383 research outputs found
Coastal and estuarine applications of multispectral photography
An evaluation of multispectral photographic techniques for optical penetration of water in the northeastern United States and the Gulf of Mexico coastal waters is presented. The spectral band (493 to 543 nanom), when exposed to place the water mass at about unit density on the photographic emulsion, was found to provide the best water penetration, independent of altitude or time of day, as long as solar glitter from the surface of the water is avoided. An isoluminous color technique was perfected, which eliminates the dimension of brightness from a multispectral color presentation
Multispectral photography for earth resources
A guide for producing accurate multispectral results for earth resource applications is presented along with theoretical and analytical concepts of color and multispectral photography. Topics discussed include: capabilities and limitations of color and color infrared films; image color measurements; methods of relating ground phenomena to film density and color measurement; sensitometry; considerations in the selection of multispectral cameras and components; and mission planning
Investigation of unconventional reconstruction and electronic properties on the Na2IrO3 surface
Na2IrO3 is an intriguing material for which spin-orbit coupling plays a key
role. Theoretical predictions, so far unverified, have been made that the
surface of Na2IrO3 should exhibit a clear signature of the quantum spin Hall
effect. We studied the surface of Na2IrO3 using scanning tunneling microscopy
and density-functional theory calculations. We observed atomic level resolution
of the surface and two types of terminations with different surface periodicity
and Na content. By comparing bias-dependent experimental topographic images to
simulated images, we determined the detailed atomistic structure of both
observed surfaces. One of these reveals a strong relaxation to the surface of
Na atoms from the subsurface region two atomic layers below. Such dramatic
structural changes at the surface cast doubt on any prediction of surface
properties based on bulk electronic structure. Indeed, using spatially resolved
tunneling spectroscopy we found no indication of the predicted quantum spin
Hall behavior
Finger somatotopy is preserved after tetraplegia but deteriorates over time
Previous studies showed reorganised and/or altered activity in the primary sensorimotor cortex after a spinal cord injury (SCI), suggested to reflect abnormal processing. However, little is known about whether somatotopically specific representations can be activated despite reduced or absent afferent hand inputs. In this observational study, we used functional MRI and a (attempted) finger movement task in tetraplegic patients to characterise the somatotopic hand layout in primary somatosensory cortex. We further used structural MRI to assess spared spinal tissue bridges. We found that somatotopic hand representations can be activated through attempted finger movements in the absence of sensory and motor hand functioning, and no spared spinal tissue bridges. Such preserved hand somatotopy could be exploited by rehabilitation approaches that aim to establish new hand-brain functional connections after SCI (e.g. neuroprosthetics). However, over years since SCI the hand representation somatotopy deteriorated, suggesting that somatotopic hand representations are more easily targeted within the first years after SCI
Theory of real space imaging of Fermi surfaces
A scanning tunneling microscope can be used to visualize in real space Fermi
surfaces with buried impurities far below substrates acting as local probes. A
theory describing this feature is developed based on the stationary phase
approximation. It is demonstrated how a Fermi surface of a material acts as a
mirror focusing electrons that scatter at hidden impurities.Comment: 10 pages, 4 figure
On the Connection of Anisotropic Conductivity to Tip Induced Space Charge Layers in Scanning Tunneling Spectroscopy of p-doped GaAs
The electronic properties of shallow acceptors in p-doped GaAs{110} are
investigated with scanning tunneling microscopy at low temperature. Shallow
acceptors are known to exhibit distinct triangular contrasts in STM images for
certain bias voltages. Spatially resolved I(V)-spectroscopy is performed to
identify their energetic origin and behavior. A crucial parameter - the STM
tip's work function - is determined experimentally. The voltage dependent
potential configuration and band bending situation is derived. Ways to validate
the calculations with the experiment are discussed. Differential conductivity
maps reveal that the triangular contrasts are only observed with a depletion
layer present under the STM tip. The tunnel process leading to the anisotropic
contrasts calls for electrons to tunnel through vacuum gap and a finite region
in the semiconductor.Comment: 11 pages, 8 figure
Local Density of States at Metal-Semiconductor Interfaces: An Atomic Scale Study
We investigate low temperature grown, abrupt, epitaxial, nonintermixed, defect-free n-type and p-type Fe/GaAs(110) interfaces by cross-sectional scanning tunneling microscopy and spectroscopy with atomic resolution. The probed local density of states shows that a model of the ideal metal-semiconductor interface requires a combination of metal-induced gap states and bond polarization at the interface which is nicely corroborated by density functional calculations. A three-dimensional finite element model of the space charge region yields a precise value for the Schottky barrier height
Band structure related wave function symmetry of amphoteric Si dopants in GaAs
Autocompensated Si-doped GaAs is studied with cross-sectional scanning
tunneling spectroscopy (X-STS). The local electronic contrasts of
substitutional Si(Ga) donors and Si(As) acceptors under the (110) cleavage
plane are imaged with high resolution. Si(Ga) donor atoms exhibit radially
symmetric contrasts. Si(As) acceptors have anisotropic features. The
anisotropic acceptor contrasts are traced back to a tunnel process at the
valence band edge. They reflect the probability density distribution of the
localized acceptor hole state.Comment: 10 pages, 3 figure
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