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Inti Sodermann, Massachusetts Institute of Technology Room: 102AB |
Thursday, March 5, 2015 8:00AM - 8:12AM |
S17.00001: Detecting topological currents in graphene superlattices Geliang Yu, Andrey Kretinin, Justin Song, Roman Gorbachev, Leonid Levitov, Konstantin Novoselov, Andre Geim Topological materials may exhibit Hall-like currents flowing transversely to the applied electric field even in the absence of a magnetic field. In graphene superlattices, which have broken inversion symmetry, topological currents originating from graphene's two valleys are predicted to flow in opposite directions and combine to produce long-range charge neutral flow. We observed this effect as a nonlocal voltage at zero magnetic field in a narrow energy range near Dirac points at distances as large as several micrometers away from the nominal current path. Locally, topological currents are comparable in strength with the applied current, indicating large valley-Hall angles. The long-range character of topological currents and their transistor-like control by means of gate voltage can be exploited for information processing based on valley degrees of freedom. [Preview Abstract] |
Thursday, March 5, 2015 8:12AM - 8:24AM |
S17.00002: Quantum Spin Hall phase in multilayer graphene Noel Garcia, Jose Luis Lado, Joaquin Fernandez-Rossier We address the question of whether multilayer graphene systems are Quantum Spin Hall (QSH) insulators. Since interlayer coupling coples $p_{z}$ orbitals to $s$ orbitals of different layers and Spin-Orbit (SO) couples $p_{z}$ orbitals with $p_{x}$ and $p_{y}$ of opposite spins, new spins mixing channels appear in the multilayer scenario that were not present in the monolayer. These new spin-mixing channels cast a doubt on the validity of the spin-conserving Kane-Mele model for multilayers and motivates our choice of a four orbital tight-binding model in the Slater-Koster approximation with intrinsic Spin-Orbit interaction. To completely determine if the QSH phase is present we calculate for different number of layers both the $Z_2$ invariant for different stackings (only for inversion symmetric systems), and the density of states at the edge of semi-infinite graphene ribbon with armchair termination. We find that systems with even number of layers are normal insulators while systems with odd number of layers are QSH insulators, regardless of the stacking. [Preview Abstract] |
Thursday, March 5, 2015 8:24AM - 8:36AM |
S17.00003: Probing layer localization in twisted graphene bilayers via cyclotron resonance Chi-Ken Lu, H.A. Fertig Electron wave functions in twisted bilayer graphene may have a strong single-layer character or be intrinsically delocalized between layers, with their nature often determined by how energetically close they are to the Dirac point. We demonstrate that in magnetic fields, cyclotron resonance spectra contain signatures that may be used to distinguish the nature of these wave functions at low energies, as well as to locate low-energy critical points in the zero-field energy spectrum. Optical absorption for two different configurations--electric field parallel and perpendicular to the bilayer--is calculated, and the configurations are shown to have different selection rules with respect to which states are connected by the perturbation. Interlayer bias further distinguishes transitions involving states of a single-layer nature from those with support in both layers. For doped systems, a sharp increase in intra-Landau-level absorption occurs with increasing field as the level passes through the zero-field saddle-point energy, where the states change character from single layer to bilayer. The effects of impurity scattering will be discussed too. [Preview Abstract] |
Thursday, March 5, 2015 8:36AM - 8:48AM |
