Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K14: Graphene: Electronic Transport and Characterization |
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Sponsoring Units: DCMP Chair: Ivan Borzenets Room: BCEC 153C |
Wednesday, March 6, 2019 8:00AM - 8:12AM |
K14.00001: Atomic-Resolution Visualization of Complex Structures in Intercalated Bilayer Graphene Jason Bonacum, Andrew O'Hara, Oleg S Ovchinnikov, Georgy Gordeev, Sonakshi Arora, Stephanie Reich, Juan Carlos Idrobo, Richard F Haglund, Sokrates T Pantelides, Kirill Bolotin Intercalation in two-dimensional materials has been widely investigated using spectroscopic, diffraction, and electrical measurements. However, these techniques generate spatially averaged data and provide information about the local atomic structure only inferentially. In this work, we deploy aberration-corrected scanning transmission electron microscopy to directly visualize the local atomic structure of FeCl3-intercalated bilayer (BLG) and few-layer graphene (FLG). The data exhibit a crystalline monolayer of FeCl3 inside BLG, atomically sharp intercalation boundaries, a variety of orientations for FeCl3 monolayers in FLG, and regions where the iron is reduced to form monolayer FeCl2. Our density-functional-theory calculations predict a low energy barrier of 0.04 meV/nm2 between different orientations of FeCl3, supporting the observation of multiple orientations. Furthermore, resonant-Raman spectroscopy yields evidence of two distinct graphene doping levels of EF=0.98eV and EF=1.06 in intercalated bilayer graphene, which may be attributed to the coexistence of FeCl3 and FeCl2. These results highlight the critical need for atomic-resolution studies of FeCl3 and similar intercalants to understand the dynamics of doping in multilayer graphene and graphene superlattices. |
Wednesday, March 6, 2019 8:12AM - 8:24AM |
K14.00002: Chiral symmetry breaking in rhombohedral multilayer graphene Narjes Kheirabadi, Andrew Hallwood, Jonathan F P Rogers, Edward McCann, Mikito Koshino We model the single-particle electronic properties of rhombohedral multilayer graphene focusing on the topological surface states that yield two flat degenerate bands in the vicinity of the Fermi level. We show that chiral-symmetry breaking tight-binding parameters produce a finite dispersion of the two low-energy bands, but do not lift their degeneracy owing to the conservation of PT symmetry. We model PT symmetry breaking due to external fields, including interlayer asymmetry due to an external gate or substrate (broken space inversion) or an external magnetic field (broken time reversal symmetry). |
Wednesday, March 6, 2019 8:24AM - 8:36AM |
K14.00003: Direct chemical potential measurements of fractional quantum Hall states in bilayer graphene Alexander Zibrov, Fangyuan Yang, Takashi Taniguchi, Kenji Watanabe, Andrea Young We report measurements of the chemical potential in high quality bilayer graphene (BLG) in the quantum Hall regime as a function of magnetic field, interlayer bias, and charge carrier density. To maintain sample quality while also enabling quantitative determination of the chemical potential of the BLG flake, we use a four plate capacitor heterostructure consisting of graphite top and bottom gates,BLG, and an integrated charge sensing graphene monolayer. The resulting devices show fractional quantum Hall states at odd-denominator filling fractions as large as 11 as well as even-denominator states at fillings ν=-5/2, -1/2, 3/2 and 7/2. Changes in the chemical potential of the BLG flake generate modulations of the charge density in the monolayer graphene sensor layer, which we detect capacitively. These modulations form the error signal for a feedback loop whose control voltage is the BLG chemical potential. We demonstrate chemical potential sensitivity better than 10uV/ |
Wednesday, March 6, 2019 8:36AM - 8:48AM |
K14.00004: Large tunable intrinsic gap in rhombohedral-stacked tetralayer graphene at half filling Shi Che, Kevin S Myhro, Yanmeng Shi, Yongjin Lee, Kevin Thilahar, Kevin Bleich, Dmitry Smirnov, Chun Ning Lau Rhombohedral-stacked tetralayer (r-4LG) has a highly unusual energy dispersion, which can be approximated as E~k4, where k is the wave vector. At half filling, the very flat energy bands in r-4LG are unstable to electronic interactions, giving rise to electronic states with spontaneous broken symmetries. Using transport measurements on suspended dual-gated devices, we observe an insulating ground state with a large interaction-induced transport gap up to 80 meV at the charge neutrality point. The energy gap is enhanced further with a perpendicular magnetic field, but closed or suppressed upon the application of an out-of-lane electric field of either polarity, increasing charge density or a critical temperature of ~40 K. This insulating gapped state is consistent to that observed in bilayer graphene (BLG) and rhombohedral-stacked trilayer graphene (r-TLG), and therefore proposed to be a layer antiferromagnet with broken time reversal symmetry. The large magnitude of the gap also suggests that, at lease in r-4LG, the band flattening effect prevails over the increasing screening and charge de-confinement in thicker graphene, in agreement with a first principle calculation. |
