Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session H31: Properties of Bilayer Graphene |
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Sponsoring Units: DCMP DMP Chair: Cory Cress, US Naval Research Laboratory Room: 294 |
Tuesday, March 14, 2017 2:30PM - 2:42PM |
H31.00001: Observation of even denominator fractional quantum hall states in dual gated bilayer graphene heterostructures Alexander Zibrov, CARLOS KOMETTER,, TAKASHI TANIGUCHI,, KENJI WATANABE,, MICHAEL ZALATEL, Andrea Young We report multi-terminal magneto-capacitance measurements of bilayer graphene (BLG) devices in which the BLG is successively enapsulated in both hBN and few-layer graphite gates. Due to the resulting dramatic increase in sample quality, we observe a rich sequence of FQH states with denominators as large as 11. In addition, we observe robust even denominator states at $\nu =-5/2$, -$1/2$, $3/2$ and, $7/2$). We find the thermodynamic gap of the $\nu=3/2$ state to be $\Delta\mu_{3/2}=4$ K at $B=14$ T, considerably larger than has been observed for even-denominator states in other experimental systems. Measuring the full capacitance matrix, we determine the layer polarization and identify both single- and multi-component regions within the quasi-degenerate space of spin, valley, and orbital quantum number. We find that the even denominator states occur in fully single component regimes, supporting their interpretation as nonabelian, Pfaffian ground states. The ease of tuning valley degeneracy in our dual gated geometry further allows us to explore level crossings between states of differing valley polarization. [Preview Abstract] |
Tuesday, March 14, 2017 2:42PM - 2:54PM |
H31.00002: Excitonic Superfluid Phase in Double Bilayer Graphene Heterostructure Jia Li, Takashi Taniguchi, Kenji Watanabe, James Hone, Cory Dean Spatially indirect excitons can be created when an electron and a hole, confined to separate layers of a double quantum well system, bind to form a composite Boson. Because there is no recombination pathway such excitons are long lived making them accessible to transport studies. With the ability to independently tune both the intralayer charge density and interlayer elecron-hole separation, graphene-Boron Nitride (BN) heterostructures provides access to the low-density, strongly interacting regime where a BEC-like phase transition into a superfluid ground state is anticipated. We observed the exciton condensate in the quantum Hall effect regime of double layer structures of bilayer graphene. Correlation between the layers is identified by quantized Hall drag appearing at matched layer densities, and the dissipationless nature of the phase is confirmed in the counterflow geometry. Independent tuning of the layer densities and interlayer bias reveals a selection rule for the condensate phase involving both the orbital and valley quantum number between the symmetry-broken states of bilayer graphene. Novel excitonic phases are also observed in higher order LLs and in the electron-hole bilayer regime. [Preview Abstract] |
Tuesday, March 14, 2017 2:54PM - 3:06PM |
H31.00003: Abstract Withdrawn
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Tuesday, March 14, 2017 3:06PM - 3:18PM |
H31.00004: Reflection of Dirac Plasmons by Topological Line Defects in Bilayer Graphene Zachariah Addison, Bor-Yuan Jiang, Guangxin Ni, Jing Shi, Xiaomeng Liu, Frank Zhao, Philip Kim, Eugene Mele, Michael Fogler, Dimitri Basov We report experimental and theoretical studies of local optical conductivity of AB-BA domain walls in bilayer graphene. Scanning near-field nanoscopy is employed to find and image these topological defects. It reveals characteristic interference patterns of graphene plasmons that are launched by the scanned tip of the nanoscope and scattering by the domain walls. To explain these observations we compute the electron structure, the carrier density, and the optical conductivity profiles across the domain walls. We find that the band structure exhibits position-dependent splitting of the Dirac points in energy and momentum, which generates an anisotropic modulation of both real and imaginary parts of the conductivity. The magnitude and the direction of the anisotropy depends on the whether the domain wall is shear or tensile. These calculations are combined with electromagnetic modeling of the tip-sample interaction and a qualitative agreement with experiment is found. [Preview Abstract] |
Tuesday, March 14, 2017 3:18PM - 3:30PM |
