1,069 research outputs found
Mapping Itinerant Electrons around Kondo Impurities
We investigate single Fe and Co atoms buried below a Cu(100) surface using
low temperature scanning tunneling spectroscopy. By mapping the local density
of states of the itinerant electrons at the surface, the Kondo resonance near
the Fermi energy is analyzed. Probing bulk impurities in this well-defined
scattering geometry allows separating the physics of the Kondo system and the
measuring process. The line shape of the Kondo signature shows an oscillatory
behavior as a function of depth of the impurity as well as a function of
lateral distance. The oscillation period along the different directions reveals
that the spectral function of the itinerant electrons is anisotropic.Comment: 5 pages, 4 figures, accepted by Physical Review Letter
UHF and VHF radar observations of thunderstorms
A study of thunderstorms was made in the Summer of 1985 with the 430-MHz and 50-MHz radars at the Arecibo Observatory in Puerto Rico. Both radars use the 300-meter dish, which gives a beam width of less than 2 degrees even at these long wavelengths. Though the radars are steerable, only vertical beams were used in this experiment. The height resolution was 300 and 150 meters for the UHF and VHF, respectively. Lightning echoes, as well as returns from precipitation and clear-air turbulence were detected with both wavelengths. Large increases in the returned power were found to be coincident with increasing downward vertical velocities at UHF, whereas at VHF the total power returned was relatively constant during the life of a storm. This was attributed to the fact that the VHF is more sensitive to scattering from the turbulence-induced inhomogeneities in the refractive index and less sensitive to scatter from precipitation particles. On occasion, the shape of the Doppler spectra was observed to change with the occurrence of a lightning discharge in the pulse volume. Though the total power and mean reflectivity weighted Doppler velocity changed little during these events, the power is Doppler frequency bins near that corresponding to the updraft did increase substantially within a fraction of a second after a discharge was detected in the beam. This suggests some interaction between precipitation and lightning
Evidence for charge orbital and spin stripe order in an overdoped manganite
We present diffraction data on a single-layered manganite
La(0.42)Sr(1.58)MnO4 with hole doping (x>0.5). Overdoped La(0.42)Sr(1.58)MnO4
exhibits a complex ordering of charges, orbitals and spins. Single crystal
neutron diffraction experiments reveal three incommensurate and one
commensurate order parameters to be tightly coupled. The position and the shape
of the distinct superstructure scattering points to a stripe arrangement in
which ferromagnetic zigzag chains are disrupted by additional Mn4+ stripes
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
Spin-wave excitations in the ferromagnetic-metallic and in the charge, orbital and spin ordered states in NdSrMnO with x0.5
Inelastic neutron scattering experiments have been performed on single
crystals of NdSrMnO with x0.5. Colossal
magnetoresistance (CMR) in the manganites arises from the interplay between a
ferromagnetic metallic and antiferromagnetic charge and orbital ordered
insulating state. Therefore, it appears important to compare these phases
concerning their underlying magnetic interaction parameters. Our investigations
of the spin-wave disperion in the AFM ordered state of
NdSrMnO exhibits a strongly anisotropic stiffness. The
sign of the anisotropy is characteristic for the site-centered model for charge
and orbital ordering in half-doped manganites. Within this model, linear
spin-wave theory yields a perfect description of the experimental dispersion.
Furthermore, magnetic excitations in the ferromagnetic metallic state of
NdSrMnO with x=0.49 and x=0.50 exhibit nearly the same
magnon dispersion which can be described with a Heisenberg model including
nearest-neighbor interactions
Insights into the Supramolecular Structure and Degradation Mechanisms of Starch from Different Botanical Sources as Affected by Extrusion-based 3D Printing
Extrusion-based 3D printing has emerged as the most versatile additive manufacturing technique for the printing of practically any material. However, 3D printing of functional materials often activates thermo-mechanical degradation, which affects the 3D shape quality. Herein, we describe the structural changes of eight different starch sources (normal or waxy) as a consequence of the temperature of an extrusion-based 3D printing system through in-depth characterization of their molecular and structural changes. The combination of size-exclusion chromatography, small-angle X-ray scattering, X-ray diffraction, dynamic viscoelasticity measurements, and in vitro digestion has offered an extensive picture of the structural and biological transformations of starch varieties. Depending on the 3D printing conditions, either gelatinization was attained (“moderate” condition) or single-amylose helix formation was induced (“extreme” condition). The stiff amylopectin crystallites in starch granules were more susceptible to thermo-mechanical degradation compared to flexible amorphous amylose. The crystalline morphology of the starch varieties varied from B-type crystallinity for the starch 3D printing at the “moderate” condition to a mixture of C- and V-type crystallinity regarding the “extreme” condition. The “extreme” condition reduced the viscoelasticity of 3D-printed starches but increased the starch digestibility rate/extent. In contrast, the “moderate” condition increased the viscoelastic moduli, decreasing the starch digestion rate/extent. This was more considerable mainly regarding the waxy starch varieties. Finally, normal starch varieties presented a well-defined shape fidelity, being able to form a stable structure, whereas waxy starches exhibited a non-well-defined structure and were not able to maintain their integrity after printing. The results of this research allow us to monitor the degradability of a variety of starch cultivars to create starch-based 3D structures, in which the local structure can be controlled based on the 3D printing parameters
Identification of storm surge events over the German Bight from atmospheric reanalysis and climate model data
A new procedure for the identification of storm surge situations for the
German Bight is developed and applied to reanalysis and global climate model
data. This method is based on the empirical approach for estimating storm
surge heights using information about wind speed and wind direction. Here, we
hypothesize that storm surge events are caused by high wind speeds from north-
westerly direction in combination with a large-scale wind storm event
affecting the North Sea region. The method is calibrated for ERA-40 data,
using the data from the storm surge atlas for Cuxhaven. It is shown that using
information of both wind speed and direction as well as large-scale wind storm
events improves the identification of storm surge events. To estimate possible
future changes of potential storm surge events, we apply the new
identification approach to an ensemble of three transient climate change
simulations performed with the ECHAM5/MPIOM model under A1B greenhouse gas
scenario forcing. We find an increase in the total number of potential storm
surge events of about 12 % [(2001–2100)–(1901–2000)], mainly based on changes
of moderate events. Yearly numbers of storm surge relevant events show high
interannual and decadal variability and only one of three simulations shows a
statistical significant increase in the yearly number of potential storm surge
events between 1900 and 2100. However, no changes in the maximum intensity and
duration of all potential events is determined. Extreme value statistic
analysis confirms no frequency change of the most severe events
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