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Search Results: 1 - 10 of 203950 matches for " Javier D. Sanchez-Yamagishi "
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Anisotropic Etching and Nanoribbon Formation in Single-Layer Graphene
Leonardo C. Campos,Vitor R. Manfrinato,Javier D. Sanchez-Yamagishi,Jing Kong,Pablo Jarillo-Herrero
Physics , 2009, DOI: 10.1021/nl900811r
Abstract: We demonstrate anisotropic etching of single-layer graphene by thermally-activated nickel nanoparticles. Using this technique, we obtain sub-10nm nanoribbons and other graphene nanostructures with edges aligned along a single crystallographic direction. We observe a new catalytic channeling behavior, whereby etched cuts do not intersect, resulting in continuously connected geometries. Raman spectroscopy and electronic measurements show that the quality of the graphene is resilient under the etching conditions, indicating that this method may serve as a powerful technique to produce graphene nanocircuits with well-defined crystallographic edges.
Tunneling in graphene-topological insulator hybrid devices
Hadar Steinberg,Lucas A. Orona,Valla Fatemi,Javier D. Sanchez-Yamagishi,Kenji Watanabe,Takashi Taniguchi,Pablo Jarillo-Herrero
Physics , 2015,
Abstract: Hybrid graphene-topological insulator (TI) devices were fabricated using a mechanical transfer method and studied via electronic transport. Devices consisting of bilayer graphene (BLG) under the TI Bi$_2$Se$_3$ exhibit differential conductance characteristics which appear to be dominated by tunneling, roughly reproducing the Bi$_2$Se$_3$ density of states. Similar results were obtained for BLG on top of Bi$_2$Se$_3$, with 10-fold greater conductance consistent with a larger contact area due to better surface conformity. The devices further show evidence of inelastic phonon-assisted tunneling processes involving both Bi$_2$Se$_3$ and graphene phonons. These processes favor phonons which compensate for momentum mismatch between the TI $\Gamma$ and graphene $K, K'$ points. Finally, the utility of these tunnel junctions is demonstrated on a density-tunable BLG device, where the charge-neutrality point is traced along the energy-density trajectory. This trajectory is used as a measure of the ground-state density of states.
Quantum Hall Effect, Screening and Layer-Polarized Insulating States in Twisted Bilayer Graphene
Javier D. Sanchez-Yamagishi,Thiti Taychatanapat,Kenji Watanabe,Takashi Taniguchi,Amir Yacoby,Pablo Jarillo-Herrero
Physics , 2011, DOI: 10.1103/PhysRevLett.108.076601
Abstract: We investigate electronic transport in dual-gated twisted bilayer graphene. Despite the sub-nanometer proximity between the layers, we identify independent contributions to the magnetoresistance from the graphene Landau level spectrum of each layer. We demonstrate that the filling factor of each layer can be independently controlled via the dual gates, which we use to induce Landau level crossings between the layers. By analyzing the gate dependence of the Landau level crossings, we characterize the finite inter-layer screening and extract the capacitance between the atomically-spaced layers. At zero filling factor, we observe magnetic and displacement field dependent insulating states, which indicate the presence of counter-propagating edge states with inter-layer coupling.
STM Spectroscopy of ultra-flat graphene on hexagonal boron nitride
Jiamin Xue,Javier Sanchez-Yamagishi,D. Bulmash,Philippe Jacquod,A. Deshpande,K. Watanabe,T. Taniguchi,Pablo Jarillo-Herrero,B. J. LeRoy
Physics , 2011, DOI: 10.1038/nmat2968
Abstract: Graphene has demonstrated great promise for future electronics technology as well as fundamental physics applications because of its linear energy-momentum dispersion relations which cross at the Dirac point. However, accessing the physics of the low density region at the Dirac point has been difficult because of the presence of disorder which leaves the graphene with local microscopic electron and hole puddles, resulting in a finite density of carriers even at the charge neutrality point. Efforts have been made to reduce the disorder by suspending graphene, leading to fabrication challenges and delicate devices which make local spectroscopic measurements difficult. Recently, it has been shown that placing graphene on hexagonal boron nitride (hBN) yields improved device performance. In this letter, we use scanning tunneling microscopy to show that graphene conforms to hBN, as evidenced by the presence of Moire patterns in the topographic images. However, contrary to recent predictions, this conformation does not lead to a sizable band gap due to the misalignment of the lattices. Moreover, local spectroscopy measurements demonstrate that the electron-hole charge fluctuations are reduced by two orders of magnitude as compared to those on silicon oxide. This leads to charge fluctuations which are as small as in suspended graphene, opening up Dirac point physics to more diverse experiments than are possible on freestanding devices.