S17.00004: Spin and valley polarized quantum Hall edge states in monolayer graphene Di Wei, Javier Sanchez-Yamagishi, Toeno van der Sar, Pablo Jarillo-Herrero, Amir Yacoby Studying edge transport in the quantum Hall regime provides insight into the nature of partially filled Landau levels. Previous reports on graphene have shown that when the four-fold degeneracy of each Landau level is lifted, spin and valley polarizations influence scattering between edges. We report progress on the fabrication of monolayer graphene devices encapsulated in hexagonal boron nitride (hBN), a technique which allows us to produce pristine graphene samples showing robust broken symmetry quantum Hall states. Additionally, graphene encapsulation allows us to create both globally and locally gated regions with sharp carrier density boundaries due to our thin hBN gate dielectrics. Here we study the transport of spin and valley polarized edge currents at these junctions. [Preview Abstract] |
Thursday, March 5, 2015 8:48AM - 9:00AM |
S17.00005: Twisting the physics in bilayer graphene Jason Luo, Javier Sanchez-Yamagishi, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero Twisted bilayer graphene is the ultimate limit of a bilayer 2DEG, where two graphene layers are stacked with an interlayer distance of only 0.34nm. The interlayer tunnel coupling can be continuously tuned by twisting the two layers, leading to different physics in the small and large twist angle limits. At small twist angles, the two layers form a large superlattice unit cell and the hybridization of the layers leads to low-energy van Hove singularities in the electronic spectrum, resulting in a strong departure from the typical monolayer graphene transport properties. At large twist angles, the system behaves as two decoupled monolayer graphene sheets, and the occupation of quantum Hall edge modes on each layer can be independently controlled. This allows for the realization of a quantum spin Hall state in twisted bilayer graphene by doping to form an electron-hole bilayer at moderate magnetic fields. I will discuss our magnetotransport measurements of high-quality twisted bilayer graphene, and how, by independently controlling the total charge density and applied electric field, we can realize different novel electronic states in this system. [Preview Abstract] |
Thursday, March 5, 2015 9:00AM - 9:12AM |
S17.00006: Ballistic transport and densiy of states of modulated bilayer graphene Liubov Zhemchuzhna, Danhong Huang, Godfrey Gumbs, Andrii Iurov The magnetic band structure for electrostatically modulated bilayer graphene is calculated. We include the $K$ and $K^\prime$ valleys. $A$ and $B$ sublattices as well as the bilayer crystalline structure. The energy eigenvalues are obtained as functions of wave vector as well as magnetic field.\footnote{Godfrey Gumbs, Andrii Iurov, Danhong Huang, and Liubov Zhemchuzhna: Phys. Rev. B {\bf 89}, 241407(R) (2014).} Our results are then employed in calculating density-of-states and ballistic conductance. Comparison with recent experimental results is presented. [Preview Abstract] |
Thursday, March 5, 2015 9:12AM - 9:24AM |
S17.00007: SU(4) symmetry breaking revealed by magneto-optical spectroscopy in epitaxial graphene Liang Z Tan, Milan Orlita, Marek Potemski, Mike Sprinkle, Claire Berger, Walter de Heer, Steven Louie, Gerard Martinez Electron-electron and electron-phonon interactions break the spin and valley degeneracies of the lowest Landau level (LL) in graphene. Multiple theoretical models have been proposed for the broken symmetry ground state. Previous tilted magnetic field transport experiments have obtained partial information on the ground state by probing the spin degree of freedom. In this work, we show that, via the valley-dependent electron-phonon interaction, symmetry breaking of the valley degree of freedom can be detected in infra-red transmission signatures close to magneto-phonon resonances. We have performed infra-red magneto-transmission experiments on multi-layer epitaxial graphene samples in magnetic fields up to 35 T. Following the main optical transition involving the lowest LL, we observe a new absorption transition increasing in intensity with magnetic fields greater than 26 T. Our theoretical calculations quantitatively explain these features, and unambiguously identify the charge density wave as the ground state in our samples. [Preview Abstract] |
Thursday, March 5, 2015 9:24AM - 9:36AM |
