536 research outputs found
Revisiting the chain magnetism in Sr14Cu24O41: Experimental and numerical results
We study the magnetism of the hole doped CuO2 spin chains in Sr14Cu24O41 by
measuring the Electron Spin Resonance (ESR) and the static magnetization M in
applied magnetic fields up to 14 T. In this compound, the dimerized ground
state and the charge order in the chains are well established. Our experimental
data suggest that at low temperatures the Curie-like increase of M as well as
the occurrence of the related ESR signal are due to a small amount of
paramagnetic centers which are not extrinsic defects but rather unpaired Cu
spins in the chain. These observations qualitatively confirm recent ab initio
calculations of the ground state properties of the CuO2 chains in Sr14Cu24O41.
Our complementary quantum statistical simulations yield that the temperature
and field dependence of the magnetization can be well described by an effective
Heisenberg model in which the ground state configuration is composed of spin
dimers, trimers, and monomers.Comment: revised versio
Phase diagram of the alternating-spin Heisenberg chain with extra isotropic three-body exchange interactions
For the time being isotropic three-body exchange interactions are scarcely
explored and mostly used as a tool for constructing various exactly solvable
one-dimensional models, although, generally speaking, such competing terms in
generic Heisenberg spin systems can be expected to support specific quantum
effects and phases. The Heisenberg chain constructed from alternating S=1 and
sigma=1/2 site spins defines a realistic prototype model admitting extra
three-body exchange terms. Based on numerical density-matrix renormalization
group (DMRG) and exact diagonalization (ED) calculations, we demonstrate that
the additional isotropic three-body terms stabilize a variety of
partially-polarized states as well as two specific non-magnetic states
including a critical spin-liquid phase controlled by two Gaussinal conformal
theories as well as a critical nematic-like phase characterized by dominant
quadrupolar S-spin fluctuations. Most of the established effects are related to
some specific features of the three-body interaction such as the promotion of
local collinear spin configurations and the enhanced tendency towards
nearest-neighbor clustering of the spins. It may be expected that most of the
predicted effects of the isotropic three-body interaction persist in higher
space dimensions.Comment: 13 pages, 17 figures, submitte
Taxonomy of Oncaeidae (Copepoda, Poecilostomatoida) from the Red Sea. - IX. Epicalymma bulbosa sp. nov., first record of the genus from the Red Sea
The oncaeid genus Epicalymma comprises small copepod species usually living at meso- and bathypelagic depth layers in oceanic areas. The genus had previously been assumed to be absent from the Red Sea, due to the unusually high deep-sea temperatures and salinities in this area. In the present account a new species, Epicalymma bulbosa, is described from the Red Sea, which appears to be the only representative of the genus in the region. The new species is the smallest Epicalymma species so far recorded, with a total body length of ∼0.32 and ∼0.29 mm in the female and male, respectively. Apart from its small size, it differs from all known Epicalymma species by an extremely long exopodal seta on P5 in both sexes, and by a free exopod segment of P5 and a very long and basally swollen spinule on the syncoxa of the maxilliped in the female. In contrast to other Epicalymma species, which are distributed between 500 and >2500 m depth, the new species occurred much shallower (100–750 m) in the Red Sea, which may be interpretated as an avoidance mechanism of the unfavourable environmental conditions in the deep Red Sea. The taxonomic status of the new species within the genus Epicalymma is discussed and the few available ecological data on Epicalymma species in the world ocean are summarized
Revisiting and modeling the magnetism of hole-doped CuO_2 spin chains in Sr{14-x}Ca_xCu_{24}O_{41}
Magnetization measurements of Sr{14-x}Ca_xCu_{24}O_{41} with 0 <= x <=12 in
magnetic fields up to 16 T show that the low-temperature magnetic response of
the CuO_2 spin chains changes strongly upon Ca doping. For x=0 quantum
statistical simulations yield that the temperature and field dependence of the
magnetization can be well described by an effective Heisenberg model in which
the ground state configuration is composed of spin dimers, trimers, and
monomers. For x>0 a constant contribution to the low-temperature magnetic
susceptibility is observed which cannot be explained in terms of simple chain
models. Alternative scenarios are outlined.Comment: 2 pages, submitted to the proceedings of the ICM, Kyoto, Japan,
August 200
Application of the finite-temperature Lanczos method for the evaluation of magnetocaloric properties of large magnetic molecules
We discuss the magnetocaloric properties of gadolinium containing magnetic
molecules which potentially could be used for sub-Kelvin cooling. We show that
a degeneracy of a singlet ground state could be advantageous in order to
support adiabatic processes to low temperatures and simultaneously minimize
