9,617 research outputs found
K+ -> pi+ nu nu(bar) and FCNC from non-universal Z' bosons
Motivated by the E787 and E949 result for K+ -> pi+ nu nu(bar) we examine the
effects of a new non-universal right-handed Z' boson on flavor changing
processes. We place bounds on the tree-level FCNC from K-K(bar) and B-B(bar)
mixing as well as from the observed CP violation in kaon decay. We discuss the
implications for K -> pi nu nu(bar), B -> X nu nu(bar) and B -> tau+ tau-. We
find that existing bounds allow substantial enhancements in the K+ -> pi+ nu
nu(bar) rate, particularly through a new one-loop Z' penguin operator.Comment: Typos corrected, references added, version to appear in PR
Dual Behavior of Antiferromagnetic Uncompensated Spins in NiFe/IrMn Exchange Biased Bilayers
We present a comprehensive study of the exchange bias effect in a model
system. Through numerical analysis of the exchange bias and coercive fields as
a function of the antiferromagnetic layer thickness we deduce the absolute
value of the averaged anisotropy constant of the antiferromagnet. We show that
the anisotropy of IrMn exhibits a finite size effect as a function of
thickness. The interfacial spin disorder involved in the data analysis is
further supported by the observation of the dual behavior of the interfacial
uncompensated spins. Utilizing soft x-ray resonant magnetic reflectometry we
have observed that the antiferromagnetic uncompensated spins are dominantly
frozen with nearly no rotating spins due to the chemical intermixing, which
correlates to the inferred mechanism for the exchange bias.Comment: 4 pages, 3 figure
Neighborhood and community interactions determine the spatial pattern of tropical tree seedling survival
Factors affecting survival and recruitment of 3531 individually mapped seedlings of Myristicaceae were examined over three years in a highly diverse neotropical rain forest, at spatial scales of 1–9 m and 25 ha. We found convincing evidence of a community compensatory trend (CCT) in seedling survival (i.e., more abundant species had higher seedling mortality at the 25-ha scale), which suggests that density-dependent mortality may contribute to the spatial dynamics of seedling recruitment. Unlike previous studies, we demonstrate that the CCT was not caused by differences in microhabitat preferences or life history strategy among the study species. In local neighborhood analyses, the spatial autocorrelation of seedling survival was important at small spatial scales (1–5 m) but decayed rapidly with increasing distance. Relative seedling height had the greatest effect on seedling survival. Conspecific seedling density had a more negative effect on survival than heterospecific seedling density and was stronger and extended farther in rare species than in common species. Taken together, the CCT and neighborhood analyses suggest that seedling mortality is coupled more strongly to the landscape-scale abundance of conspecific large trees in common species and the local density of conspecific seedlings in rare species. We conclude that negative density dependence could promote species coexistence in this rain forest community but that the scale dependence of interactions differs between rare and common species
Mn valence instability in La2/3Ca1/3MnO3 thin films
A Mn valence instability on La2/3Ca1/3MnO3 thin films, grown on LaAlO3
(001)substrates is observed by x-ray absorption spectroscopy at the Mn L-edge
and O K-edge. As-grown samples, in situ annealed at 800 C in oxygen, exhibit a
Curie temperature well below that of the bulk material. Upon air exposure a
reduction of the saturation magnetization, MS, of the films is detected.
Simultaneously a Mn2+ spectral signature develops, in addition to the expected
Mn3+ and Mn4+ contributions, which increases with time. The similarity of the
spectral results obtained by total electron yield and fluorescence yield
spectroscopy indicates that the location of the Mn valence anomalies is not
confined to a narrow surface region of the film, but can extend throughout the
whole thickness of the sample. High temperature annealing at 1000 C in air,
immediately after growth, improves the magnetic and transport properties of
such films towards the bulk values and the Mn2+ signature in the spectra does
not appear. The Mn valence is then stable even to prolonged air exposure. We
propose a mechanism for the Mn2+ ions formation and discuss the importance of
these observations with respect to previous findings and production of thin
films devices.Comment: Double space, 21 pages, 6 figure
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