S17.00008: Combination of conductance oscillation in the quantum Hall regime and charge trap flash memory phenomena in graphene field effect transistor Yoojoo Yun, Haeyong Kang, Joong-gyu Kim, Jeongmin Park, Thuy Kieu Truong, Nahee Park, Jeong-gyun Kim, Yourack Lee, Hoyeol Yun, Sang Wook Lee, Young Hee Lee, Dongseok Suh We present the feature of conductance oscillation in the quantum Hall regime graphene FET on top of large gate-voltage hysteresis (up to 100 V). A mono-layer graphene was put on the hBN flake on the wet silicon oxide/silicon substrate. At 300 K, the normal conductance versus gate-voltage curve was observed showing the charge neutrality point without hysteresis. At 2 K, however, there was a huge conductance hysteresis during the sweep of gate-voltage, which could be attributed to the characteristics of charge-trap memory behavior because of defects located inside the dielectric playing a role of charge-trap sites. Even though the hysteresis during gate-voltage sweeping was enormous, in our device having hBN for graphene device preventing the deteriorating impacts from the defective SiO$_{\mathrm{2}}$, the conductance oscillation during the gate-voltage sweep was observed from the magnetic field 4 T. In summary, the results proved that the combination of quantum Hall related transport phenomena and the charge-trap memory operation was achieved successfully without affecting each other in our graphene-on-hBN FET device. [Preview Abstract] |
Thursday, March 5, 2015 9:36AM - 9:48AM |
S17.00009: Transport in graphene on boron nitride Ashley DaSilva, Jeil Jung, Shaffique Adam, Allan MacDonald The lattice mismatch and twist angle of graphene on boron nitride contributes to a long wavelength moire pattern in the atomic positions. This superlattice structure and the sublattice symmetry breaking in the hexagonal boron nitride layer lead to observable transport features in the graphene layer. We show a decreased conductivity at the Dirac point due to an opening of the band gap. There is also a decreased conductivity when the Fermi level is tuned to four carriers per moire unit cell, which is the position of the secondary Dirac points in perfectly matched and sublattice symmetric hexagonal bilayers. We show both intraband and interband contributions to the conductivity, the latter is peaked when the Fermi level is tuned inside the gaps, either at the Dirac point or the secondary Dirac points. [Preview Abstract] |
Thursday, March 5, 2015 9:48AM - 10:00AM |
S17.00010: Graphene-based quantum Hall resistance standards grown by chemical vapor deposition on silicon carbide Rebeca Ribeiro-Palau, Fabien Lafont, Dimitris Kazazis, Adrien Michon, Olivier Couturaud, Christophe Consejo, Benoit Jouault, Wilfrid Poirier, Felicien Schopfer Replace GaAs-based quantum Hall resistance standards (GaAs-QHRS) by a more convenient one, based on graphene (Gr-QHRS), is an ongoing goal in metrology. The new Gr-QHRS are expected to work in less demanding experimental conditions than GaAs ones. It will open the way to a broad dissemination of quantum standards, potentially towards industrial end-users, and it will support the implementation of a new International System of Units based on fixed fundamental constants. Here, we present accurate quantum Hall resistance measurements in large graphene Hall bars, grown by the hybrid scalable technique of propane/hydrogen chemical vapor deposition (CVD) on silicon carbide (SiC). This new Gr-QHRS shows a relative accuracy of $1\times10^{-9}$ of the Hall resistance under the lowest magnetic field ever achieved in graphene. These experimental conditions surpass those of the most wildely used GaAs-QHRS. These results confirm the promises of graphene for resistance metrology applications and emphasizes the quality of the graphene produced by the CVD on SiC for applications as demanding as the resistance metrology. [Preview Abstract] |
Thursday, March 5, 2015 10:00AM - 10:12AM |
S17.00011: Observation of conducting edge states in graphene at zero magnetic field Monica Allen, Ion Fulga, Oles Shtanko, Kenji Watanabi, Takashi Taniguchi, Pablo Jarillo-Herrero, Anton Akhmerov, Leonid Levitov, Amir Yacoby The electronic nature of edge states confined to the boundaries of a graphene crystal remains an outstanding question. Proposals range from Anderson localization to chiral zero-energy edge modes, but a full microscopic picture of edge transport remains elusive. We directly image current transmission in real space by coupling superconducting electrodes to a graphene crystal and measuring quantum interference as a function of applied magnetic flux. To obtain a more quantitative