Mesoscopic transport phenomena in epitaxial graphene nanostructures : a surface science approach

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The quarter median

We introduce and discuss a multivariate version of the classical median that is based on an equipartition property with respect to quarter spaces. These arise as pairwise intersections of the half-spaces associated with the coordinate hyperplanes of an orthogonal basis. We obtain results on existence, equivariance, and asymptotic normality

Discrete mixture representations of spherical distributions

We obtain discrete mixture representations for parametric families of probability distributions on Euclidean spheres, such as the von Mises–Fisher, the Watson and the angular Gaussian families. In addition to several special results we present a general approach to isotropic distribution families that is based on density expansions in terms of special surface harmonics. We discuss the connections to stochastic processes on spheres, in particular random walks, discrete mixture representations derived from spherical diffusions, and the use of Markov representations for the mixing base to obtain representations for families of spherical distributions

Project #52: Health Alliance Plan - COVID Vaccination Initiative

Since the initial Michigan COVID-19 surge in early 2020, HAP has been collaborating with a variety of partners to educate members on COVID-19 while preparing for COVID mitigation efforts, including vaccination of our member population to improve the health of our membership. Starting in January 2021, HAP employed a variety of data-driven tactics focusing on a goal of 70% membership vaccine coverage (at least once vaccine administered) for the ages 16+ population by Oct. 1, 2021. This overall initial goal incrementally expanded as younger age groups became vaccine eligible and intensified with a special equity focus for Medicaid and Medicare/Medicaid duals member vaccination in underserved, socially vulnerable populations. As a result of these efforts, HAP reached the 70% vaccine coverage goal for the 16+ population as of early December 2021 and continues to focus vaccine efforts to increase vaccine coverage for subgroups of younger members, encourage booster dosing, and improve vaccination equity in high-risk groups. This initiative has focused on achieving high rates of vaccination coverage for HAP members to lessen the risk of hospitalizations, deaths, and post-covid complications related to the coronavirus pandemic in Michigan. Strategically, this initiative helped genuinely engage our health plan teams in the fight against COVID-19 and continues to amplify HAP’s overall corporate mission to transform HAP to improve the health of our neighbors.https://scholarlycommons.henryford.com/qualityexpo2022/1007/thumbnail.jp

Local transport measurements on epitaxial graphene

Growth of large-scale graphene is still accompanied by imperfections. By means of a four-tip STM/SEM the local structure of graphene grown on SiC(0001) was correlated with scanning electron microscope images and spatially resolved transport measurements. The systematic variation of probe spacings and substrate temperature has clearly revealed two-dimensional transport regimes of Anderson localization as well as of diffusive transport. The detailed analysis of the temperature dependent data demonstrates that the local on-top nano-sized contacts do not induce significant strain to the epitaxial graphene films.Comment: 3 figure

Plasmon damping below the Landau regime: the role of defects in epitaxial graphene

The sheet plasmon in epitaxially grown graphene layers on SiC(0001) and the influence of surface roughness have been investigated in detail by means of low-energy electron diffraction (LEED) and electron energy loss spectroscopy (EELS). We show that the existence of steps or grain boundaries in this epitaxial system is a source of strong damping, while the dispersion is rather insensitive to defects. To the first order, the lifetime of the plasmons was found to be proportional to the average terrace length and to the plasmon wavelength. A possible reason for this surprisingly efficient plasmon damping may be the close coincidence of phase (and group) velocities of the plasmons ( almost linear dispersion) with the Fermi velocity of the electrons. Therefore, uncorrelated defects like steps only have to act as a momentum source to effectively couple plasmons to the electron-hole continuum

Plasmons in Pb nanowire arrays on Si(557): Between one and two dimensions

The plasmon dispersion in arrays of nanowires of Pb close to an average Pb coverage of one monolayer was determined on the Si(557) surface using electron energy loss spectroscopy with both high energy and momentum resolution. While we find purely one-dimensional (1D) plasmon losses at a Pb concentration of 1.31 monolayers (ML), measured with respect to the Si(111) surface concentration, the 1.2 and 1.4 ML coverages exhibit wavelength-dependent transitions from 1D to anisotropic 2D properties. However, due to the high anisotropy in the system at all coverages, the dispersion curves exhibit 1D characteristics in both directions. This behavior seems to be related to the Pb-induced refacetting of the Si(557) surface, which depends on Pb coverage. It changes both effective system sizes and coupling strength between miniterraces. © 2011 American Physical Society.Ministry of Education, Culture, Sports, Science, and Technology, Japa

Ballistic bipolar junctions in chemically gated graphene ribbons

The realization of ballistic graphene pn-junctions is an essential task in order to study Klein tunneling phenomena. Here we show that intercalation of Ge under the buffer layer of pre-structured SiC-samples succeeds to make truly nano-scaled pn-junctions. By means of local tunneling spectroscopy the junction width is found to be as narrow as 5 nm which is a hundred times smaller compared to electrically gated structures. The ballistic transmission across the junction is directly proven by systematic transport measurements with a 4-tip STM. Various npn- and pnp-junctions are studied with respect to the barrier length. The pn-junctions are shown to act as polarizer and analyzer with the second junction becoming transparent in case of a fully ballistic barrier. This can be attributed to the almost full suppression of electron transmission through the junction away from normal incidence.DFG/SPP/145