Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F33: Edge States and Nanoribbons of 2D Materials |
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Sponsoring Units: DCMP Chair: Ben Chuang, US Naval Research Laboratory Room: 296 |
Tuesday, March 14, 2017 11:15AM - 11:27AM |
F33.00001: Geometry-dependent magnetoresistance between edge states of graphene layers V\'{\i}ctor Manuel Garc\'{\i}a-Su\'arez, Amador Garc\'{\i}a-Fuente, Diego Carrascal, Enrique Burzuri, Michel Calame, Herre van de Zant, Jaime Ferrer We thoroughly characterize the rich transport properties of 2D materials separated by a nanogap and terminated with magnetic edges. We find that the low-bias conductance is spin dependent and varies with the orientation of the magnetic configuration of the edges and their precise geometry. When the edges are straight the $I-V$ curve is perfectly ohmic but the magnetoresistance is negligible because the magnetic states of the borders do not couple to the inner states. The introduction of imperfections such as wedges or protrusions couples the edge states to the inner states and produces a sizeable magnetoresistance effect, which translates into a giant negative magnetoresistance when both sides of the junction have wedges in front of each other. We also find a clear negative differential resistance and a strong spin filtering-rectification for this configuration and also for configurations with wedges facing straight edges. The analysis of the $I-V$ curves allows us then to univocally determine the type of edges and their magnetic properties. We apply our model to the specific case of zigzag graphene edges and find a very good qualitative agreement between our results and the simulated ab-initio results. [Preview Abstract] |
Tuesday, March 14, 2017 11:27AM - 11:39AM |
F33.00002: Electronic transport along graphene grain boundaries in quantizing magnetic fields Madeleine Phillips, Eugene Mele We study ballistic electronic transport through grain boundaries in single layer graphene in the presence of a quantizing perpendicular magnetic field. For all the geometries studied, in addition to the chiral edge states on the sample boundaries, we find additional propagating electronic states confined to the grain boundary. When contacted to external electrodes, the current carried by the exterior edge states can be redirected into the grain boundary. Furthermore, edge state deflection leaves a signature in the shot noise: energies where quantum hall edge states are preserved correspond to a Fano factor of zero, while the Fano factor approaches one at energies where edge states are efficiently deflected into the grain boundary. We find that this switching can be selectively turned on or off by a uniform gate potential and study how the edge to grain boundary transmission is controlled by kinematic backscattering constraints in the grain boundary. [Preview Abstract] |
Tuesday, March 14, 2017 11:39AM - 11:51AM |
F33.00003: Mach-Zehnder interferometry using broken symmetry quantum Hall edges in graphene Di Wei, Toeno van der Sar, Javier Sanchez-Yamagishi, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero, Bertrand Halperin, Amir Yacoby Graphene has emerged as a unique platform for studying electron optics, particularly in the presence of a magnetic field. Here, we engineer a Mach-Zehnder interferometer using quantum Hall edge states that co-propagate along a single gate-defined NP interface. We use encapsulated monolayer graphene, clean enough to lift the four-fold spin and valley degeneracy. In order to create two separate co-propagating paths, we exploit the suppression of edge state scattering along gate defined edges, and use scattering sites at the ends of the NP interface to form our beam splitters. We observe conductance oscillations as a function of magnetic and electric field indicative of coherent transport, and measure values consistent with spin-selective scattering. We can tune our interferometer to regimes of high visibility ($>$98$\%$), surpassing the values reported for GaAs quantum-well Mach-Zehnder interferometers. These results demonstrate a promising method to observe interference between fractional charges in graphene. [Preview Abstract] |
Tuesday, March 14, 2017 11:51AM - 12:03PM |
F33.00004: Experimental observation of edge transport in graphene nanostructures Amogh Kinikar, T. Phanindra Sai, Semonti Bhattacharyya, Adhip Agarwala, Tathagata Biswas, Sanjoy K. Sarker, H. R. Krishnamurthy, Manish Jain, Vijay B. Shenoy, Arindam Ghosh The zizzag edges of graphene, whether single or few layers, host zero energy gapless states and are perfect 1D ballistic conductors. Conclusive observations of electrical conduction through edge states has been elusive. We report the observation of edge bound transport in atomic-scale constrictions of single and multilayer suspended graphene created stochastically by nanomechanical exfoliation of graphite. We observe that the conductance is quantized in near multiples of e$^{\mathrm{2}}$/h. Non-equilibrium transport shows a split zero bias anomaly and, the magneto-conductance is hysteretic; indicating that the electron transport is through spin polarized edge states in the presence of electron-electron interaction. Atomic force microscope scans on the graphite surface post exfoliation reveal that the final constriction is usually a single layer graphene with a constricting angle of 30$^{\mathrm{o}}$. Tearing along crystallographic angles suggests the tears occur along zigzag and armchair configurations with high fidelity of the edge morphology. [Preview Abstract] |
