242 research outputs found
Adapting Planck's route to investigate the thermodynamics of the spin-half pyrochlore Heisenberg antiferromagnet
The spin-half pyrochlore Heisenberg antiferromagnet (PHAF) is one of the most
challenging problems in the field of highly frustrated quantum magnetism.
Stimulated by the seminal paper of M.~Planck [M.~Planck, Verhandl. Dtsch. phys.
Ges. {\bf 2}, 202-204 (1900)] we calculate thermodynamic properties of this
model by interpolating between the low- and high-temperature behavior. For that
we follow ideas developed in detail by B.~Bernu and G.~Misguich and use for the
interpolation the entropy exploiting sum rules [the ``entropy method'' (EM)].
We complement the EM results for the specific heat, the entropy, and the
susceptibility by corresponding results obtained by the finite-temperature
Lanczos method (FTLM) for a finite lattice of sites as well as by the
high-temperature expansion (HTE) data. We find that due to pronounced
finite-size effects the FTLM data for are not representative for the
infinite system below . A similar restriction to
holds for the HTE designed for the infinite PHAF. By contrast, the EM provides
reliable data for the whole temperature region for the infinite PHAF. We find
evidence for a gapless spectrum leading to a power-law behavior of the specific
heat at low and for a single maximum in at . For the
susceptibility we find indications of a monotonous increase of
upon decreasing of reaching at . Moreover, the EM
allows to estimate the ground-state energy to .Comment: 17 pages, 24 figure
Enhanced magnetocaloric effect in frustrated magnetic molecules with icosahedral symmetry
We investigate the magnetocaloric properties of certain antiferromagnetic
spin systems that have already been or very likely can be synthesized as
magnetic molecules. It turns out that the special geometric frustration which
is present in antiferromagnets that consist of corner-sharing triangles leads
to an enhanced magnetocaloric effect with high cooling rates in the vicinity of
the saturation field. These findings are compared with the behavior of a simple
unfrustrated spin ring as well as with the properties of the icosahedron. To
our surprise, also for the icosahedron large cooling rates can be achieved but
due to a different kind of geometric frustration.Comment: 5 pages, 8 figures, more information at
http://obelix.physik.uni-osnabrueck.de/~schnack
Metamagnetic phase transition of the antiferromagnetic Heisenberg icosahedron
The observation of hysteresis effects in single molecule magnets like
Mn-acetate has initiated ideas of future applications in storage
technology. The appearance of a hysteresis loop in such compounds is an outcome
of their magnetic anisotropy. In this Letter we report that magnetic hysteresis
occurs in a spin system without any anisotropy, specifically, where spins
mounted on the vertices of an icosahedron are coupled by antiferromagnetic
isotropic nearest-neighbor Heisenberg interaction giving rise to geometric
frustration. At T=0 this system undergoes a first order metamagnetic phase
transition at a critical field \Bcrit between two distinct families of ground
state configurations. The metastable phase of the system is characterized by a
temperature and field dependent survival probability distribution.Comment: 4 pages, 4 figures, submitted to Physical Review Letter
Frustration effects in magnetic molecules
Besides being a fascinating class of new materials, magnetic molecules
provide the opportunity to study concepts of condensed matter physics in zero
dimensions. This contribution will exemplify the impact of molecular magnetism
on concepts of frustrated spin systems. We will discuss spin rings and the
unexpected rules that govern their low-energy behavior. Rotational bands, which
are experimentally observed in various molecular magnets, provide a useful,
simplified framework for characterizing the energy spectrum, but there are also
deviations thereof with far-reaching consequences. It will be shown that
localized independent magnons on certain frustrated spin systems lead to giant
magnetization jumps, a new macroscopic quantum effect. In addition a
frustration-induced metamagnetic phase transitions will be discussed, which
demonstrates that hysteresis can exist without anisotropy. Finally, it is
demonstrated that frustrated magnetic molecules could give rise to an enhanced
magnetocaloric effect.Comment: 6 pages, 4 figures; submitted to J. Low. Temp. Phys.; proceedings of
the Sixth International Symposium on Crystalline Organic Metals,
Superconductors, and Ferromagnets (ISCOM
Are taxonomy details of relevance to ecologists? An example from microcopepods of the Red Sea
The marine microcopepod family Oncaeidae in the Red Sea has been the subject of comprehensive ecological studies over the past 15 years, providing for the first time insights into their community structure, vertical distribution and feeding ecology. Owing to taxonomic problems in species identification, however, many of the earlier ecological results were based on provisionally named species or morphotypes. A recent, ongoing taxonomic study of Red Sea Oncaeidae resulted in a considerable increase in the estimated numbers of species, since many of the species had not been described before. The present paper focuses on the potential significance of an improved taxonomic resolution of oncaeids with respect to various ecological aspects in this area, such as indicator species, community analysis and vertical distribution. The progress in our knowledge of the diversity of Red Sea Oncaeidae is summarized, including latest findings on the taxonomy and zoogeography of very small species (<0.5 mm), and the importance of sibling species in the family is pointed out. The south–north gradient in species diversity of Oncaeidae within the Red Sea appears to be greater than previously assumed, since several of the newly described species were restricted to the southern part. The number of endemic species among Red Sea oncaeids is very low, however, most of the new species being also recorded outside the Red Sea. New quantitative data on the abundance and vertical distribution of selected oncaeid siblings obtained during a recent cruise in the northern Red Sea are provided to exemplify the changes in the knowledge of oncaeid community structure attributable to the improved taxonomic resolution. The potential ecological importance of a more differentiated consideration of oncaeid species in marine microcopepod communities is discusse
Frustration-induced exotic properties of magnetic molecules
Geometric frustration of interacting spin systems is the driving force of a
variety of fascinating phenomena in low-dimensional magnetism. In this
contribution I will review recent results on frustration-induced effects in
magnetic molecules, i.e. zero-dimensional magnetic systems, as well as in a
recently synthesized frustrated molecule-based spin tube, i.e. a
one-dimensional spin system.Comment: 5 pages, 9 eps figures; proceedings of the symposium on "Spin- and
charge-correlations in molecule-based materials", October 2005, Koenigstein
(Taunus), German
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