6,811 research outputs found
The self-trapped hole in caesium halides
The equilibrium lattice configuration, electronic excitation energies and activation energies for hopping motion are calculated for a self-trapped hole in simple cubic CsCl, CsBr and CsI. The defect is regarded as a X2- molecular ion (X=Cl, Br, I) whose bond-length has been modified by the crystalline environment. Agreement with the experimental ultraviolet transition energies is good. Excitation energies deduced from measurement of g-shifts in CsBr and CsI are too low, a feature common to all alkali bromides and iodides, and attributed to the approximations involved in their deviation. The initial calculations predict lower activation energies of 90 degrees jumps than for 180 degrees jumps, in contrast with what is observed in CsI. An alternative model is presented, which reproduces the correct trend. Comparison of the actual numbers with experiment is hampered by the fact that the latter are done at low temperature (60-90K), the calculations being done in the high-temperature limit
ELECTRONIC STRUCTURE AND LUMINESCENCE OF CSI:NA
Calculations are performed on several aspects of the luminescence of pure CsI and CsI:Na. These include electronic-structure calculations by both pseudopotential and semi-empirical molecular-orbital methods, as well as lattice-configuration studies. The results suggest that the main observed emission in CsI:Na at 2.95 eV involves the recombination of a self-trapped exciton immediately adjacent to the substitutional Na impurity
Thermoelectricity of EuCu{2}(Ge{1-x}Si{x}){2} intermetallics
The evolution of the thermopower EuCu{2}(Ge{1-x}Si{x}){2} intermetallics,
which is induced by the Si-Ge substitution, is explained by the Kondo
scattering of conduction electrons on the Eu ions which fluctuate between the
magnetic 2+ and non-magnetic 3+ Hund's rule configurations. The Si-Ge
substitution is equivalent to chemical pressure which modifies the coupling and
the relative occupation of the {\it f} and conduction states.Comment: 2 pages, Proceedings of the SCES 2005 confernece. Physica B (2006),
in pres
Mid-infrared size survey of Young Stellar Objects: Description of Keck segment-tilting experiment and basic results
The mid-infrared properties of pre-planetary disks are sensitive to the
temperature and flaring profiles of disks for the regions where planet
formation is expected to occur. In order to constrain theories of planet
formation, we have carried out a mid-infrared (wavelength 10.7 microns) size
survey of young stellar objects using the segmented Keck telescope in a novel
configuration. We introduced a customized pattern of tilts to individual mirror
segments to allow efficient sparse-aperture interferometry, allowing full
aperture synthesis imaging with higher calibration precision than traditional
imaging. In contrast to previous surveys on smaller telescopes and with poorer
calibration precision, we find most objects in our sample are partially
resolved. Here we present the main observational results of our survey of 5
embedded massive protostars, 25 Herbig Ae/Be stars, 3 T Tauri stars, 1 FU Ori
system, and 5 emission-line objects of uncertain classification. The observed
mid-infrared sizes do not obey the size-luminosity relation found at
near-infrared wavelengths and a companion paper will provide further modelling
analysis of this sample. In addition, we report imaging results for a few of
the most resolved objects, including complex emission around embedded massive
protostars, the photoevaporating circumbinary disk around MWC 361A, and the
subarcsecond binaries T Tau, FU Ori and MWC 1080.Comment: Accepted by Astrophysical Journal. 38 pages. 9 figure
KOI-1003: A new spotted, eclipsing RS CVn binary in the Kepler field
Using the high-precision photometry from the Kepler space telescope,
thousands of stars with stellar and planetary companions have been observed.
The characterization of stars with companions is not always straightforward and
can be contaminated by systematic and stellar influences on the light curves.
Here, through a detailed analysis of starspots and eclipses, we identify
KOI-1003 as a new, active RS CVn star---the first identified with data from
Kepler. The Kepler light curve of this close binary system exhibits the
system's primary transit, secondary eclipse, and starspot evolution of two
persistent active longitudes. The near equality of the system's orbital and
rotation periods indicates the orbit and primary star's rotation are nearly
synchronized ( days; days). By assuming the secondary star is on the main sequence, we suggest
the system consists of a subgiant primary and
a main-sequence companion. Our work gives a
distance of pc and an age of Gyr,
parameters which are discrepant with previous studies that included the star as
a member of the open cluster NGC 6791.Comment: 21 pages, 19 figures, accepted to Ap
Strong Near-Infrared Emission Interior to the Dust-Sublimation Radius of Young Stellar Objects MWC275 and AB Aur
Using the longest optical-interferometeric baselines currently available, we
have detected strong near-infrared (NIR) emission from inside the
dust-destruction radius of Herbig Ae stars MWC275 and AB Aur. Our
sub-milli-arcsecond resolution observations unambiguously place the emission
between the dust-destruction radius and the magnetospheric co-rotation radius.
We argue that this new component corresponds to hot gas inside the
dust-sublimation radius, confirming recent claims based on spectrally-resolved
interferometry and dust evaporation front modeling.Comment: 12 pages, 4 figures, Accepted for publication in ApJ
Born Again Protoplanetary Disk Around Mira B
The Mira AB system is a nearby (~107 pc) example of a wind accreting binary
star system. In this class of system, the wind from a mass-losing red giant
star (Mira A) is accreted onto a companion (Mira B), as indicated by an
accretion shock signature in spectra at ultraviolet and X-ray wavelengths.
Using novel imaging techniques, we report the detection of emission at
mid-infrared wavelengths between 9.7 and 18.3 m from the vicinity of Mira
B but with a peak at a radial position about 10 AU closer to the primary Mira
A. We interpret the mid-infrared emission as the edge of an optically-thick
accretion disk heated by Mira A. The discovery of this new class of accretion
disk fed by M-giant mass loss implies a potential population of young planetary
systems in white-dwarf binaries which has been little explored, despite being
relatively common in the solar neighborhood.Comment: Accepted for Ap
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