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Search Results: 1 - 10 of 10556 matches for " Matthew Yankowitz "
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Graphene on Hexagonal Boron Nitride
Matthew Yankowitz,Jiamin Xue,Brian J. LeRoy
Physics , 2014, DOI: 10.1088/0953-8984/26/30/303201
Abstract: The field of graphene research has developed rapidly since its first isolation by mechanical exfoliation in 2004. Due to the relativistic Dirac nature of its charge carriers, graphene is both a promising material for next-generation electronic devices and a convenient low-energy testbed for intrinsically high-energy physical phenomena. Both of these research branches require the facile fabrication of clean graphene devices so as not to obscure its intrinsic physical properties. Hexagonal boron nitride has emerged as a promising substrate for graphene devices, as it is insulating, atomically flat and provides a clean charge environment for the graphene. Additionally, the interaction between graphene and boron nitride provides a path for the study of new physical phenomena not present in bare graphene devices. This review focuses on recent advancements in the study of graphene on hexagonal boron nitride devices from the perspective of scanning tunneling microscopy with highlights of some important results from electrical transport measurements.
Local Spectroscopic Characterization of Spin and Layer Polarization in WSe$_2$
Matthew Yankowitz,Devin McKenzie,Brian J. LeRoy
Physics , 2015, DOI: 10.1103/PhysRevLett.115.136803
Abstract: We report scanning tunneling microscopy (STM) and spectroscopy (STS) measurements of monolayer and bilayer WSe$_2$. We measure a band gap of 2.21 $\pm$ 0.08 eV in monolayer WSe$_2$, which is much larger than the energy of the photoluminescence peak indicating a large excitonic binding energy. We additionally observe significant electronic scattering arising from atomic-scale defects. Using Fourier transform STS (FT-STS), we map the energy versus momentum dispersion relations for monolayer and bilayer WSe$_2$. Further, by tracking allowed and forbidden scattering channels as a function of energy we infer the spin texture of both the conduction and valence bands. We observe a large spin-splitting of the valence band due to strong spin-orbit coupling, and additionally observe spin-valley-layer coupling in the conduction band of bilayer WSe$_2$.
Local Spectroscopy of the Electrically Tunable Band Gap in Trilayer Graphene
Matthew Yankowitz,Fenglin Wang,Chun Ning Lau,Brian J. LeRoy
Physics , 2013, DOI: 10.1103/PhysRevB.87.165102
Abstract: The stacking order degree of freedom in trilayer graphene plays a critical role in determining the existence of an electric field tunable band gap. We present spatially-resolved tunneling spectroscopy measurements of dual gated Bernal (ABA) and rhombohedral (ABC) stacked trilayer graphene devices. We demonstrate that while ABA trilayer graphene remains metallic, ABC trilayer graphene exhibits a widely tunable band gap as a function of electric field. However, we find that charged impurities in the underlying substrate cause substantial spatial fluctuation of the gap size. Our work elucidates the microscopic behavior of trilayer graphene and its consequences for macroscopic devices.
Evolution of the electronic band structure of twisted bilayer graphene upon doping
Shengqiang Huang,Matthew Yankowitz,Kanokporn Chattrakun,Arvinder Sandhu,Brian J. LeRoy
Physics , 2015,
Abstract: The electronic band structure of twisted bilayer graphene depends on the twist angle between the two layers. In particular, van Hove singularities occur in the density of states when the two sets of monolayer bands cross. An enhanced Raman G peak is observed when the excitation laser is resonant with the energy separation of these singularities. Using Raman spectroscopy, we monitor the evolution of the electronic band structure for various twist angles upon charge doping. The variation of the Raman G peak area with charge density reveals changes in the electronic band structure, and a decrease in the energy of the resonant scattering pathway is inferred from the twist angle dependence. This decrease is due to a charge density asymmetry between the two layers. Our results demonstrate that the electronic and optical properties of twisted bilayer graphene can be controlled by doping.
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.