Wednesday, March 6, 2019 8:48AM - 9:00AM |
K14.00005: Energetics of complex phase diagram in a tunable bilayer graphene probed by quantum capacitance Manabendra Kuiri, Anindya Das Bilayer graphene provides a unique platform to explore the rich physics in quantum Hall effect. The unusual combination of spin, valley and orbital degeneracy leads to interesting symmetry broken states with electric and magnetic field. Conventional transport measurements like resistance measurements have been performed to probe the different ordered states in bilayer graphene. However, not much work has been done to directly map the energetics of those states in bilayer graphene. Here, we have carried out the magneto capacitance measurements with electric and magnetic field in a hexagonal boron nitride encapsulated dual gated bilayer graphene device. In presence of perpendicular magnetic field, we observe Landau level crossing in our magneto-capacitance measurements with electric field. The gap closing and reopening of the lowest Landau level with electric and magnetic field shows the transition from one ordered state to another one. Further more we observe the collapsing of the Landau levels near the band edge at higher electric field ( D > 0.5 V/nm), which was predicted theoretically. The complete energetics of the Landau levels of bilayer graphene with electric and magnetic field in our experiment paves the way to unravel the nature of ground states of the system. |
Wednesday, March 6, 2019 9:00AM - 9:12AM |
K14.00006: Quantitative Measurement of Valley Polarization in a Multicomponent Fractional Quantum Hall System Fangyuan Yang, Alexander Zibrov, Takashi Taniguchi, Kenji Watanabe, Michael Zaletel, Andrea Young In bilayer graphene, the electronic structure at high magnetic fields are enhanced by spin, valley and orbital degrees of freedom. In particular, the valley quantum number can be directly mapped onto layer polarization, allowing for direct capacitive probes. In this talk, I will present quantitative measurements of valley polarization in high quality bilayer graphene quantum Hall system. The valley polarization is obtained by measuring anti-symmetric combinations of top- and bottom- gate capacitances to the graphene bilayer. We improve on past approaches by using a multilayer heterostructure in which the bilayer is encapsulated in successive layers of hexagonal boron nitride (hBN), monolayer graphene, hBN, and graphite, allowing full control of density and layer polarization in both the bilayer and in the monolayer graphene gates. Our layer resolved measurements provide new quantitative insight into the ground states and excitations of both integer and fractional quantum Hall phases in bilayer graphene. |
Wednesday, March 6, 2019 9:12AM - 9:24AM |
K14.00007: Study of mode-projected electron-phonon coupling in graphite by time -resolved ARPES MengXing Na, Arthur K Mills, Fabio Boschini, Matteo Michiardi, Benjamin Nosarzewski, Ryan P Day, Elia Razzoli, Alexander Sheyerman, Giorgio Levy, Michael Schneider, Sergey Zhdanovich, Thomas Devereaux, Alexander Kemper, David J Jones, Andrea Damascelli Time-resolved spectroscopies have significantly contributed to the study of the dynamical properties of conventional and quantum materials. By following the dynamics of excited electrons, one can follow the dominant scattering processes and discern the flow of energy from the electronic system to other degrees of freedom. Time and angle-resolved photoemission spectroscopy (TR-ARPES) allow one to access electron dynamics in a momentum resolved way. In this work, a balance of energy and time resolution revealed a peak associated with optical excitation. In addition, we observe quantized energy-loss scattering processes, which comes from the emission of an optical phonon from electrons in the direct-transition peak (DTP). The characteristic time scale associated with the transfer of spectral weight from the DTP to the phonon-scattering-peak can then be directly related to the mode-projected electron-phonon matrix element. |
Wednesday, March 6, 2019 9:24AM - 9:36AM |