H31.00005: Tunneling Spectroscopy of Quantum Hall States in Bilayer Graphene Ke Wang, Achim Harzheim, Kenji Watanabe, Takashi Taniguchi, Philip Kim In the quantum Hall (QH) regime, ballistic conducting paths along the physical edges of a sample appear, leading to quantized Hall conductance and vanishing longitudinal magnetoconductance. These QH edge states are often described as ballistic compressible strips separated by insulating incompressible strips, the spatial profiles of which can be crucial in understanding the stability and emergence of interaction driven QH states. In this work, we present tunneling transport between two QH edge states in bilayer graphene. Employing locally gated device structure, we guide and control the separation between the QH edge states in bilayer graphene. Using resonant Landau level tunneling as a spectroscopy tool, we measure the energy gap in bilayer graphene as a function of displacement field and probe the emergence and evolution of incompressible strips. [Preview Abstract] |
Tuesday, March 14, 2017 3:30PM - 3:42PM |
H31.00006: Quantum Spin Hall Effect in Twisted Bilayer Graphene Francesca Finocchiaro, Francisco Guinea, Pablo San-Jose Motivated by a recent experiment (Sanchez-Yamagishi et.al, arXiv:1602.06815) reporting evidence of helical spin-polarized edge states in layer-biased twisted bilayer graphene under a magnetic flux, we study the possibility of stabilising a Quantum Spin Hall (QSH) phase in such a system, without Zeeman or spin-orbit couplings, and with a QSH gap induced instead by electronic interactions. We analyse how magnetic flux, electric field, interlayer rotation angle, and interactions (treated at a mean field level) combine to produce a pseudo-QSH with broken time-reversal symmetry, and spin-polarized helical edge states. The effect is a consequence of a robust interaction-induced ferrimagnetic ordering of the Quantum Hall ground state under an interlayer bias, provided the two rotated layers are effectively decoupled at low energies. We discuss in detail the electronic structure and the constraints on system parameters, such as the angle, interactions and magnetic flux, required to reach the pseudo-QSH phase. We find, in particular, that purely local electronic interactions are not sufficient to account for the experimental observations, which demand at least nearest-neighbour interactions to be included. [Preview Abstract] |
Tuesday, March 14, 2017 3:42PM - 3:54PM |
H31.00007: Flat-band magnetism and spiral magnetic order in twisted bilayer graphene Luis Gonzalez-Arraga, Jose Lado, Francisco Guinea, Pablo San-Jose The Fermi velocity in Moire superlattices of twisted bilayer graphene diminishes with decreasing twisting angle, so that below 1 degree, it is almost suppressed, and flat bands arise at the Fermi level. The flat bands correspond to quasi-localized states within the regions of AA-stacking. Electron confinement is enhanced by the application of an interlayer electrostatic bias. In this regime, AA regions behave at low energies as a triangular network of weakly connected quantum dots. We study the effects of electron-electron repulsions on these localized states within a Hubbard mean-field approach. Within the minimal supercell, we find that the magnetic moments of AA regions may become ferromagnetic above a critical Hubbard coupling, and may be efficiently controlled by an interlayer bias. The relative ordering of neighbouring AA regions is dominated by the inter-cell exchange, which has antiferromagnetic character. Due to the triangular symmetry of the moiré pattern, the exchange results in a unique spiral magnetic order, despite the absence of spin-orbit coupling in the system. Such magnetic properties are rather unique amongst known 2D materials. Electrical spin-tuneability, and negligible stray fields from the spiral order make this system a promising platform for data storage. [Preview Abstract] |
Tuesday, March 14, 2017 3:54PM - 4:06PM |
H31.00008: Binding Energy of Interlayer Excitons in Twisted Bilayer Graphene HIRAL PATEL, Lujie Huang, Cheol-Joo Kim, Jiwoong Park, Matt Graham When two sheets of graphene are stacked at off axis angle, 2p orbitals hybridize, giving angle-tunable absorption resonances. By comparing the ultrafast intra-band and multi-photon transient absorption spectra, our results agree best with recent theoretical simulations which predict that overlapping interlayer 2p orbitals interfere deconstructively to produce symmetrized bound exciton states that are decoupled from lower lying continuum states. We further map out the excited state manifold of exciton fine states using 2-photon photoluminescence microscopy. Our spectral analysis suggests that the observed photoluminescence emission from a bright exciton state is thermally populated by a lower-lying, long lived dark-exciton state. For this dark state, both our ultrafast transient absorption and two-photon photoluminescence studies support a large binding energy of the order of 0.6 eV. We believe twisted bilayer graphene is the first 2D metallic material that can form stable, strongly bound excitons upon resonant excitation. Such strong excitonic effects coupled with enhanced hot carrier lifetimes suggest possible new routes for hot carrier extraction from an otherwise predictable 2D metallic system. [Preview Abstract] |