Intrinsic disorder in graphene on transition metal dichalcogenide heterostructures
Matthew Yankowitz,Stefano Larentis,Kyounghwan Kim,Jiamin Xue,Devin McKenzie,Shengqiang Huang,Marina Paggen,Mazhar N. Ali,Robert J. Cava,Emanuel Tutuc,Brian J. LeRoy
Physics , 2014, DOI: 10.1021/nl5047736
Abstract: The electronic properties of two-dimensional materials such as graphene are extremely sensitive to their environment, especially the underlying substrate. Planar van der Waals bonded substrates such as hexagonal boron nitride (hBN) have been shown to greatly improve the electrical performance of graphene devices by reducing topographic variations and charge fluctuations compared to amorphous insulating substrates}. Semiconducting transition metal dichalchogenides (TMDs) are another family of van der Waals bonded materials that have recently received interest as alternative substrates to hBN for graphene as well as for components in novel graphene-based device heterostructures. Additionally, their semiconducting nature permits dynamic gate voltage control over the interaction strength with graphene. Through local scanning probe measurements we find that crystalline defects intrinsic to TMDs induce scattering in graphene which results in significant degradation of the heterostructure quality, particularly compared to similar graphene on hBN devices.
Electric Field Control of Soliton Motion and Stacking in Trilayer Graphene
Matthew Yankowitz,Joel I-Jan Wang,A. Glen Birdwell,Yu-An Chen,K. Watanabe,T. Taniguchi,Philippe Jacquod,Pablo San-Jose,Pablo Jarillo-Herrero,Brian J. LeRoy
Physics , 2014, DOI: 10.1038/nmat3965
Abstract: The crystal structure of a material plays an important role in determining its electronic properties. Changing from one crystal structure to another involves a phase transition which is usually controlled by a state variable such as temperature or pressure. In the case of trilayer graphene, there are two common stacking configurations (Bernal and rhombohedral) which exhibit very different electronic properties. In graphene flakes with both stacking configurations, the region between them consists of a localized strain soliton where the carbon atoms of one graphene layer shift by the carbon-carbon bond distance. Here we show the ability to move this strain soliton with a perpendicular electric field and hence control the stacking configuration of trilayer graphene with only an external voltage. Moreover, we find that the free energy difference between the two stacking configurations scales quadratically with electric field, and thus rhombohedral stacking is favored as the electric field increases. This ability to control the stacking order in graphene opens the way to novel devices which combine structural and electrical properties.
Band Structure Mapping of Bilayer Graphene via Quasiparticle Scattering
Matthew Yankowitz,Joel I-Jan Wang,Suchun Li,A. Glen Birdwell,Yu-An Chen,Kenji Watanabe,Takashi Taniguchi,Su Ying Quek,Pablo Jarillo-Herrero,Brian J. LeRoy
Physics , 2014, DOI: 10.1063/1.4890543
Abstract: A perpendicular electric field breaks the layer symmetry of Bernal-stacked bilayer graphene, resulting in the opening of a band gap and a modification of the effective mass of the charge carriers. Using scanning tunneling microscopy and spectroscopy, we examine standing waves in the local density of states of bilayer graphene formed by scattering from a bilayer/trilayer boundary. The quasiparticle interference properties are controlled by the bilayer graphene band structure, allowing a direct local probe of the evolution of the band structure of bilayer graphene as a function of electric field. We extract the Slonczewski-Weiss-McClure model tight binding parameters as $\gamma_0 = 3.1$ eV, $\gamma_1 = 0.39$ eV, and $\gamma_4 = 0.22$ eV.
HPV Prevalence and Concordance in the Cervix and Oral Cavity of Pregnant Women
E. M. Smith,J. M. Ritchie,J. Yankowitz,D. Wang,L. P. Turek,T. H. Haugen
Infectious Diseases in Obstetrics and Gynecology , 2004, DOI: 10.1080/10647440400009896
Abstract: Objectives: This investigation examined human papillomavirus (HPV) in pregnant women in order to characterize viral prevalence, types and concordance between infection in the cervix and in the oral cavity.
Tunneling Spectroscopy of Graphene-Boron Nitride Heterostructures
F. Amet,J. R. Williams,A. G. F. Garcia,M. Yankowitz,K. Watanabe,T. Taniguchi,D. Goldhaber-Gordon
Physics , 2011, DOI: 10.1103/PhysRevB.85.073405
Abstract: We report on the fabrication and measurement of a graphene tunnel junction using hexagonal-boron nitride as a tunnel barrier between graphene and a metal gate. The tunneling behavior into graphene is altered by the interactions with phonons and the presence of disorder. We extract prop- erties of graphene and observe multiple phonon-enhanced tunneling thresholds. Finally, differences in the measured properties of two devices are used to shed light on mutually-contrasting previous results of scanning tunneling microscopy in graphene.
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