Tuesday, March 14, 2017 12:03PM - 12:15PM |
F33.00005: Symmetries and band gaps in nanoribbons ZHIWEI ZHANG, YITENG TIAN, GAYANATH FERNANDO, ARMEN KOCHARIAN In ideal graphene-like systems, time reversal and sublattice symmetries preserve the degeneracies at the Dirac point(s). We have examined such degeneracies in the band structure as well as the transport properties in various arm-twisted (graphene-related) nanoribbons. A twist angle is defined such that at 0 degrees the ribbon is a rectangular ribbon and at 60 degrees the ribbon is cut from a honeycomb lattice. Using model Hamiltonians and first principles calculations in these nanoribbons with Z$_2$ topology, we have monitored the band structure as a function of the twist angle $\theta$. In twisted ribbons, it turns out that the introduction of an extra hopping term leads to a gap opening. We have also calculated the size and temperature broadening effects in similar ribbons in addition to Rashba-induced transport properties. [Preview Abstract] |
Tuesday, March 14, 2017 12:15PM - 12:27PM |
F33.00006: Edge state structure of the bilayer graphene zero Landau level Haoxin Zhou, Alexander Zibrov, Eric Spanton, Takashi Taniguchi, Kenji Watanabe, Andrea Young In the eight-fold degenerate zero-energy Landau level (ZLL). Bernal stacked bilayer graphene exhibits numerous phase transitions between gapped states with different spin, valley, and orbital polarization. I will describe combined transport and capacitance measurements in dual graphite-gated devices performed in tilted magnetic fields. In addition to a quantum spin Hall state at zero density, we find evidence for protected, counter-propagating edge states near half filling of the ZLL. I will discuss these results with different models for symmetry breaking within the ZLL. [Preview Abstract] |
Tuesday, March 14, 2017 12:27PM - 12:39PM |
F33.00007: Electronic magnetic properties of slipped bilayer graphene nanoribbon Bi-Ru Wu We use a first-principles method for investigating the electronic and magnetic properties of slipped bilayer graphene nanoribbon. The band dispersion in the low energy region are dominated by the stacking manner, because that alters the interlayer interaction so much. The magnetic properties of zigzag edged bilayer graphene nanoribbon depend on the interaction between edges belong to different layers. It was found that a transition of antiferromagnetic semiconductor to nonmagnetic metal occurs during the slipping process. The armchair edged bilayer graphene exhibits as nonmagnetic semiconductor with a rule similar to the armchair edged monolayer graphene nanoribbon. This founding will provide an opportunity for tuning the electronic properties and magnetic properties of the devices made by bilayer graphene nanoribbon. [Preview Abstract] |
Tuesday, March 14, 2017 12:39PM - 12:51PM |
F33.00008: Electronic properties of hybrid graphane/boron nitride nanoribbons with hydrogen vacancies. Chi-Hsuan Lee, Chih-Kai Yang Electronic properties of hybrid graphane/boron nitride nanoribbons with hydrogen vacancies are investigated using density functional calculations. Two types of vacancies, line and chain, are studied. They reveal different electronic and magnetic properties. Formation of vacancies at different locations is also considered. Interaction between two separate chain vacancies within a graphane nanoribbon is also compared with that of a BN ribbon. The results should be useful for application in nanoelectronic devices. . [Preview Abstract] |
Tuesday, March 14, 2017 12:51PM - 1:03PM |
F33.00009: Engineering Monolayer Graphene Nanoribbons with Boron Nitride. Rong Yang Graphene nanostructures are potential building blocks for nanoelectronic and spintronic devices. However, the production of monolayer graphene nanostructures with high-quality and well-defined zigzag edges remains a challenge. In this talk, we report the engineering of electronic grade monolayer graphene nanostructures on hexagonal boron nitride (h-BN) substrates either by an top-down etching technique or by an bottom-up growth way. As for the top-down way, we found that hydrogen plasma etching of monolayer graphene on h-BN is highly anisotropic due to the inert and ultra-flat nature of the h-BN surface, resulting in zigzag edge formation. The as-fabricated zigzag-edged monolayer graphene nanoribbons with widths below 30 nm show high carrier mobility (2000 cm$^{\mathrm{2}}$/ Vs, with on/off ratio of \textgreater 10$^{\mathrm{2}})$ and width-dependent energy gaps at liquid helium temperature. Besides, we can also obtain monolayer graphene nanoribbons (GNRs) on h-BN by a bottom-up epitaxy growth. We found that GNRs grow preferentially from the atomic steps of h-BN, forming in-plane heterostructures. As-grown GNRs on h-BN have high quality with a carrier mobility of \textasciitilde 20000 cm$^{\mathrm{2}}$/ Vs for 100-nm-wide GNRs at a temperature of 1.7 K. More interesting, a moire pattern induced quasi-one-dimensional superlattice with a periodicity of 15nm for GNR/h-BN was also observed, indicating zero crystallographic twisting angle between GNRs and h-BN substrate. The superlattice induced band structure modification is confirmed by our transport results. [Preview Abstract] |