disturbing dipolar interactions. Since the Hilbert spaces of such spin systems
assume very large dimensions we evaluate the necessary thermodynamic
observables by means of the Finite-Temperature Lanczos Method.Comment: 7 pages, 10 figures, invited for the special issue of EPJB on "New
trends in magnetism and magnetic materials
Taxonomic diversity and identification problems of oncaeid microcopepods in the Mediterranean Sea
The species diversity of the pelagic microcopepod
family Oncaeidae collected with nets of 0.1-mm mesh
size was studied at 6 stations along a west-to-east transect
in the Mediterranean Sea down to a maximum depth of
1,000 m. A total of 27 species and two form variants have
been identified, including three new records for the
Mediterranean. In addition, about 20, as yet undescribed,
new morphospecies were found (mainly from the genera
Epicalymma and Triconia) which need to be examined
further. The total number of identified oncaeid species was
similar in the Western and Eastern Basins, but for some cooccurring
sibling species, the estimated numerical dominance
changed. The deep-sea fauna of Oncaeidae, studied
at selected depth layers between 400 m and the near-bottom
layer at >4,200 m depth in the eastern Mediterranean
(Levantine Sea), showed rather constant species numbers
down to ∼3,000 m depth. In the near-bottom layers, the
diversity of oncaeids declined and species of Epicalymma
strongly increased in numerical importance. The taxonomic
status of all oncaeid species recorded earlier in the
Mediterranean Sea is evaluated: 19 out of the 46 known
valid oncaeid species are insufficiently described, and most
of the taxonomically unresolved species (13 species) have
originally been described from this area (type locality). The
deficiencies in the species identification of oncaeids cast
into doubt the allegedly cosmopolitan distribution of some
species, in particular those of Mediterranean origin. The
existing identification problems even of well-described
oncaeid species are exemplified for the Oncaea mediacomplex,
including O. media Giesbrecht, O. scottodicarloi
Heron & Bradford-Grieve, and O. waldemari Bersano &
Boxshall, which are often erroneously identified as a single
species (O. media). The inadequacy in the species identification
of Oncaeidae, in particular those from the Atlantic
and Mediterranean, is mainly due to the lack of reliable
identification keys for Oncaeidae in warm-temperate and/or
tropical seas. Future efforts should be directed to the
construction of identification keys that can be updated
according to the latest taxonomic findings, which can be
used by the non-expert as well as by the specialist. The
adequate consideration of the numerous, as yet undescribed,
microcopepod species in the world oceans, in
particular the Oncaeidae, is a challenge for the study of the
structure and function of plankton communities as well as
for global biodiversity estimates
Heat capacity uncovers physics of a frustrated spin tube
We report on refined experimental results concerning the low-temperature
specific heat of the frustrated spin tube material [(CuCl2tachH)3Cl]Cl2. This
substance turns out to be an unusually perfect spin tube system which allows to
study the physics of quasi-one dimensional antiferromagnetic structures in
rather general terms. An analysis of the specific heat data demonstrates that
at low enough temperatures the system exhibits a Tomonaga-Luttinger liquid
behavior corresponding to an effective spin-3/2 antiferromagnetic Heisenberg
chain with short-range exchange interactions. On the other hand, at somewhat
elevated temperatures the composite spin structure of the chain is revealed
through a Schottky-type peak in the specific heat located around 2 K. We argue
that the dominating contribution to the peak originates from gapped magnon-type
excitations related to the internal degrees of freedom of the rung spins.Comment: 4+ pages, 6 figure
Rotational modes in molecular magnets with antiferromagnetic Heisenberg exchange
In an effort to understand the low temperature behavior of recently
synthesized molecular magnets we present numerical evidence for the existence
of a rotational band in systems of quantum spins interacting with
nearest-neighbor antiferromagnetic Heisenberg exchange. While this result has
previously been noted for ring arrays with an even number of spin sites, we
find that it also applies for rings with an odd number of sites as well as for
all of the polytope configurations we have investigated (tetrahedron, cube,
octahedron, icosahedron, triangular prism, and axially truncated icosahedron).
It is demonstrated how the rotational band levels can in many cases be
accurately predicted using the underlying sublattice structure of the spin
array. We illustrate how the characteristics of the rotational band can provide
valuable estimates for the low temperature magnetic susceptibility.Comment: 14 pages, 7 figures, to be published in Phys. Rev.
Наноалмазы как идеальные наноносители для циансодежащих цитостатиков
Цианосодержащие цитостатики - новый класс открытых нами лекарств, которые благодаря цианогруппам хорошо закрепляются на наноалмазах, с увеличением активности
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