Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session B25: Graphene II: Electronic Properties and QHE |
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Sponsoring Units: DMP Chair: Ziqiang Wang, Boston College Room: 327 |
Monday, March 16, 2009 11:15AM - 11:27AM |
B25.00001: Quantum Hall effect of massless Dirac fermions in a vanishing magnetic field Kentaro Nomura, Shinsei Ryu, Mikito Koshino, Christopher Mudry, Akira Furusaki Graphene displays an unconventional quantization of the Hall conductivity when subjected to a magnetic field. The Hall conductivity is measured to be a half-integer in units of 4 times the conductance quantum. We have undertaken a numerical study of the quantum Hall effect of massless Dirac fermions in two-dimensions, and found the following remarkable effect. A negative and a positive quantized Hall plateaus survive even in the limit of vanishing magnetic fields provided inter-valley scattering is negligible. This conclusion is based on our finding that all but one critical states between the different quantized Hall plateaus float away from the charge neutral point as the Landau level mixing becomes stronger. The exception is the state at the neutral point that remains critical whatever the disorder strength is, or equivalently no matter how weak the magnetic field is. K. Nomura, S. Ryu, M. Koshino, C. Mudry, A. Furusaki, Phys. Rev. Lett. 100, 246806 (2008). [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B25.00002: New quantum Hall ferromagnetic states in graphene bilayers Rene Cote, Yafis Barlas, Jules Lambert, Allan H. MacDonald, Kentaro Nomura Quantum Hall ferromagnetic states are expected in graphene bilayers because of the degeneracy of the eight Landau levels which appear near the neutral system Fermi level. Working within the Hartree-Fock approximation, we derive the phase diagram of the two-dimensional electron gas (2DEG) at integer filllings $\nu=-4,-3,...,3,4$ as a function of the magnetic field and an external potential difference between the layers. We show that coulomb interaction leads to broken symmetry ground states that lift the degeneracies associated with spin, valley pseudospin and orbital pseudospin (the $n=0$ and $n=1$ orbital Landau level states are degenerate in graphene bilayers). The phase diagram of the 2DEG in this system is very rich and contains states with interlayer and/or orbital coherence that can be abruptly modified by an interlayer potential difference. We show that some of the broken symmetry states have collective excitations with unusual dispersion relations due to the coupling between valley and orbital pseudospins whose fluctuations give rise to electric dipoles. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 12:15PM |
B25.00003: Novel electronic and transport properties of graphene superlattices Invited Speaker: Charge carriers in graphene show linear and isotropic energy dispersion relation and chiral behavior, like massless neutrinos in particle physics. Because of these novel properties, many interesting and unconventional phenomena occur in graphene. On the other hand, since the 1970's, metallic and semiconducting superlattice structures -- man-made crystals -- have been extensively studied regarding to their fundamental electronic and optical properties as well as many applications. In this talk, I will present calculations on the properties of charge carriers in graphene under an external periodic potential (graphene superlattices) which are found to be greatly different from those of conventional two-dimensional electron gases in similar conditions [1-3]. I will discuss the anisotropies in the group velocity around the Dirac point and in the gap opening at the supercell Brillouin zone boundary [1]. Next, I will focus on the special cases where the group velocity along one direction becomes zero [1,2], emphasizing the phenomena of pseudospin collapse and possible electron beam supercollimation effects in these systems [2]. Finally, I will discuss the properties of a new generation of massless Dirac fermions at the supercell Brillouin zone boundaries and their experimental implications [3]. \\[4pt] [1] C. -H. Park, L. Yang, Y. -W. Son, M. L. Cohen, and S. G. Louie, Nature Phys. 4, 870 (2008). \\[0pt] [2] C. -H. Park, Y. -W. Son, L. Yang, M. L. Cohen, and S. G. Louie, Nano Lett. 8, 2920 (2008). \\[0pt] [3] C. -H. Park, L. Yang, Y. -W. Son, M. L. Cohen, and S. G. Louie, Phys. Rev. Lett. 101, 126804 (2008). [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B25.00004: Quantum Hall wavefunction for Dirac fermions in high magnetic field Feng Cai, Ziqiang Wang A salient feature of planar Dirac fermion system is the presence of zero modes in high magnetic field with non-trivial topological properties. Based on this fact we develop a field theoretic approach to derive the ground state wavefunction in the $n=0$ Landau level. We discuss possible connections to the unconventional quantum Hall states observed in graphene. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B25.00005: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 12:39PM - 12:51PM |
B25.00006: Edge states in Graphene: from gapped flat band to gapless chiral modes Shengyuan Yang, Wang Yao, Qian Niu We study edge-states in graphene systems where a bulk energy gap is opened by inversion symmetry breaking. We find that the edge-bands dispersion can be controlled by potentials applied on the boundary row. Under certain boundary potentials, gapless edge-states with valley-dependent velocity are found, exactly analogous to the spin-dependent gapless chiral edge-states in quantum spin Hall systems. The connection of the edge-states to bulk topological properties is revealed. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B25.00007: Excitonic supersolid in quantum Hall graphene bilayers Yogesh Joglekar, Chang-hua Zhang We study the ground state of two graphene sheets separated by a distance $d$ in the quantum Hall regime where the top layer has electrons and the bottom layer has holes as carriers. We obtain a rich mean-field phase diagram as a function of distance $d$ and the partial filling factor $\nu_e=\nu_h=\nu$ for different Landau levels. We find that the ground state in high Landau levels at large $d$ is a generalized Wigner crystal that includes anisotropic stripe and bubble states, and at small $d$ the ground state is a uniform excitonic condensate. We show that for a wide range of partial filling factors $0\le \nu \leq 1/2$, at intermediate values of $d$, the ground state has interlayer phase coherence as well as a lattice structure, i.e. it is an excitonic supersolid. We discuss the predictions for signatures of such a state in transport and optical experiments. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B25.00008: Transport on quantum antidot made of 4-terminal graphene ribbons Andrea Latge, Carlos Ritter, Pedro Orellana, Monica Pacheco Electronic and transport properties of four-terminal graphene ribbons are discussed taking into account different configurations of quantum antidot potentials, designed at a central conductor. In general, the formation of these antidot potentials promotes a reorganization of the carriers, leading to an electronic localization at the neighboring vacancy sites. Depending upon the position, extension, and symmetries of such antidots, one may find delocalization along the structure due to the formation of new allowed paths. Here we discuss the origin of conductance dips, maximum and complete transport suppressions, within the microscopic scenario of the electronic localization, and using real-space Green function formalism. For such analysis we construct local electronic density of states mapping for different antidot configurations. The results are discussed in comparison with equivalent two-lead devices and perfect structures. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B25.00009: Gate-field effect in multilayer graphenes Mikito Koshino We study the electronic properties of the multilayer graphenes in presence of the external electric field perpendicular to the layers. We calculate the electronic potential of each layer taking account of the screening effect, to obtain the self-consistent band structure within the effective mass approximation. We calculate the conductivity using the self-consistent Born approximation and analyze its external-field dependence for every layer number. We also compute the optical absorption spectra, which is found to be strongly modified by the electric field. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B25.00010: Chemical Doping and Electron-Hole Conduction Asymmetry in Graphene Devices Roksana Golizadeh Mojarad, Damon Farmer, Vasili Perebeinos, Yu-Ming Lin, George S. Tulevski, James C. Tsang, Ali Afzali, Phaedon Avouris We investigate polyethylene imine and diazonium salts as stable, complementary dopants on graphene. Transport in graphene devices doped with these molecules exhibits asymmetry in electron and hole conductance. The conductance of one carrier is preserved, while the conductance of the other carrier decreases. Simulations based on nonequilibrium Green's function formalism suggest that the origin of this asymmetry is imbalanced carrier injection from the graphene electrodes caused by misalignment of the electrode and channel neutrality points. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B25.00011: Sub-lithographic Patterning of Extended Arrays of Graphene Nanostructures Ke Li, Wei Han, Sarah C. Parks, Wenzhong Bao, John Ciraldo, Chun Ning Lau, Roland Kawakami, Ezekiel Johnston-Halperin Quasi-one-dimensional graphene nanoribbons (GNRs) with narrow width (w $\le $ 10 nm) and smooth edges have been shown to exhibit bandgaps due to quantum confinement and edge effects. Current fabrication methods of GNRs include electron beam lithography and chemical synthesis. However, the lithographic approach has difficulties in reaching true nanometer-scale widths while the chemical approach lacks fidelity in GNR length and width control. The recent development of sub-lithographic patterning using the superlattice nanowire pattern transfer (SNAP) technique provides a novel approach to fabricating ultra-long ($>$ 1 mm) GNRs with width down to 7 nm. In addition, repeating SNAP at 90\r{ } with respect to the 1st patterning potentially allows graphene rectangular antidot arrays with N$_{dot}$=160,000 and dot density up to 10$^{11}$ /cm$^{2}$ (dot-to-dot spacing 15 nm; dot area 8 nm $\times $ 8 nm). This novel sub-lithographic patterning technique should enable tailored graphene nanostructures and high throughput manufacturing of GNR-based nano-devices for next generation nanoelectronic applications. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B25.00012: Quantum Hall Edge Transport Across Graphene monolayer-bilayer junctions Yue Zhao, Mikito Koshino, Philip Kim We experimentally studied the transport property of a graphene monolayer-bilayer junction in the Quantum Hall (QH) regime. Both the monolayer graphene (MG) and the bilayer graphene (BG) develop their own landau levels under high magnetic field. While the transport measurement shows their distinct QH effect in bulk part of the MG and BG respectively, the transport measurement across their interface exhibits unusual transverse transport behaviors. The transverse resistance across the MG BG interface is asymmetric for opposite sides of the hall bar, and its polarity can be changed by reversing the magnetic field direction. When the QH plateaus of MG and BG overlap, quantized resistance will appear only on one side of the hall bar electrode pairs across the junction. These experimental observations can be ascribed to the QH edge state transport across the MG/BG interface. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B25.00013: Dissipation and Criticality in the Lowest Landau Level of Graphene Pallab Goswami, Xun Jia, Sudip Chakravarty The lowest Landau level of graphene is studied numerically by considering a tight-binding Hamiltonian with disorder. The Hall conductance $\sigma_{xy}$ and the longitudinal conductance $\sigma_{xx}$ are computed. We demonstrate that bond disorder can produce a plateaulike feature centered at $\nu=0$, while the longitudinal conductance is nonzero in the same region, reflecting a band of extended states between $\pm E_c$, whose magnitude depends on the disorder strength. The critical exponent corresponding to the localization length at the edges of this band is found to be $2.47\pm 0.04$. When both bond disorder and a finite mass term exist the localization length exponent varies continuously between $\sim 1.0$ and $\sim 7/3$. [Preview Abstract] |
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