K14.00008: Magnetic Effects on Topological Chiral Channels in Bilayer Graphene John Maier, Aaron Winn, Andrew Y Jiao, Jeffrey C.Y. Teo In this paper, we study the effect of a magnetic field on topological chiral channels of bilayer graphene at electric domain walls. The persistence of chiral edge states is attributed to the difference in valley Chern number in the regions of opposite electric field. We explore the regime of large electric and magnetic fields perpendicular to the lattice. The magnetic field shifts the channel away from our electric interface in a way that is inconsistent with the semiclassical expectation from the Lorentz force. Moreover, the magnetic field causes an imbalanced layer occupation preference to the chiral channels. These behaviors can be understood in the limits that either the electric or the magnetic field dominates. We numerically show in the general case that the system can be well-approximated as a weighted sum of the two limits. |
Wednesday, March 6, 2019 9:36AM - 9:48AM |
K14.00009: Energy drag and shear viscosity in interacting quantum systems William Berdanier, Thomas Scaffidi, Christoph Karrasch, Joel Moore There has recently been a wave of interest in the hydrodynamical behavior of quantum many-body systems. Using `Coulomb drag’ physics as a guide, we discuss analogues of shear viscosity in quantum systems on discrete lattices. We argue that, although particle-hole symmetric materials have zero Coulomb drag, they nevertheless show a thermal equivalent in the form of drag between energy currents. Using a combination of perturbation theory and techniques from integrability, we give analytical predictions for this effect in coupled one-dimensional wires and compare them to DMRG studies on the Hubbard model. We comment on the generalization of these results to higher dimensional systems. |
Wednesday, March 6, 2019 9:48AM - 10:00AM |
K14.00010: Spatial measurements of ballistic and hydrodynamic signatures in Graphene Asaf Rozen, Shahal Ilani The study of electric current, dominated by electron-electron interactions, has recently gained interest owing to a new generation of ultra clean materials. In this regime, the flow behaves as viscous hydrodynamics fluid, in contrast to non-interacting flow types such as diffusive and ballistic. In this work, we use a scanning single electron transistor (SET) to image the local potential landscape produced by flowing electrons in an expanding channel of graphene. This geometry is ideal to reveal the spatial manifestation of the quantum contact resistance as well as non-local aspects of ballistic and hydrodynamic flows. In this talk we will present our recent scanning measurements that shed new light on all the above aspects of electron flow. |
Wednesday, March 6, 2019 10:00AM - 10:12AM |
K14.00011: Probing exciton condensates in double layer graphene using the corbino geometry Yihang Zeng, J.I.A. Li, Olivia Ghosh, Kenji Watanabe, Takashi Taniguchi, Cory Dean, James Hone Double layer graphene system with a thin tunnel barrier was recently reported to host exciton condensates and novel interlayer fractional quantum hall (FQH) states in the presence of a high magnetic field. The corbino geometry, consisting normally of two concentric rings, provides a direct transport probe of the bulk response, without the contribution of sample edges. In such geometry, combing the parallel flow and counter flow experimental setup, we observe the perfect drag phenomenon, namely the current in each of the two layers has exactly the same intensity but opposite direction. It demonstrates that all the current in the system is carried by excitons and provides an important evidence of the existence of exciton condensate. For the first time, we demonstrate with perfect drag that the exciton coupling remains robust at high landau level where the enhanced screening effect between electrons reduces the interlayer coupling. We study the exciton coupling as a function of interlayer bias, temperature, magnetic field and dc bias. Evidence of a fractional exciton condensate, occurring at total filling fraction of 1/3, is discussed. |
Wednesday, March 6, 2019 10:12AM - 10:24AM |
