21,042 research outputs found
Phase Field Crystals as a Coarse-Graining in Time of Molecular Dynamics
Phase field crystals (PFC) are a tool for simulating materials at the atomic
level. They combine the small length-scale resolution of molecular dynamics
(MD) with the ability to simulate dynamics on mesoscopic time scales. We show
how PFC can be interpreted as the result of applying coarse-graining in time to
the microscopic density field of molecular dynamics simulations. We take the
form of the free energy for the phase field from the classical density
functional theory of inhomogeneous liquids and then choose coefficients to
match the structure factor of the time coarse-grained microscopic density
field. As an example, we show how to construct a PFC free energy for Weber and
Stillinger's two-dimensional square crystal potential which models a system of
proteins suspended in a membrane.Comment: 5 pages, 4 figures, typos corrected, more explanation in parts,
equilib vs non-equilib clarifie
Controls on carbon and energy exchange by a black spruce - moss ecosystem: Testing the mathematical model Ecosys with data from the BOREAS experiment
Imaging X-ray spectrometer
An X-ray spectrometer for providing imaging and energy resolution of an X-ray source is described. This spectrometer is comprised of a thick silicon wafer having an embedded matrix or grid of aluminum completely through the wafer fabricated, for example, by thermal migration. The aluminum matrix defines the walls of a rectangular array of silicon X-ray detector cells or pixels. A thermally diffused aluminum electrode is also formed centrally through each of the silicon cells with biasing means being connected to the aluminum cell walls and causes lateral charge carrier depletion between the cell walls so that incident X-ray energy causes a photoelectric reaction within the silicon producing collectible charge carriers in the form of electrons which are collected and used for imaging
Site specific spin dynamics in BaFe2As2: tuning the ground state by orbital differentiation
The role of orbital differentiation on the emergence of superconductivity in
the Fe-based superconductors remains an open question to the scientific
community. In this investigation, we employ a suitable microscopic spin probe
technique, namely Electron Spin Resonance (ESR), to investigate this issue on
selected chemically substituted BaFeAs single crystals. As the
spin-density wave (SDW) phase is suppressed, we observe a clear increase of the
Fe 3 bands anisotropy along with their localization at the FeAs plane. Such
an increase of the planar orbital content interestingly occurs independently on
the chemical substitution responsible for suppressing the SDW phase. As a
consequence, the magnetic fluctuations combined with the resultant particular
symmetry of the Fe 3 bands are propitious ingredients to the emergence of
superconductivity in this class of materials.Comment: 6 pages, 5 figure
Dynamic Behavior of Soils From Field and Laboratory Tests
Both geophysical and laboratory tests were used to determine the dynamic shear moduli of medium stiff to hard clays and silts at six sites. The geophysical measurements included conventional downhole tests conducted at low strain levels and modified cross-hole impulse tests conducted over a wide range of strains. Laboratory resonant column and cyclic triaxial tests were performed on soil samples retrieved from borings made at the sites. The results from both the field and laboratory tests of the medium stiff to stiff sols showed fairly good agreement. The laboratory test results of the hard clays and silts, however, were typically half the values of the field tests at corresponding strain levels. This would suggest the need for adjusting the laboratory results performed on hard cohesive soils. Also, the field test results suggest that as soil stiffness increases, the modulus attenuation rate with strain decreases
Characterization of the residual stresses in spray-formed steels using neutron diffraction
Neutron diffraction was used to characterize the residual stresses in an as-sprayed tube-shaped steel preform. The measured residual stress distributions were compared with those simulated using finite element method by taking into account the effects of the thermal history, porosity and different phases of the sprayed preform. The porosity was measured using X-ray microcomputed tomography. The study revealed for the first time the correlation between the distribution of porosity and residual stress developed in the as-sprayed preform
- …