Tuesday, March 14, 2017 4:06PM - 4:18PM |
H31.00009: Topologically protected states at stacking boundaries in bilayer graphene Marta Pelc, Wlodzimierz Jask\'olski, Leonor Chico, Andres Ayuela Recent experiments [Nature 520, 650 (2010)] confirm the existence of gapless states at domain walls created in gated bilayer graphene with the stacking change from AB to BA. The significance of these states is due to their topological protection, valley-polarization and contribution to conductance along the domain wall [Phys. Rev. B 92, 085433 (2015)]. Current theoretical models predict the appearance of such states only at domain walls which preserve the sublattice order. We show that the appearance of the topologically protected states in stacking domain walls can be much more common in bilayer graphene, since they can also emerge at grain boundaries with atomic-scale topological defects. We focus on a bilayer system in which one of the layers contains a line of octagon–double pentagon defects that mix graphene sublattices [Nanoscale 8, 6079 (2016)]. We demonstrate that gap states are not only preserved, but also, their number changes by inverting the gate polarization, yielding an asymmetric conductance along the grain boundary under gate reversal. This effect should be detectable in transport measurements and could be exploited in electrical switches. [Preview Abstract] |
Tuesday, March 14, 2017 4:18PM - 4:30PM |
H31.00010: Hofstatder Physics and Many-body Interactions in Twisted Bilayer Graphene Yuan Cao, Jason Luo, Valla Fatemi, Shiang Fang, Javier Sanchez-Yamagishi, Kenji Watanabe, Takashi Taniguchi, Efthimios Kaxiras, Pablo Jarillo-Herrero Twisted bilayer graphene (TwBLG) consists of two twistingly stacked sheets of graphene spaced only 0.34 nm apart. The interlayer interactions in TwBLG are strongly dependent on the twist angle and can vastly affect the electronic properties of the system. Recently, we have experimentally observed the existence of novel insulating states in TwBLG with small twist angles, which result from the interplay between interlayer hybridization and the periodicity of the underlying moir\'{e} pattern[1]. However, the transport gaps of the insulating states are much larger than existing theory predictions, suggesting possible effects of many-body interactions and/or mechanical effects. Additionally, we also observe for the first time the Hofstadter butterfly in TwBLG with a sub-degree twist angle in a high magnetic field, where the quantum Hall effect in this regime shows major differences compared with previously measured samples. [1] Y. Cao, J. Y. Luo, V. Fatemi, S. Fang, J. D. Sanchez-Yamagishi, K. Watanabe, T. Taniguchi, E. Kaxiras, and P. Jarillo-Herrero, Phys. Rev. Lett. 117, 116804 (2016). [Preview Abstract] |
Tuesday, March 14, 2017 4:30PM - 4:42PM |
H31.00011: Thermal Conductivity of Twisted Bilayer Graphene Nanoribbons from Non-equilibrium Molecular Dynamics Study. Chenyang Li, Shanshan Su, Supeng Ge, Roger Lake Misorientation of the two layers of bilayer graphene affects both the electronic properties and the vibrational modes or phonons. The phonon density of modes is little affected by misorientation, however, zone-folding can allow new Umklapp scattering processes that could affect the phonon transport and thermal conductivity. To investigate this, we use NEMD molecular dynamics simulations as implemented in LAMMPS to study the thermal conductivity of the misoriented graphene bilayers. Seven commensurate misorientation angles varying from 6.01º to 48.36º have modeled and analyzed to understand how the misorientation angle affects the thermal conductivity of relatively wide (\textasciitilde 10 nm) misoriented bilayer graphene nanoribbons (m-BLGNRs). Within numerical accuracy, we find that the thermal conductivity of the m-BLGNRs for all of the simulated commensurate angles have the same thermal conductivity with AB stacked and AA stacked BLGNRs. These results indicate that neither the misorientation angle nor the stacking order affect the thermal conductivity of BLGNRs. [Preview Abstract] |
Tuesday, March 14, 2017 4:42PM - 4:54PM |
