253 research outputs found
Dynamical Structure Factors for Dimerized Spin Systems
We discuss the transition strength between the disordered ground state and
the basic low-lying triplet excitation for interacting dimer materials by
presenting theoretical calculations and series expansions as well as inelastic
neutron scattering results for the material KCuCl_3. We describe in detail the
features resulting from the presence of two differently oriented dimers per
unit cell and show how energies and spectral weights of the resulting two modes
are related to each other. We present results from the perturbation expansion
in the interdimer interaction strength and thus demonstrate that the wave
vector dependence of the simple dimer approximation is modified in higher
orders. Explicit results are given in 10th order for dimers coupled in 1D, and
in 2nd order for dimers coupled in 3D with application to KCuCl_3 and TlCuCl_3.Comment: 17 pages, 6 figures, part 2 is based on cond-mat/021133
Random Bond Effect in the Quantum Spin System (TlK)CuCl
The effect of exchange bond randomness on the ground state and the
field-induced magnetic ordering was investigated through magnetization
measurements in the spin-1/2 mixed quantum spin system
(TlK)CuCl for . Both parent compounds TlCuCl and
KCuCl are coupled spin dimer systems, which have the singlet ground state
with excitation gaps K and 31 K, respectively. Due to
bond randomness, the singlet ground state turns into the magnetic state with
finite susceptibility, nevertheless, the excitation gap remains. Field-induced
magnetic ordering, which can be described by the Bose condensation of excited
triplets, magnons, was observed as in the parent systems. The phase transition
temperature is suppressed by the bond randomness. This behavior may be
attributed to the localization effect.Comment: 19 pages, 7 figures, 12 eps files, revtex, will appear in PR
Field-Induced Magnetic Ordering in the Quantum Spin System KCuCl
KCuCl is a three-dimensional coupled spin-dimer system and has a singlet
ground state with an excitation gap K. High-field
magnetization measurements for KCuCl have been performed in static magnetic
fields of up to 30 T and in pulsed magnetic fields of up to 60 T. The entire
magnetization curve including the saturation region was obtained at K.
From the analysis of the magnetization curve, it was found that the exchange
parameters determined from the dispersion relations of the magnetic excitations
should be reduced, which suggests the importance of the renormalization effect
in the magnetic excitations. The field-induced magnetic ordering accompanied by
the cusplike minimum of the magnetization was observed as in the isomorphous
compound TlCuCl. The phase boundary was almost independent of the field
direction, and is represented by the power law. These results are consistent
with the magnon Bose-Einstein condensation picture for field-induced magnetic
ordering.Comment: 9 pages, 7 figures, 9 eps files, revtex styl
Microscopic model for the magnetization plateaus in NH4CuCl3
A simple model consisting of three distinct dimer sublattices is proposed to
describe the magnetism of NH4CuCl3. It explains the occurrence of magnetization
plateaus only at 1/4 and 3/4 of the saturation magnetization. The field
dependence of the excitation modes observed by ESR measurements is also
explained by the model. The model predicts that the magnetization plateaus
should disappear under high pressure.Comment: 4 pages, 5 figures, REVTeX
The disordered-free-moment phase: a low-field disordered state in spin-gap antiferromagnets with site dilution
Site dilution of spin-gapped antiferromagnets leads to localized free
moments, which can order antiferromagnetically in two and higher dimensions.