Tuesday, March 14, 2017 1:03PM - 1:15PM |
F33.00010: Triplet {\$}p{\$}-wave pairing correlation in low doped zigzag graphene nanoribbons Tianxing Ma, Fan Yang, Zhong-Bing Huang, Hai-Qing Lin We reveal an edge spin triplet {\$}p-{\$}wave superconducting pairing correlation in slightly doped zigzag graphene nanoribbons. By employing combined random phase approximation, the finite temperature determinant quantum Monte Carlo approach, and the ground state constrained path quantum Monte Carlo method, it is shown such a spin triplet pairing is mediated by the ferromagnetic fluctuations caused by the flat band at the edge. The edge spin susceptibility and effective pairing interactions strongly increase as the on-site Coulomb interaction increases, indicating the importance of electron-electron correlation. It is also found that the doping dependent ground state {\$}p{\$}-wave pairing correlation bears some similarity to the famous superconducting dome in the phase diagram of high-temperature superconductor, while the spin correlation at the edge is weakened as the system is doped away from half filling. [Preview Abstract] |
Tuesday, March 14, 2017 1:15PM - 1:27PM |
F33.00011: Substitutional impurities in halfmetallic graphene/h-BN nanoribbons. Andrea Latgé, Marcio Costa, Carlos Leon Zigzag graphene nanoribbons (ZGNRs) present a half-metallic response for a critical external electric field valuewith possibilities for spin-filter applications. On the other hand, embedded ZGNRs in zigzag hexagonal boron nitride (ZBNNR) exhibit half-metallicity without the presence of an external field. In this work, we analyze electronic properties of a mixed nanoribbon system (ZBNR/ZGNR/ZBNR), using a Hubbard model Hamiltonian within a mean field approximation. Due to different electronegativities of the boron and nitrogen atoms, an electric field is induced across the ZGNR strip, breaking the spin degeneracy of the electronic band structure. Edge potentials as corrections for on-site energies are investigated, and also how they are affected due to the ZBNNR/ZGNR interfaces. Substitutional impurities are found as a mechanism to enhance half-metallic response. Here we analyze the effect of N and B impurities along the GNR and also through the interfaces. Energetic stabilities of the different configurations studied were included. We found that energy gap sizes may be properly engineered by controlling the spatial doping process and that binding energy impurity calculations may be used to study impurity diffusion processes along the mixed nanoribbons. [Preview Abstract] |
Tuesday, March 14, 2017 1:27PM - 1:39PM |
F33.00012: Nonlinear edge conduction in monolayer WTe2 Tauno Palomaki, Zaiyao Fei, Wenjin Zhao, Paul Nguyen, Bosong Sun, Xiaodong Xu, David Cobden Monolayer WTe$_{\mathrm{2}}$ was recently predicted to have an inverted band structure, giving rise to helical edge states degenerate with bulk bands. [Qian \textit{et al.} Science 346, 1344-1347 (2014)] Although it is expected to be semimetal, experimentally we find insulating behavior in the bulk below approximately 100 K, while edge conduction remains. At lower temperatures (below \textasciitilde 20 K) we often observe a pronounced dip in the edge conductance at zero bias which is highly gate dependent. I will discuss our recent progress on understanding this zero bias anomaly through its bias voltage, gate, length, temperature, and magnetic field dependence. Possible effects of the contacts, magnetic scatterers in the edge, and electron correlations will be discussed. [Preview Abstract] |
Tuesday, March 14, 2017 1:39PM - 1:51PM |
F33.00013: Intrinsic Half-Metallicity in Atomically Thin Zigzag Tungsten Dichalcogenide Nanoribbons Ping Cui, Jin-Ho Choi, Jiang Zeng, Zhenyu Li, Changgan Zeng, Chih-Kang Shih, Zhenyu Zhang Realization of half-metallicity has been serving as a central research emphasis in development of next-generation spintronic devices. To date, only three-dimensional (3D) half-metals have been achieved, while their 2D counterparts remain to be materialized despite of extensive efforts on various predictive designs. This standing challenge is largely due to stringent requirements to establish ferromagnetic order in 2D materials. Here we use first-principles approaches to uncover that atomically thin zigzag tungsten dichalcogenide WX$_{\mathrm{2}}$ (X $=$ S, Se) nanoribbons stand as the first known intrinsic 2D half-metallic systems, without the typical approach of invoking an external electric field, chemical modification, or carrier doping. The readily accessible half-metallicity is attributed to distinctly different structural reconstructions along the two edges, insulating along one edge, metallic along the other, the latter characterized by the propagation of a robust spin-polarized electron transmission channel. These findings are expected to offer unprecedented opportunities in spintronics purely based on 2D materials. [Preview Abstract] |
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