405 research outputs found
Hvordan fastlægger vi oprensningskriterier for grundvandstruende forureninger?:Overvejelser på lokal- og oplandsskala
Clinical components and associated behavioural aspects of a complex healthcare intervention : Multi-methods study of selective decontamination of the digestive tract in critical care
Copyright © 2013 Australian College of Critical Care Nurses Ltd. Published by Elsevier Ltd. All rights reserved.Peer reviewedPostprin
Imaging and Dynamics of Light Atoms and Molecules on Graphene
Observing the individual building blocks of matter is one of the primary
goals of microscopy. The invention of the scanning tunneling microscope [1]
revolutionized experimental surface science in that atomic-scale features on a
solid-state surface could finally be readily imaged. However, scanning
tunneling microscopy has limited applicability due to restrictions, for
example, in sample conductivity, cleanliness, and data aquisition rate. An
older microscopy technique, that of transmission electron microscopy (TEM) [2,
3] has benefited tremendously in recent years from subtle instrumentation
advances, and individual heavy (high atomic number) atoms can now be detected
by TEM [4 - 7] even when embedded within a semiconductor material [8, 9].
However, detecting an individual low atomic number atom, for example carbon or
even hydrogen, is still extremely challenging, if not impossible, via
conventional TEM due to the very low contrast of light elements [2, 3, 10 -
12]. Here we demonstrate a means to observe, by conventional transmision
electron microscopy, even the smallest atoms and molecules: On a clean
single-layer graphene membrane, adsorbates such as atomic hydrogen and carbon
can be seen as if they were suspended in free space. We directly image such
individual adatoms, along with carbon chains and vacancies, and investigate
their dynamics in real time. These techniques open a way to reveal dynamics of
more complex chemical reactions or identify the atomic-scale structure of
unknown adsorbates. In addition, the study of atomic scale defects in graphene
may provide insights for nanoelectronic applications of this interesting
material.Comment: 9 pages manuscript and figures, 9 pages supplementary informatio
Characterization of chlorinated solvent contamination in limestone using innovative FLUTe® technologies in combination with other methods in a line of evidence approach
Energy spectra of fractional quantum Hall systems in the presence of a valence hole
The energy spectrum of a two-dimensional electron gas (2DEG) in the
fractional quantum Hall regime interacting with an optically injected valence
band hole is studied as a function of the filling factor and the
separation between the electron and hole layers. The response of the 2DEG
to the hole changes abruptly at of the order of the magnetic length
. At , the hole binds electrons to form neutral () or
charged () excitons, and the photoluminescence (PL) spectrum probes the
lifetimes and binding energies of these states rather than the original
correlations of the 2DEG. The ``dressed exciton'' picture (in which the
interaction between an exciton and the 2DEG was proposed to merely enhance the
exciton mass) is questioned. Instead, the low energy states are explained in
terms of Laughlin correlations between the constituent fermions (electrons and
's) and the formation of two-component incompressible fluid states in the
electron--hole plasma. At , the hole binds up to two Laughlin
quasielectrons (QE) of the 2DEG to form fractionally charged excitons
QE. The previously found ``anyon exciton'' QE is shown to be
unstable at any value of . The critical dependence of the stability of
different QE complexes on the presence of QE's in the 2DEG leads to the
observed discontinuity of the PL spectrum at or .Comment: 16 pages, 14 figures, submitted to PR
Quantum point contact on graphite surface
The conductance through a quantum point contact created by a sharp and hard
metal tip on the graphite surface has features which to our knowledge have not
been encountered so far in metal contacts or in nanowires. In this paper we
first investigate these features which emerge from the strongly directional
bonding and electronic structure of graphite, and provide a theoretical
understanding for the electronic conduction through quantum point contacts. Our
study involves the molecular-dynamics simulations to reveal the variation of
interlayer distances and atomic structure at the proximity of the contact that
evolves by the tip pressing toward the surface. The effects of the elastic
deformation on the electronic structure, state density at the Fermi level, and
crystal potential are analyzed by performing self-consistent-field
pseudopotential calculations within the local-density approximation. It is
found that the metallicity of graphite increases under the uniaxial compressive
strain perpendicular to the basal plane. The quantum point contact is modeled
by a constriction with a realistic potential. The conductance is calculated by
representing the current transporting states in Laue representation, and the
variation of conductance with the evolution of contact is explained by taking
the characteristic features of graphite into account. It is shown that the
sequential puncturing of the layers characterizes the conductance.Comment: LaTeX, 11 pages, 9 figures (included), to be published in Phys. Rev.
B, tentatively scheduled for 15 September 1998 (Volume 58, Number 12
Structure determination of the (1×2) and (1×3) reconstructions of Pt(110) by low-energy electron diffraction
The atomic geometry of the (1×2) and (1×3) structures of the Pt(100) surface has been determined from a low-energy electron-diffraction intensity analysis. Both structures are found to be of the missing-row type, consisting of (111) microfacets, and with similar relaxations in the subsurface layers. In both reconstructions the top-layer spacing is contracted by approximately 20% together with a buckling of about 0.17 Å in the third layer and a small lateral shift of about 0.04 Å in the second layer. Further relaxations down to the fourth layer were detectable. The surface relaxations correspond to a variation of interatomic distances, ranging from -7% to +4%, where in general a contraction of approximately 3% for the distances parallel to the surface occurs. The Pendry and Zanazzi-Jona R factors were used in the analysis, resulting in a minimum value of RP=0.36 and RZJ=0.26 for 12 beams at normal incidence for the (1×2) structure, and similar agreement for 19 beams of the (1×3) structure. The (1×3) structure has been reproducibly obtained after heating the crystal in an oxygen atmosphere of 5×10-6 mbar at 1200 K for about 30 min and could be removed by annealing at 1800 K for 45 min after which the (1×2) structure appeared again. Both reconstructed surfaces are clean within the detection limits of the Auger spectrometer. CO adsorption lifts the reconstruction in both structures. After desorption at 500 K the initial structures appear again, indicating that at least one of the reconstructions does not represent the equilibrium structure of the clean surface and may be stabilized by impurities
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