Here we show how a weak magnetic field drives this order-by-disorder state into
a novel disordered-free-moment phase, characterized by the formation of local
singlets between neighboring moments and by localized moments aligned
antiparallel to the field. This disordered phase is characterized by the
absence of a gap, as it is the case in a Bose glass. The associated
field-driven quantum phase transition is consistent with the universality of a
superfluid-to-Bose-glass transition. The robustness of the
disordered-free-moment phase and its prominent features, in particular a series
of pseudo-plateaus in the magnetization curve, makes it accessible and relevant
to experiments.Comment: 4 pages, 4 figure
Ehrenfest relations and magnetoelastic effects in field-induced ordered phases
Magnetoelastic properties in field-induced magnetic ordered phases are
studied theoretically based on a Ginzburg-Landau theory. A critical field for
the field-induced ordered phase is obtained as a function of temperature and
pressure, which determine the phase diagram. It is found that magnetic field
dependence of elastic constant decreases discontinuously at the critical field,
Hc, and that it decreases linearly with field in the ordered phase (H>Hc). We
found an Ehrenfest relation between the field dependence of the elastic
constant and the pressure dependence of critical field. Our theory provides the
theoretical form for magnetoelastic properties in field- and pressure-induced
ordered phases.Comment: 7 pages, 3 figure
Yeast and Human Frataxin Are Processed to Mature Form in Two Sequential Steps by the Mitochondrial Processing Peptidase
Frataxin is a nuclear-encoded mitochondrial protein which is deficient in Friedreich’s ataxia, a hereditary neurodegenerative disease. Yeast mutants lacking the yeast frataxin homologue (Yfh1p) show iron accumulation in mitochondria and increased sensitivity to oxidative stress, suggesting that frataxin plays a critical role in mitochondrial iron homeostasis and free radical toxicity. Both Yfh1p and frataxin are synthesized as larger precursor molecules that, upon import into mitochondria, are subject to two proteolytic cleavages, yielding an intermediate and a mature size form. A recent study found that recombinant rat mitochondrial processing peptidase (MPP) cleaves the mouse frataxin precursor to the intermediate but not the mature form (Koutnikova, H., Campuzano, V., and Koenig, M. (1998) Hum. Mol. Gen. 7, 1485–1489), suggesting that a different peptidase might be required for production of mature size frataxin. However, in the present study we show that MPP is solely responsible for maturation of yeast and human frataxin. MPP first cleaves the precursor to intermediate form and subsequently converts the intermediate to mature size protein. In this way, MPP could influence frataxin function and indirectly affect mitochondrial iron homeostasis
Yeast and Human Frataxin Are Processed to Mature Form in Two Sequential Steps by the Mitochondrial Processing Peptidase
Frataxin is a nuclear-encoded mitochondrial protein which is deficient in Friedreich’s ataxia, a hereditary neurodegenerative disease. Yeast mutants lacking the yeast frataxin homologue (Yfh1p) show iron accumulation in mitochondria and increased sensitivity to oxidative stress, suggesting that frataxin plays a critical role in mitochondrial iron homeostasis and free radical toxicity. Both Yfh1p and frataxin are synthesized as larger precursor molecules that, upon import into mitochondria, are subject to two proteolytic cleavages, yielding an intermediate and a mature size form. A recent study found that recombinant rat mitochondrial processing peptidase (MPP) cleaves the mouse frataxin precursor to the intermediate but not the mature form (Koutnikova, H., Campuzano, V., and Koenig, M. (1998) Hum. Mol. Gen. 7, 1485–1489), suggesting that a different peptidase might be required for production of mature size frataxin. However, in the present study we show that MPP is solely responsible for maturation of yeast and human frataxin. MPP first cleaves the precursor to intermediate form and subsequently converts the intermediate to mature size protein. In this way, MPP could influence frataxin function and indirectly affect mitochondrial iron homeostasis
Pasture Management in the US Midwest – An Assessment of Current Practices and Future Opportunities
Managed grazing offers significant potential to improve the sustainability of livestock farms in the US Midwest, however the benefits of managed grazing are largely influenced by the management practices employed on farm. The objective of this study was to gain an understanding of current grazing practices on Midwest farms and to identify the knowledge and support needs of graziers. A total of 185 responses were received from a range of different enterprises including dairy, beef, and sheep production. Results show a substantial degree of variation in grazing management practices between respondents and highlights significant scope for improvement on farms particularly in the areas of pasture measurement and budgeting, and grazing infrastructure. Reported benefits of managed grazing included lower environmental impact, better pasture and animal performance, better animal health and welfare, and lower costs. Challenges with managed grazing included time and labor input, maintaining pasture quantity and quality during the grazing season, adverse weather conditions such as excessive rain and drought, and animal health challenges such as heat stress, parasites and in some cases coyotes. The study highlighted opportunities for research and extension providers to better support farmers with information and advice and identified knowledge gaps in areas such as pasture species selection, soil fertility, grazing infrastructure, pasture budgeting, legumes, and pasture measurement. The study successfully gained an insight into graziers in the Midwest, the outputs of which, will be valuable to a number of key stakeholders going forward, including researchers, extension agents, farmers and policy makers
Point-contact investigations of challenging superconductors: two-band MgB2, antiferromagnetic HoNi2B2C, heavy fermion UPd2Al3, paramagnetic MgCNi3
An overview on recent efforts in point-contact (PC) spectroscopy of title
superconductors is given. Distinct phonon features and
crystalline-electric-field effects are observed in PC spectra of HoNi2B2C.
Results of study of superconducting (SC) gap and excess current versus
temperature and magnetic field reflecting specific multi-band electronic
structure in MgB2 are presented. The nature of the extremely nonlinear I(V)
curves in the antiferromagnetic (AF) and SC state are elucidated for UPd2Al3
break-junctions and MgCNi3 point contacts.Comment: 2 pages, 4 figs., presented on SCES`04 (26-30 July, Karlsruhe,
Germany), accepted for publicatio
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