259 research outputs found
Effect of epitaxial strain on ferroelectric polarization in multiferroic BiFeO3 films
Multiferroic BiFeO3 epitaxial films with thickness ranging from 40 nm to 960
nm were grown by pulsed laser deposition on SrTiO3 (001) substrates with SrRuO3
bottom electrodes. X-ray characterization shows that the structure evolves from
angularly-distorted tetragonal with c/a ~ 1.04 to more bulk-like distorted
rhombohedral (c/a ~ 1.01) as the strain relaxes with increasing thickness.
Despite this significant structural evolution, the ferroelectric polarization
along the body diagonal of the distorted pseudo-cubic unit cells, as calculated
from measurements along the normal direction, barely changes.Comment: Legend in Fig.3 corrected and et
Development of boron calibration via hybrid comparator method in prompt gamma activation analysis
The prompt gamma activation analysis (PGAA) facility at the Nuclear Engineering Teaching Laboratory at The University of Texas at Austin was utilized to quantify boron concentrations in boron carbide semiconductor films deposited on silicon substrates. Calibration was complicated by the unique and varying sample geometries analyzed. In addition, there was a dearth of solid materials available with quantified boron concentrations having comparable or readily modifiable dimensions to exploit for calibration purposes. Therefore, a novel hybrid comparator method was developed for the quantification of boron utilizing aluminum as an inexpensive and easily machinable reference material. Aluminum samples were manufactured with high tolerances to match the geometry of each sample of interest. Each boron carbide film sample and its congruent aluminum sample were measured in the PGAA system. The measured aluminum responses and relevant nuclear parameters were used to standardize the measurements. A boron standard was created using a procedure derived from a similar approach used by the National Institute of Standards and Technology. Quality control measurements using this standard show that the method provided accuracy to within 5% for boron quantification
Some Characteristics of Broadband Delta-Sigma Modulation
This paper presents an analysis using correlation techniques
of an idealized Delta-Sigma Modulation system. An
analytical assumption of errors with a marginally Gaussian
distribution is shown to yield accurate results for broadband
modulation with a maximum input-output cross correlation.
It is also shown that this maximum is greatest for the degenerate
case of only "hard limiting" with no feedback and no
integration. A case of highly correla~ed inputs for Delta-
Sigma Modulation is also discussed to compare it with broad-
band performance and "hard limiting.
The pursuit of isotopic and molecular fire tracers in the polar atmosphere and cryosphere
We present an overview of recent multidisciplinary, multi-institutional efforts to identify and date major sources of combustion aerosol in the current and paleoatmospheres. The work was stimulated, in part, by an atmospheric particle \u27sample of opportunity\u27 collected at Summit, Greenland in August 1994, that bore the 14C imprint of biomass burning. During the summer field seasons of 1995 and 1996, we collected air filter, surface snow and snowpit samples to investigate chemical and isotopic evidence of combustion particles that had been transported from distant fires. Among the chemical tracers employed for source identification are organic acids, potassium and ammonium ions, and elemental and organic components of carbonaceous particles. Ion chromatography, performed by members of the Climate Change Research Center (University of New Hampshire), has been especially valuable in indicating periods at Summit that were likely to have been affected by the long range transport of biomass burning aerosol. Univariate and multivariate patterns of the ion concentrations in the snow and ice pinpointed surface and snowpit samples for the direct analysis of particulate (soot) carbon and carbon isotopes. The research at NIST is focusing on graphitic and polycyclic aromatic carbon, which serve as almost certain indicators of fire, and measurements of carbon isotopes, especially 14C, to distinguish fossil and biomass combustion sources. Complementing the chemical and isotopic record, are direct \u27visual\u27 (satellite imagery) records and less direct backtrajectory records, to indicate geographic source regions and transport paths. In this paper we illustrate the unique way in which the synthesis of the chemical, isotopic, satellite and trajectory data enhances our ability to develop the recent history of the formation and transport of soot deposited in the polar snow and ice
Temperature Driven Structural Phase Transition in Tetragonal-Like BiFeO3
Highly-strained BiFeO3 exhibits a "tetragonal-like, monoclinic" crystal
structure found only in epitaxial films (with an out-of-plane lattice parameter
exceeding the in-plane value by >20%). Previous work has shown that this phase
is properly described as a M monoclinic structure at room temperature
[with a (010) symmetry plane, which contains the ferroelectric
polarization]. Here we show detailed temperature-dependent x-ray diffraction
data that evidence a structural phase transition at ~100C to a high-temperature
M phase ["tetragonal-like" but with a (1-10) symmetry plane].
These results indicate that the ferroelectric properties and domain structures
of strained BiFeO will be strongly temperature dependent.Comment: 10 pages, 3 figure
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Wide-range strain tunability provided by epitaxial LaAl1−xScxO3 template films
The dielectric diamagnetic LaAl1− xScxO3 (LASO) (x=0–1) is proposed for adjusting of the biaxial in-plane lattice parameter of oxide substrates in the wide range from 3.79 to 4.05 Å (6.5%). This range includes the pseudocubic lattice parameters of most of the currently investigated complex oxides. The in-plane lattice parameter of strain-relaxed LASO films depends linearly on the composition, and these films grow with a smooth surface. On several different LASO-buffered substrates, ferromagnetic La0.7Sr0.3MnO3 (LSMO) films have been grown in predetermined strain states. A series of 30 nm thick LSMO films on LASO-buffered LaSrAlO4(001) demonstrates that continuously controlled coherent strains in a wide range, in this case from − 1 to +0.6%, can be obtained for the functional oxide films grown on LASO
Assessment of Non-traditional Isotopic Ratios by Mass Spectrometry for Analysis of Nuclear Activities. Annual Report 2011
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