8,880 research outputs found
Magneto-optics in pure and defective Ga_{1-x}Mn_xAs from first-principles
The magneto-optical properties of GaMnAs including their most
common defects were investigated with precise first--principles
density-functional FLAPW calculations in order to: {\em i}) elucidate the
origin of the features in the Kerr spectra in terms of the underlying
electronic structure; {\em ii}) perform an accurate comparison with
experiments; and {\em iii}) understand the role of the Mn concentration and
occupied sites in shaping the spectra. In the substitutional case, our results
show that most of the features have an interband origin and are only slightly
affected by Drude--like contributions, even at low photon energies. While not
strongly affected by the Mn concentration for the intermediately diluted range
( 10%), the Kerr factor shows a marked minimum (up to 1.5) occurring
at a photon energy of 0.5 eV. For interstitial Mn, the calculated
results bear a striking resemblance to the experimental spectra, pointing to
the comparison between simulated and experimental Kerr angles as a valid tool
to distinguish different defects in the diluted magnetic semiconductors
framework.Comment: 10 pages including 2 figures, submitted to Phys. Rev.
Letter between E. S. Jones and W. J. Kerr
Letters concerning a position in botany at Utah Agricultural College
Emergence of the Shackleton Range from beneath the Antarctic Ice Sheet due to glacial erosion
This paper explores the long-term evolution of a subglacial fjord landscape in the Shackleton Range, Antarctica. We propose that prolonged ice-sheet erosion across a passive continental margin caused troughs to deepen and lower the surrounding ice-sheet surface, leaving adjacent mountains exposed. Geomorphological evidence suggests a change in the direction of regional ice flow accompanied emergence. Simple calculations suggest that isostatic compensation caused by the deepening of bounding ice-stream troughs lowered the ice-sheet surface relative to the mountains by ~800m. Use of multiple cosmogenic isotopes on bedrock and erratics (26Al, 10Be, 21Ne) provides evidence that overriding of the massif and the deepening of the adjacent troughs occurred earlier than the Quaternary. Perhaps this occurred in the mid-Miocene, as elsewhere in East Antarctica in the McMurdo Dry Valleys and the Lambert basin. The implication is that glacial erosion instigates feedback that can change ice-sheet thickness, extent, and direction of flow. Indeed, as the subglacial troughs evolve over millions of years, they increase topographic relief; and this changes the dynamics of the ice sheet. © 2013 Elsevier B.V
The energy budget in Rayleigh-Benard convection
It is shown using three series of Rayleigh number simulations of varying
aspect ratio AR and Prandtl number Pr that the normalized dissipation at the
wall, while significantly greater than 1, approaches a constant dependent upon
AR and Pr. It is also found that the peak velocity, not the mean square
velocity, obeys the experimental scaling of Ra^{0.5}. The scaling of the mean
square velocity is closer to Ra^{0.46}, which is shown to be consistent with
experimental measurements and the numerical results for the scaling of Nu and
the temperature if there are strong correlations between the velocity and
temperature.Comment: 5 pages, 3 figures, new version 13 Mar, 200
Discovery of Gamma-ray Pulsations from the Transitional Redback PSR J1227-4853
The 1.69 ms spin period of PSR J1227-4853 was recently discovered in radio
observations of the low-mass X-ray binary XSS J12270-4859 following the
announcement of a possible transition to a rotation-powered millisecond pulsar
state, inferred from decreases in optical, X-ray, and gamma-ray flux from the
source. We report the detection of significant (5) gamma-ray pulsations
after the transition, at the known spin period, using ~1 year of data from the
Large Area Telescope on board the Fermi Gamma-ray Space Telescope. The
gamma-ray light curve of PSR J1227-4853 can be fit by one broad peak, which
occurs at nearly the same phase as the main peak in the 1.4 GHz radio profile.
The partial alignment of light-curve peaks in different wavebands suggests that
at least some of the radio emission may originate at high altitude in the
pulsar magnetosphere, in extended regions co-located with the gamma-ray
emission site. We folded the LAT data at the orbital period, both pre- and
post-transition, but find no evidence for significant modulation of the
gamma-ray flux. Analysis of the gamma-ray flux over the mission suggests an
approximate transition time of 2012 November 30. Continued study of the pulsed
emission and monitoring of PSR J1227-4853, and other known redback systems, for
subsequent flux changes will increase our knowledge of the pulsar emission
mechanism and transitioning systems.Comment: 5 figures, 1 table, accepted for publication in ApJ, updated to
reflect accepted version and add additional coautho
The latitudinal distribution of ozone to 35 km altitude from ECC ozonesonde observations, 1982-1990
Electrochemical concentration cell (ECC) ozone-sonde observations, made in recent years at ten stations whose locations range from the Arctic to Antarctica, have yielded a self-consistent ozone data base from which mean seasonal and annual latitudinal ozone vertical distributions to 35 km have been derived. Ozone measurement uncertainties are estimated, and results are presented in the Bass-Paur (1985) ozone absorption coefficient scale adopted for use with Dobson ozone spectrophotometers January 1, 1992. The data should be useful for comparison with model calculations of the global distribution of atmospheric ozone, for serving as apriori statistical information in deriving ozone vertical distributions from satellite and Umkehr observations, and for improving the satellite and Umkehr ozone inversion algorithms. Attention is drawn to similar results based on a less comprehensive data set published in Ozone in the Atmosphere, Proceedings of the 1988 Quadrennial Ozone Symposium where errors in data tabulations occurred for three of the stations due to inadvertent transposition of ozone partial pressure and air temperature values
Quasilocal Energy for a Kerr black hole
The quasilocal energy associated with a constant stationary time slice of the
Kerr spacetime is presented. The calculations are based on a recent proposal
\cite{by} in which quasilocal energy is derived from the Hamiltonian of
spatially bounded gravitational systems. Three different classes of boundary
surfaces for the Kerr slice are considered (constant radius surfaces, round
spheres, and the ergosurface). Their embeddings in both the Kerr slice and flat
three-dimensional space (required as a normalization of the energy) are
analyzed. The energy contained within each surface is explicitly calculated in
the slow rotation regime and its properties discussed in detail. The energy is
a positive, monotonically decreasing function of the boundary surface radius.
