276 research outputs found
High temperature susceptibility in electron doped Ca1-xYxMnO3: Double Exchange vs Superexchange
We present a study of the magnetic properties of the electron doped
manganites Ca1-xYxMnO3 (for 0<=x<=0.25) in the paramagnetic regime. For the
less doped samples (x<=0.1) the magnetic susceptibility, c(T), follows a
Curie-Weiss (CW) law only for T > 450 K and, below this temperature, c^-1(T)
shows a ferrimagnetic-like curvature. We approached the discussion of these
results in terms of a simple mean-field model where double exchange,
approximated by a ferromagnetic Heisenberg-like interaction between Mn3+ and
Mn4+ ions, competes with classical superexchange. For higher levels of doping
(x>=0.15), the CW behaviour is observed down to the magnetic ordering
temperature (Tmo) and a better description of c(T) was obtained by assuming
full delocalization of the eg electrons. In order to explore the degree of
delocalization as a function of T and x, we analyzed the problem through
Montecarlo simulations. Within this picture we found that at high T the
electrons doped are completely delocalized but, when Tmo is approached, they
form magnetic polarons of large spin that cause the observed curvature in
c^-1(T) for x<=0.1.Comment: 15 pages, 10 figures, Submitted to J. Physics: Condensed Matter
(06/28/02
How weather-proof is the construction sector? : empirical evidence from Germany
With the purpose to reduce winter unemployment and to promote all-season employment in the constructions sector, Germany maintains an extensive bad weather allowance system. Since the mid 1990s, these regulations have been subject to several reforms that resemble the range of approaches for employment promotion which can be found in other European countries. We analyse the effect of these reforms on individual unemployment risks using large individual administrative data merged with information about local weather conditions and the business cycle. We find a weaker direct link between seasonal layoffs and actual weather than broadly assumed, since most of the layoffs take place at fixed dates. The reforms under consideration have economically plausible effects; Regulations that limit an employer's financial burden reduce transitions to unemployment and render it less weather-dependent
Sailing into a dilemma : an economic and legal analysis of an EU trading scheme for maritime emissions
On the basis of a joint economic and legal analysis, we evaluate the effects of a “regional” (European)
emission trading scheme aiming at reducing emissions of international shipping. The focus lies on the
question which share of emissions from maritime transport activities to and from the EU can and
should be included in such a system. Our findings suggest that the attempt to implement an EU
maritime ETS runs into a dilemma. It is not possible to design a system that achieves emission
reductions in a cost efficient manner and is compatible with international law
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