K14.00012: Direct visualization of electrostatic gating on the band structure of two-dimensional materials via Angled Resolved Photo Emission Spectroscopy (ARPES) Frederic Joucken, Jose Avila, Zhehao Ge, Eberth Quezada, Hemian Yi, John L Davenport, Kenji Watanabe, Takashi Taniguchi, Maria C. Asensio, Jairo Velasco Jr. The ability to manipulate the band structure of two-dimensional (2D) materials via electrostatic gating is crucial for the realization of technological applications and study of fundamental physics with these materials. ARPES is the standard experimental technique for band structure visualization, however, to this date, the direct visualization of electrostatic gating on the band structure of 2D materials is lacking. Several obstacles have precluded the incorporation of electrostatic gating with ARPES measurements such as sample size, surface uniformity, and a working gate electrode. To address these obstacles, we performed ARPES experiments with submicron spatial resolution on a heterostructure of bilayer graphene (BLG) and hexagonal boron nitride (hBN) that is back gated with an underlying graphite flake. We found that such a device geometry enabled direct visualization of the BLG bands in conjunction with their shift via electrostatic gating. We will discuss the latest experimental progress towards the direct visualization of electrostatic gate modulation of BLG bands. |
Wednesday, March 6, 2019 10:24AM - 10:36AM |
K14.00013: RKKY Interaction in Graphene Landau Levels Jinlyu Cao, Herbert Fertig, Shixiong Zhang We consider the problem of RKKY coupling of classical magnetic moments on graphene when the conduction electrons are in Landau level states. This can be due to a real applied field, or to non-uniform strain. The problem cannot be fully handled in the usual perturbative approach due to the large degeneracy of the Landau levels. For zero chemical potential and two impurities, we demonstrate that four bound states break away from zero energy, with energies dependent of the relative orientations of the spins, two of which are below the Fermi energy. In strained graphene, these two filled states create an effective spin-spin interaction (∝Jsd) in addition to the standard RKKY (∝J2sd). Moreover, this contribution is active on only one of the graphene sublattices. At the mean-field level, this sublattice has stronger ferromagnetic coupling than the other, while the two sublattices are anti-ferromagnetically coupled. This can potentially give rise to a ferrimagnetic phase with a broken O(3) symmetry. By comparison, the RKKY interaction of dilute impurities in unstrained graphene in a real magnetic field (perpendicular to sample) allows for a canted antiferromagnetic state with broken U(1) symmetry, and an accompanying Kosterlitz-Thouless transition. |
Wednesday, March 6, 2019 10:36AM - 10:48AM |
K14.00014: Texture Transitions of Quantum Hall Skryme Crystals Near Charge Neutrality in Graphene Braden Feshami, Herbert Fertig How does the quantum Hall canted antiferromagnetic (CAF) groundstate of graphene transition into the ferromagnet when the system is doped away from zero filling factor? We study this question within a self-consistent Hartree-Fock theory, in which regular Skyrme crystals are stabilized by long range Coulomb interactions. In addition to this we include a Hubbard on-site term in the Hamiltonian which breaks the symmetry in the valley degree of freedom, and stabilizes a CAF state when the Zeeman energy is not too large. The full set of possible symmetry-breaking terms in the interactions are realized by incorporating an active window of non-zero Landau levels in our Hartree-Fock state. We explore how textures in the various discrete degrees of freedom evolve with the system parameters, and discuss the consequences of this for the phase diagram within the space of the on-site Hubbard parameter, Zeeman strength, and filling factor. |
Wednesday, March 6, 2019 10:48AM - 11:00AM |
K14.00015: Equilibrium-state currents in Quantum Hall graphene quantum dots and pn junctions Cyprian Lewandowski, Aviram Uri, Eli Zeldov Charge carriers in a quantum Hall state, when subjected to a spatially dependent electrostatic confining potential, arrange themselves in a characteristic pattern of alternating compressible and incompressible regions. By delineating the individual contribution of each Landau level (LL) to electric currents in these regions, we elucidate the microscopic origin of the currents flowing in thermodynamic equilibrium. We focus on the case of a graphene quantum dot and a pn junction geometry. We find, both theoretically and experimentally, non-zero currents flowing in both compressible and incompressible regions, which alternate in sign between neighboring regions. Whilst all occupied Landau levels contribute to the incompressible currents equally, as expected, the compressible currents receive a contribution only from the last occupied LL with the contribution proportional to its energy. This peculiar dependence of compressible currents on the LL energy is best exemplified by the 0th LL, which due to graphene’s band structure, is expected to carry no current in the compressible region. This behavior is in agreement with experimental observations. |
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