It approaches the Arnowitt-Deser-Misner (ADM) mass at spatial infinity and
reduces to (twice) the irreducible mass at the horizon of the Kerr black hole.
The expressions possess the correct static limit and include negative
contributions due to gravitational binding. The energy at the ergosurface is
compared with the energies at other surfaces. Finally, the difficulties
involved in an estimation of the energy in the fast rotation regime are
discussed.Comment: 22 pages, Revtex, Alberta-Thy-18-94. (the approximations in Section
IV have been improved. To appear in Phys. Rev. D
Nonnegatively curved homogeneous metrics obtained by scaling fibers of submersions
We consider invariant Riemannian metrics on compact homogeneous spaces G/H
where an intermediate subgroup K between G and H exists, so that the
homogeneous space G/H is the total space of a Riemannian submersion. We study
the question as to whether enlarging the fibers of the submersion by a constant
scaling factor retains the nonnegative curvature in the case that the
deformation starts at a normal homogeneous metric. We classify triples of
groups (H,K,G) where nonnegative curvature is maintained for small
deformations, using a criterion proved by Schwachh\"ofer and Tapp. We obtain a
complete classification in case the subgroup H has full rank and an almost
complete classification in the case of regular subgroups.Comment: 23 pages; minor revisions, to appear in Geometriae Dedicat
JHK Observations of Faint Standard Stars in the Mauna Kea Near-Infrared Photometric System
JHK photometry in the Mauna Kea Observatory (MKO) near-IR system is presented
for 115 stars. Of these, 79 are UKIRT standards and 42 are LCO standards. The
average brightness is 11.5 mag, with a range of 10 to 15. The average number of
nights each star was observed is 4, and the average of the internal error of
the final results is 0.011 mag. These JHK data agree with those reported by
other groups to 0.02 mag. The measurements are used to derive transformations
between the MKO JHK photometric system and the UKIRT, LCO and 2MASS systems.
The 2MASS-MKO data scatter by 0.05 mag for redder stars: 2MASS-J includes H2O
features in dwarfs and MKO-K includes CO features in giants. Transformations
derived for stars whose spectra contain only weak features cannot give accurate
transformations for objects with strong absorption features within a filter
bandpasses. We find evidence of systematic effects at the 0.02 mag level in the
photometry of stars with J<11 and H,K<10.5. This is due to an underestimate of
the linearity correction for stars observed with the shortest exposure times;
very accurate photometry of stars approaching the saturation limits of infrared
detectors which are operated in double-read mode is difficult to obtain. Four
stars in the sample, GSPC S705-D, FS 116 (B216-b7), FS 144 (Ser-EC84) and FS 32
(Feige 108), may be variable. 84 stars in the sample have 11< J< 15 and
10.5<H,K<15, are not suspected to be variable, and have magnitudes with an
estimated error <0.027 mag; 79 of these have an error of <0.020 mag. These
represent the first published high-accuracy JHK stellar photometry in the MKO
photometric system; we recommend these objects be employed as primary standards
for that system [abridged].Comment: Accepted for publication in MNRAS, 14 pages, 5 Figure
The Proper Motion of SgrA*: I. First VLBA Results
We observed Sgr A* and two extragalactic radio sources nearby in angle with
the VLBA over a period of two years and measured relative positions with an
accuracy approaching 0.1 mas. The apparent proper motion of Sgr A* relative to
J1745-283 is 5.90 +/- 0.4 mas/yr, almost entirely in the plane of the Galaxy.
The effects of the orbit of the Sun around the Galactic Center can account for
this motion, and any residual proper motion of Sgr A*, with respect to
extragalactic sources, is less than about 20 km/s. Assuming that Sgr A* is at
rest at the center of the Galaxy, we estimate that the circular rotation speed
in the Galaxy at the position of the Sun is 219 +/- 20 km/s, scaled by Ro/8.0
kpc.
Current observations are consistent with Sgr A* containing all of the nearly
2.6 x 10^6 solar masses, deduced from stellar proper motions, in the form of a
massive black hole. While the low luminosity of Sgr A*, for example, might
possibly have come from a contact binary containing of order 10 solar masses,
the lack of substantial motion rules out a "stellar" origin for Sgr A*. The
very slow speed of Sgr A* yields a lower limit to the mass of Sgr A* of about
1,000 solar masses. Even for this mass, Sgr A* appears to be radiating at less
than 0.1 percent of its Eddington limit
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