Emergence of Superlattice Dirac Points in Graphene on Hexagonal Boron Nitride
Matthew Yankowitz,Jiamin Xue,Daniel Cormode,Javier D. Sanchez-Yamagishi,K. Watanabe,T. Taniguchi,Pablo Jarillo-Herrero,Philippe Jacquod,Brian J. LeRoy
Physics , 2012, DOI: 10.1038/nphys2272
Abstract: The Schr\"odinger equation dictates that the propagation of nearly free electrons through a weak periodic potential results in the opening of band gaps near points of the reciprocal lattice known as Brillouin zone boundaries. However, in the case of massless Dirac fermions, it has been predicted that the chirality of the charge carriers prevents the opening of a band gap and instead new Dirac points appear in the electronic structure of the material. Graphene on hexagonal boron nitride (hBN) exhibits a rotation dependent Moir\'e pattern. In this letter, we show experimentally and theoretically that this Moir\'e pattern acts as a weak periodic potential and thereby leads to the emergence of a new set of Dirac points at an energy determined by its wavelength. The new massless Dirac fermions generated at these superlattice Dirac points are characterized by a significantly reduced Fermi velocity. The local density of states near these Dirac cones exhibits hexagonal modulations indicating an anisotropic Fermi velocity.
Chemical reactivity imprint lithography on graphene: Controlling the substrate influence on electron transfer reactions
Qing Hua Wang,Zhong Jin,Ki Kang Kim,Andrew J. Hilmer,Geraldine L. C. Paulus,Chih-Jen Shih,Moon-Ho Ham,Javier D. Sanchez-Yamagishi,Kenji Watanabe,Takashi Taniguchi,Jing Kong,Pablo Jarillo-Herrero,Michael S. Strano
Physics , 2012, DOI: 10.1038/nchem.1421
Abstract: The chemical functionalization of graphene enables control over electronic properties and sensor recognition sites. However, its study is confounded by an unusually strong influence of the underlying substrate. In this paper, we show a stark difference in the rate of electron transfer chemistry with aryl diazonium salts on monolayer graphene supported on a broad range of substrates. Reactions proceed rapidly when graphene is on SiO_2 and Al_2O_3 (sapphire), but negligibly on alkyl-terminated and hexagonal boron nitride (hBN) surfaces. The effect is contrary to expectations based on doping levels and can instead be described using a reactivity model accounting for substrate-induced electron-hole puddles in graphene. Raman spectroscopic mapping is used to characterize the effect of the substrates on graphene. Reactivity imprint lithography (RIL) is demonstrated as a technique for spatially patterning chemical groups on graphene by patterning the underlying substrate, and is applied to the covalent tethering of proteins on graphene.
Long wavelength local density of states oscillations near graphene step edges
Jiamin Xue,Javier Sanchez-Yamagishi,K. Watanabe,T. Taniguchi,Pablo Jarillo-Herrero,Brian J. LeRoy
Physics , 2011, DOI: 10.1103/PhysRevLett.108.016801
Abstract: Using scanning tunneling microscopy and spectroscopy, we have studied the local density of states (LDOS) of graphene over step edges in boron nitride. Long wavelength oscillations in the LDOS are observed with maxima parallel to the step edge. Their wavelength and amplitude are controlled by the energy of the quasiparticles allowing a direct probe of the graphene dispersion relation. We also observe a faster decay of the LDOS oscillations away from the step edge than in conventional metals. This is due to the chiral nature of the Dirac fermions in graphene.
