Experimental determination of the state-dependent enhancement of the electron-positron momentum density in solids

The state-dependence of the enhancement of the electron-positron momentum density is investigated for some transition and simple metals (Cr, V, Ag and Al). Quantitative comparison with linearized muffin-tin orbital calculations of the corresponding quantity in the first Brillouin zone is shown to yield a measurement of the enhancement of the s, p and d states, independent of any parameterizations in terms of the electron density local to the positron. An empirical correction that can be applied to a first-principles state-dependent model is proposed that reproduces the measured state-dependence very well, yielding a general, predictive model for the enhancement of the momentum distribution of positron annihilation measurements, including those of angular correlation and coincidence Doppler broadening techniques

Fermi surface of the colossal magnetoresistance perovskite La_{0.7}Sr_{0.3}MnO_{3}

Materials that exhibit colossal magnetoresistance (CMR) are currently the focus of an intense research effort, driven by the technological applications that their sensitivity lends them to. Using the angular correlation of photons from electron-positron annihilation, we present a first glimpse of the Fermi surface of a material that exhibits CMR, supported by ``virtual crystal'' electronic structure calculations. The Fermi surface is shown to be sufficiently cubic in nature that it is likely to support nesting.Comment: 5 pages, 5 PS figure

Fermi Surface as the Driving Mechanism for Helical Antiferromagnetic Ordering in Gd-Y Alloys

The first direct experimental evidence for the Fermi surface (FS) driving the helical antiferromagnetic ordering in a gadolinium-yttrium alloy is reported. The presence of a FS sheet capable of nesting is revealed, and the nesting vector associated with the sheet is found to be in excellent agreement with the periodicity of the helical ordering.Comment: 4 pages, 4 figure

The electronic structure of {\em R}NiC2_2 intermetallic compounds

First-principles calculations of the electronic structure of members of the $R$NiC$_2$ series are presented, and their Fermi surfaces investigated for nesting propensities which might be linked to the charge-density waves exhibited by certain members of the series ($R$ = Sm, Gd and Nd). Calculations of the generalized susceptibility, $\chi_{0}({\bf q},\omega)$, show strong peaks at the same ${\bf q}$-vector in both the real and imaginary parts for these compounds. Moreover, this peak occurs at a wavevector which is very close to that experimentally observed in SmNiC$_2$. In contrast, for LaNiC$_2$ (which is a superconductor below 2.7K) as well as for ferromagnetic SmNiC$_2$, there is no such sharp peak. This could explain the absence of a charge-density wave transition in the former, and the destruction of the charge-density wave that has been observed to accompany the onset of ferromagnetic order in the latter.Comment: 5 pages, 7 figures. Accepted for publication in Phys. Rev.

Rainfall is associated with social behaviour in Seychelles warblers

Species are facing environmental challenges caused by rapidly changing environments induced by climate change. In tropical regions, climate change causes increasing frequencies of extreme weather,like droughts or extreme rainfall. Natural selection is usually slow and adaptations through phenotypic plasticity are limited, thus species might need more mechanisms to cope with change, like cooperative breeding. Rainfall is an important cue for onset of breeding in many tropical bird species, so young are born when food abundance is highest. We explore the effect of rainfall on the social behaviour and life history of the insectivorous Seychelles warblers (Acrocephalus sechellensis), a facultative cooperative58breeder, using long-term data. We expect that with increasing amounts of rain food resources will increase, and thus reproductive output and survival will increase. If so, in dry periods, reproductive output of breeding pairs will be limited, but cooperative breeding may counteract this negative effect because helpers assist with feeding young. We expect that in dry periods breeding pairs produce more offspring of the helping sex, who stay in their natal territory and become helpers in the following year. Rainfall positively affected insect abundance. With increasing rainfall, survival of individuals during the breeding season and reproductive output, especially for cooperatively breeding groups, increased. With more rainfall, also more daughters (the helping sex) were produced, resulting in more group formation the year after. Thus, in contrast to our expectations, Seychelles warblers form social groups in favourable conditions, indicating that group living may be costly and thus not a solution to coping with changing environments. This study showed that the interaction between the environment and life histories,including social behaviour, is complex, and important to consider when studying the impact of changing environments on species surviva