H31.00012: Towards valley valve operations in bilayer graphene devices Jing Li, Zhenxi Yin, Jun Zhu The application of a perpendicular electric field in bilayer graphene opens a band gap, the sign of which depends on the direction of the electric field. Recently, we have demonstrated the existence of one-dimensional edge channels (aka kink states) at the line junction of two oppositely gated bilayer graphene, i.e. $E_L\cdot E_R < 0$ [1]. Reversing the helicity of the electric field, i.e. $E_L$ to -$E_L$ and $E_R$ to -$E_R$ reverses the direction of the kink states. Two back-to-back kink states thus form a valve. The current transmission is expected to be high/low when the kink states have the same/opposite helicity. This novel valleytronics concept can potentially enable gate-controlled transmission of ballistic one-dimensional channels. Here we show the fabrication of a quad-split-gate structure, where two kink states with independently controlled helicity interact at a cross section. We discuss the operations of the valve. [1] J. Li et al, Nat. Nano. 10.1038/nnano.2016.158. [Preview Abstract] |
Tuesday, March 14, 2017 4:54PM - 5:06PM |
H31.00013: Bilayer graphene under pressure: Electron-hole Symmetry Breaking, Valley Hall Effect, and Landau Levels Jorge O. Sofo, Francisco Muñoz, Hector P. Ojeda Collado, Gonzalo Usaj, Carlos A. Balseiro The electronic structure of bilayer graphene under pressure develops an enhancement of the trigonal warping and a splitting of the parabolic touching bands at the K point of the reciprocal space into four Dirac cones, one at K and three along the T symmetry lines. As pressure is increased, these cones separate in reciprocal space and in energy, breaking the electron-hole symmetry. Due to their energy separation, their opposite Berry curvature can be observed in valley Hall effect experiments, in the structure of the Landau levels, and in the rotation of the polarization angle of light. Based on the electronic structure obtained by Density Functional Theory, we develop a low energy Hamiltonian that describes the effects of pressure on measurable quantities such as the Hall conductivity, the Landau levels of the system, and the optical conductivity. [Preview Abstract] |
Tuesday, March 14, 2017 5:06PM - 5:18PM |
H31.00014: Tunable zero-line modes via magnetic field in bilayer graphene Ke Wang, Zhenhua Qiao Zero-line modes appear in bilayer graphene at the internal boundary between two opposite vertical electrostatic confinements. These one-dimensional modes are metallic along the boundary and exhibit quantized conductance in the absence of inter-valley scattering. However, experimental results show that the conductance is around 0.5 e$^{2}$/h rather than quantized. This observation can be explained from our numerical results, which suggest that the scattering between zero-line mode and bound states and the presence of atomic scale disorders that provide inter-valley scattering can effectively reduce the conductance to about 0.5 e$^{2}$/h. We further find that out-of-plane magnetic field can strongly suppress these scattering mechanisms and gives rise to nearly quantized conductance. On one hand, the presence of magnetic field makes bound states become Landau levels, which reduces the scattering between zero-line mode and bound states. On the other hand, the wave function distributions of oppositely propagating zero-line modes at different valleys are spatially separated, which can strongly suppress the inter-valley scattering. Specifically speaking, the conductance can be increased to 3.2 e$^{2}$/h at 8 T even when the atomic Anderson type disorders are considered. [Preview Abstract] |
Tuesday, March 14, 2017 5:18PM - 5:30PM |
H31.00015: Transport signatures of secondary moir\'{e}s in triply-stacked graphene Jason Luo, Yuan Cao, Stephen Carr, Shiang Fang, Kenji Watanabe, Takashi Taniguchi, Efthimios Kaxiras, Pablo Jarillo-Herrero Although two-layer van der Waals heterostructures already exhibit a vast range of electronic and topological behaviors, stacking beyond two layers provides us with even more possibilities to explore new quantum phenomena. While the longest moir\'{e} wavelength arising from stacking two layers of similar lattice constants is controlled by the lattice periodicity, this longest wavelength involves higher harmonics of the periodic electronic distribution when stacking three or more layers. We fabricated high quality, dual-gated triply-stacked graphene devices encapsulated by hexagonal boron nitride, where the rotational misalignments between the three layers of graphene are controlled independently. We investigate via transport measurements the existence of these secondary moir\'{e}s, and the nature of interlayer hopping and hybridization in triply-stacked graphene. [Preview Abstract] |
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