41 research outputs found
Two-magnon Raman scattering in insulating cuprates: Modifications of the effective Raman operator
Calculations of Raman scattering intensities in spin 1/2 square-lattice
Heisenberg model, using the Fleury-Loudon-Elliott theory, have so far been
unable to describe the broad line shape and asymmetry of the two magnon peak
found experimentally in the cuprate materials. Even more notably, the
polarization selection rules are violated with respect to the
Fleury-Loudon-Elliott theory. There is comparable scattering in and
geometries, whereas the theory would predict scattering in only
geometry. We review various suggestions for this discrepency and
suggest that at least part of the problem can be addressed by modifying the
effective Raman Hamiltonian, allowing for two-magnon states with arbitrary
total momentum. Such an approach based on the Sawatzsky-Lorenzana theory of
optical absorption assumes an important role of phonons as momentum sinks. It
leaves the low energy physics of the Heisenberg model unchanged but
substantially alters the Raman line-shape and selection rules, bringing the
results closer to experiments.Comment: 7 pages, 6 figures, revtex. Contains some minor revisions from
previous versio
The On-orbit Calibrations for the Fermi Large Area Telescope
The Large Area Telescope (LAT) on--board the Fermi Gamma ray Space Telescope
began its on--orbit operations on June 23, 2008. Calibrations, defined in a
generic sense, correspond to synchronization of trigger signals, optimization
of delays for latching data, determination of detector thresholds, gains and
responses, evaluation of the perimeter of the South Atlantic Anomaly (SAA),
measurements of live time, of absolute time, and internal and spacecraft
boresight alignments. Here we describe on orbit calibration results obtained
using known astrophysical sources, galactic cosmic rays, and charge injection
into the front-end electronics of each detector. Instrument response functions
will be described in a separate publication. This paper demonstrates the
stability of calibrations and describes minor changes observed since launch.
These results have been used to calibrate the LAT datasets to be publicly
released in August 2009.Comment: 60 pages, 34 figures, submitted to Astroparticle Physic
Combined Effect of pH and Sodium Chloride on the Heat-induced Aggregation of Whole Egg Proteins
Assessing trend and variation of Arctic sea-ice extent during 1979â2012 from a latitude perspective of ice edge
Arctic sea-ice extent (in summer) has been shrinking since the 1970s. However, we have little knowledge of the detailed spatial variability of this shrinking. In this study, we examine the (latitudinal) ice extent along each degree of longitude, using the monthly Arctic ice index data sets (1979â2012) from the National Snow and Ice Data Center. Statistical analysis suggests that: (1) for summer months (JulyâOctober), there was a 34-year declining trend in sea-ice extent at most regions, except for the Canadian Arctic Archipelago, Greenland and Svalbard, with retreat rates of 0.0562â0.0898 latitude degree/year (or 6.26â10.00 km/year, at a significance level of 0.05); (2) for sea ice not geographically muted by the continental coastline in winter months (JanuaryâApril), there was a declining trend of 0.0216â0.0559 latitude degree/year (2.40â6.22 km/year, at a significance level of 0.05). Regionally, the most evident sea-ice decline occurred in the Chukchi Sea from August to October, Baffin Bay and Greenland Sea from January to May, Barents Sea in most months, Kara Sea from July to August and Laptev Sea and eastern Siberian Sea in August and September. Trend analysis also indicates that: (1) the decline in summer ice extent became significant (at a 0.05 significance level) since 1999 and (2) winter ice extent showed a clear changing point (decline) around 2000, becoming statistically significant around 2005. The PacificâSiberian sector of the Arctic accounted for most of the summer sea-ice decline, while the winter recovery of sea ice in the Atlantic sector tended to decrease
Achieving fast high-resolution 3D imaging by combining synchrotron x-ray microCT, advanced algorithms, and high performance data management
Synchrotrons like the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory (LBNL) are an extremely bright source of X-rays. In recent years, this brightness has been coupled to large increases in detector speeds (including CMOS and sCMOS detectors) to enable microCT 3D imaging at unprecedented speeds and resolutions. The micro-CT Beamline at the ALS has been used by geologi