Massive Dirac fermions and Hofstadter butterfly in a van der Waals heterostructure
B. Hunt,J. D. Sanchez-Yamagishi,A. F. Young,K. Watanabe,T. Taniguchi,P. Moon,M. Koshino,P. Jarillo-Herrero,R. C. Ashoori
Physics , 2013, DOI: 10.1126/science.1237240
Abstract: Van der Waals heterostructures comprise a new class of artificial materials formed by stacking atomically-thin planar crystals. Here, we demonstrate band structure engineering of a van der Waals heterostructure composed of a monolayer graphene flake coupled to a rotationally-aligned hexagonal boron nitride substrate. The spatially-varying interlayer atomic registry results both in a local breaking of the carbon sublattice symmetry and a long-range moir\'e superlattice potential in the graphene. This interplay between short- and long-wavelength effects results in a band structure described by isolated superlattice minibands and an unexpectedly large band gap at charge neutrality, both of which can be tuned by varying the interlayer alignment. Magnetocapacitance measurements reveal previously unobserved fractional quantum Hall states reflecting the massive Dirac dispersion that results from broken sublattice symmetry. At ultra-high fields, integer conductance plateaus are observed at non-integer filling factors due to the emergence of the Hofstadter butterfly in a symmetry-broken Landau level.
Tunable symmetry breaking and helical edge transport in a graphene quantum spin Hall state
A. F. Young,J. D. Sanchez-Yamagishi,B. Hunt,S. H. Choi,K. Watanabe,T. Taniguchi,R. C. Ashoori,P. Jarillo-Herrero
Physics , 2013, DOI: 10.1038/nature12800
Abstract: Low-dimensional electronic systems have traditionally been obtained by electrostatically confining electrons, either in heterostructures or in intrinsically nanoscale materials such as single molecules, nanowires, and graphene. Recently, a new paradigm has emerged with the advent of symmetry-protected surface states on the boundary of topological insulators, enabling the creation of electronic systems with novel properties. For example, time reversal symmetry (TRS) endows the massless charge carriers on the surface of a three-dimensional topological insulator with helicity, locking the orientation of their spin relative to their momentum. Weakly breaking this symmetry generates a gap on the surface, resulting in charge carriers with finite effective mass and exotic spin textures. Analogous manipulations of the one-dimensional boundary states of a two-dimensional topological insulator are also possible, but have yet to be observed in the leading candidate materials. Here, we demonstrate experimentally that charge neutral monolayer graphene displays a new type of quantum spin Hall (QSH) effect, previously thought to exist only in TRS topological insulators, when it is subjected to a very large magnetic field angled with respect to the graphene plane. Unlike in the TRS case, the QSH presented here is protected by a spin-rotation symmetry that emerges as electron spins in a half-filled Landau level are polarized by the large in-plane magnetic field. The properties of the resulting helical edge states can be modulated by balancing the applied field against an intrinsic antiferromagnetic instability, which tends to spontaneously break the spin-rotation symmetry. In the resulting canted antiferromagnetic (CAF) state, we observe transport signatures of gapped edge states, which constitute a new kind of one-dimensional electronic system with tunable band gap and associated spin-texture.
Syndromic Surveillance Using Veterinary Laboratory Data: Algorithm Combination and Customization of Alerts
Fernanda C. Dórea, Beverly J. McEwen, W. Bruce McNab, Javier Sanchez, Crawford W. Revie
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0082183
Abstract: Background Syndromic surveillance research has focused on two main themes: the search for data sources that can provide early disease detection; and the development of efficient algorithms that can detect potential outbreak signals. Methods This work combines three algorithms that have demonstrated solid performance in detecting simulated outbreak signals of varying shapes in time series of laboratory submissions counts. These are: the Shewhart control charts designed to detect sudden spikes in counts; the EWMA control charts developed to detect slow increasing outbreaks; and the Holt-Winters exponential smoothing, which can explicitly account for temporal effects in the data stream monitored. A scoring system to detect and report alarms using these algorithms in a complementary way is proposed. Results The use of multiple algorithms in parallel resulted in increased system sensitivity. Specificity was decreased in simulated data, but the number of false alarms per year when the approach was applied to real data was considered manageable (between 1 and 3 per year for each of ten syndromic groups monitored). The automated implementation of this approach, including a method for on-line filtering of potential outbreak signals is described. Conclusion The developed system provides high sensitivity for detection of potential outbreak signals while also providing robustness and flexibility in establishing what signals constitute an alarm. This flexibility allows an analyst to customize the system for different syndromes.
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