picture, we employ Fourier techniques to extract the real space current distribution with nanoscale precision. We observe robust confinement of current to the edges of the crystal approaching the Dirac point and show that relative edge and bulk contributions are tunable via electrostatic gating. A strong candidate consistent with our data is the proposal of chiral edge modes that arise from sublattice symmetry breaking at the edge, sustained in all crystallographic edge orientations except atomically perfect armchair. Our techniques also open the door to fast spatial imaging of current distributions along more complicated networks of domains in larger crystals. [Preview Abstract] |
Thursday, March 5, 2015 10:12AM - 10:24AM |
S17.00012: ABSTRACT WITHDRAWN |
Thursday, March 5, 2015 10:24AM - 10:36AM |
S17.00013: Experimental evidence of one-dimensional edge states at the line junction of two oppositely biased bilayer graphene J. Li, K. Watanabe, T. Taniguchi, J. Zhu A one-dimensional edge (kink) state is predicted to exist at the line junction of two gapped bilayer graphene with opposite electric field bias or the stacking fault of AB-BA stacked bilayer regions. The conductance of the kink state is expected to be quantized at 4$e^2/h$ in the absence of K-K$^\prime$ valley mixing, counting spin and layer degeneracy. This novel 1D system has not been realized experimentally due to fabrication challenges. Here we report evidence of the kink state in split dual-gated bilayer graphene, where the top and bottom splits are approximately 70 nm and precisely aligned. $h$-BN encapsulation ensures the high quality of the device, which allows us to make the bulk bilayer graphene very insulating at moderate E-fields of less than 0.3V/nm. The junction resistance $R_j$ exhibits drastic contrast between low resistances of several tens of $K\Omega$ when the two bilayers are oppositely biased, versus high resistances of several $M\Omega$ when the two E-fields have the same polarity. The low-resistance states are weakly insulating in temperature dependence and their resistances drop substantially in a perpendicular magnetic field. We discuss the nature of the kink state and possible reasons that $R_j$ deviates from the single-particle prediction of $4h/e^2$. [Preview Abstract] |
Thursday, March 5, 2015 10:36AM - 10:48AM |
S17.00014: Semiconducting transport characteristics of monolayer graphene through substrate-induced functionalization Po-Hsiang Wang, Lo-Yueh Chang, Fu-Yu Shih, Po-Hsun Ho, Chia-Hao Chen, Chun-Wei Chen, Wei-Hua Wang We report semiconducting transport behaviors of monolayer graphene functionalized through chemically reactive substrates. In contrast to pristine graphene, graphene on activated SiO$_{\mathrm{2}}$/Si substrates exhibits a transport gap at cryogenic temperature, nonlinear transfer characteristics, and insulating transport behaviors. Raman spectroscopy was performed to provide evidence of sp$^{\mathrm{3}}$ hybridization of graphene and confirms the presence of chemical bonding in the graphene samples. Moreover, we observe hopping transport characteristics at cryogenic temperature. Our study points toward an alternative method to control the functionalization of graphene and its transport behaviors. [Preview Abstract] |
Thursday, March 5, 2015 10:48AM - 11:00AM |
S17.00015: Tunable Optoelectronic Properties of Semiconducting Graphene Marc Dvorak, Zhigang Wu If patterned properly, structural modifications on graphene can induce intervalley scattering between Dirac points and open a sizeable band gap. Such two-dimensional semiconductors could offer great tunability in electronic and optical properties. We performed calculations based on many-body perturbation theory using Green's functions to obtain quasiparticle energies and optical absorption spectra for these semiconducting graphene structures, focusing on the role of defect size, type, and geometric configuration. Our results show a strong renormalization of the band gap over Kohn-Sham energy levels and exciton binding energies greater than 0.4 eV. By stacking monolayer defected graphene with various defect sizes and configurations, one could create an excellent photovoltaic absorber layer that efficiently absorbs photons with energy larger than 1.0 eV. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700