7,667 research outputs found
Origin of transition metal clustering tendencies in GaAs based dilute magnetic semiconductors
While isovalent doping of GaAs (e.g. by In) leads to a repulsion between the
solute atoms, two Cr, Mn, or Fe atoms in GaAs are found to have lower energy
than the well-separated pair, and hence attract each other. The strong bonding
interaction between levels with t2 symmetry on the transition metal (TM) atoms
results in these atoms exhibiting a strong tendency to cluster. Using
first-principles calculations, we show that this attraction is maximal for Cr,
Mn and Fe while it is minimal for V. The difference is attributed to the
symmetry of the highest occupied levels. While the intention is to find
possible choices of spintronic materials that show a reduced tendency to
cluster, one finds that the conditions that minimize clustering tendencies also
minimize the stabilization of the magnetic state.Comment: To appear in Appl. Phys. Let
Generalization of color by chickens: experimental observations and a Bayesian model
Sensory generalization influences animals' responses to novel stimuli. Because color forms a perceptual continuum, it is a good subject for studying generalization. Moreover, because different causes of variation in spectral signals, such as pigmentation, gloss, and illumination, have differing behavioral significance, it may be beneficial to have adaptable generalization. We report on generalization by poultry chicks following differential training to rewarded (T+) and unrewarded (Tâ) colors, in particular on the phenomenon of peak shift, which leads to subjects preferring stimuli displaced away from Tâ. The first three experiments test effects of learning either a fine or a coarse discrimination. In experiments 1 and 2, peak shift occurs, but contrary to some predictions, the shift is smaller after the animal learned a fine discrimination than after it learned a coarse discrimination. Experiment 3 finds a similar effect for generalization on a color axis orthogonal to that separating T+ from Tâ. Experiment 4 shows that generalization is rapidly modified by experience. These results imply that the scale of a âperceptual rulerâ is set by experience. We show that the observations are consistent with generalization following principles of Bayesian inference, which forms a powerful framework for understanding this type of behavior
Characterization of the known T type dwarfs towards the Sigma Orionis cluster
(Abridged) A total of three T type candidates (SOri70, SOri73, and
SOriJ0538-0213) lying in the line of sight towards Sigma Orionis were
characterized by means of near-infrared photometric, astrometric, and
spectroscopic studies. H-band methane images were collected for all three
sources and an additional sample of 15 field T type dwarfs using LIRIS/WHT.
J-band spectra of resolution of ~500 were obtained for SOriJ0538-0213 with
ISAAC/VLT, and JH spectra of resolution of ~50 acquired with WFC3/HST were
employed for the spectroscopic classification of SOri70 and 73. Proper motions
with a typical uncertainty of +/-3 mas/yr and a time interval of ~7-9 yr were
derived. Using the LIRIS observations of the field T dwarfs, we calibrated this
imager for T spectral typing via methane photometry. The three SOri objects
were spectroscopically classified as T4.5+/-0.5 (SOri73), T5+/-0.5
(SOriJ0538-0213), and T7 (SOri70). The similarity between the
observed JH spectra and the methane colors and the data of field ultra-cool
dwarfs of related classifications suggests that SOri70, 73, and
SOriJ053804.65-021352.5 do not deviate significantly in surface gravity in
relation to the field. Additionally, the detection of KI at ~1.25 microns in
SOriJ0538-0213 points to a high-gravity atmosphere. Only the K-band reddish
nature of SOri70 may be consistent with a low gravity atmosphere. The proper
motions of SOri70 and 73 are measurable and are larger than that of the cluster
by >3.5 sigma. The proper motion of SOriJ0538-0213 is consistent with a null
displacement. These observations suggest that none of the three T dwarfs are
likely Sigma Orionis members, and that either planetary-mass objects with
masses below ~4 MJup may not exist free-floating in the cluster or they may lie
at fainter near-infrared magnitudes than those of the targets (this is H>20.6
mag), thus remaining unidentified to date.Comment: Accepted for publication in A&A (2014), corrected typo
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