31 research outputs found
Weak ferromagnetism and field-induced spin reorientation in K2V3O8
Magnetization and neutron diffraction measurements indicate long-range
antiferromagnetic ordering below TN=4 K in the 2D, S=1/2 Heisenberg
antiferromagnet K2V3O8. The ordered state exhibits ``weak ferromagnetism'' and
novel, field-induced spin reorientations. These experimental observations are
well described by a classical, two-spin Heisenberg model incorporating
Dzyaloshinskii-Moriya interactions and an additional c-axis anisotropy. This
additional anisotropy can be accounted for by inclusion of the symmetric
anisotropy term recently described by Kaplan, Shekhtman, Entin-Wohlman, and
Aharony. This suggests that K2V3O8 may be a very unique system where the
qualitative behavior relies on the presence of this symmetric anisotropy.Comment: 5 pages, 4 ps figures, REVTEX, submitted to PR
Interference of a first-order transition with the formation of a spin-Peierls state in alpha'-NaV2O5?
We present results of high-resolution thermal-expansion and specific-heat
measurements on single crystalline alpha'-NaV2O5. We find clear evidence for
two almost degenerate phase transitions associated with the formation of the
dimerized state around 33K: A sharp first-order transition at T1=(33+-0.1)K
slightly below the onset of a second-order transition at T2onset around
(34+-0.1)K. The latter is accompanied by pronounced spontaneous strains. Our
results are consistent with a structural transformation at T1 induced by the
incipient spin-Peierls (SP) order parameter above T2=TSP.Comment: 5 pages, 7 figure
NaV_2O_5 as a quarter-filled ladder compound
A new X-ray diffraction study of the one-dimensional spin-Peierls compound
\alpha-NaV_2O_5 reveals a centrosymmetric (Pmmn) crystal structure with one
type of V site, contrary to the previously postulated non-centrosymmetric
P2_1mn structure with two types of V sites (V^{+4} and V^{+5}). Density
functional calculations indicate that NaV_2O_5 is a quarter-filled ladder
compound with the spins carried by V-O-V molecular orbitals on the rungs of the
ladder. Estimates of the charge-transfer gap and the exchange coupling agree
well with experiment and explain the insulating behavior of NaV_2O_5 and its
magnetic properties.Comment: Final version for PRL, value of U correcte
Dynamical properties of the spin-Peierls compound \alpha'--NaV2O5
Dynamical properties of the novel inorganic spin-Peierls compound
\alpha'--NaV2O5 are investigated using a one-dimensional dimerized Heisenberg
model. By exact diagonalizations of chains with up to 28 sites, supplemented by
a finite-size scaling analysis, the dimerization parameter \delta is determined
by requiring that the model reproduces the experimentally observed spin gap
\Delta. The dynamical and static spin structure factors are calculated. As for
CuGeO3, the existence of a low energy magnon branch separated from the
continuum is predicted. The present calculations also suggest that a large
magnetic Raman scattering intensity should appear above an energy threshold of
1.9 \Delta. The predicted photoemission spectrum is qualitatively similar to
results for an undimerized chain due to the presence of sizable short-range
antiferromagnetic correlations.Comment: 4 pages, latex, minor misprints corrected and a few references adde
Produção de edificações sustentáveis: desafios e alternativas
A discussão do projeto e execução de edificações sustentáveis vem sendo abordada por diferentes autores, em geral com foco no desempenho da edificação e vida útil dos materiais. Entretanto, para atender aos requisitos de desempenho ambiental das edificações, existe também a necessidade de se introduzir mudanças na organização e gestão do processo projetual. Este artigo apresenta os resultados de uma pesquisa que investigou como a incorporação dos requisitos da sustentabilidade ambiental podem influenciar as práticas de projeto de arquitetura e engenharia. Reconhecendo a influência francesa na produção de edificações sustentáveis no Brasil - em decorrência da adoção por algumas empresas brasileiras do referencial Aqua (adaptado do HQE® francês) - , a pesquisa foi realizada junto a empresas que atuam naquele país. Os resultados apontam que a melhoria do processo de projeto depende do estabelecimento de um sistema de gestão que possa auxiliar os profissionais a lidarem com o grande número requisitos relacionados com a edificação sustentável. Também indicam a necessidade de revisão dos esquemas adotados até então, de forma a viabilizar a introdução da interoperabilidade entre os diferentes profissionais de projeto, desde a concepção arquitetônica, que deve se tornar responsabilidade de toda a equipe de projeto
Raman Scattering in the Inorganic Spin-Peierls System alpha'-Na_{1-delta}V_2O_5
We have studied the spin-Peierls (SP) transition in alpha'-Na_{1-delta}V_2O_5
(delta = 0, 0.01 and 0.1) by means of Raman scattering. At room temperature, we
observe six A_1 and three A_2 phonon modes and a broad Raman band. Below
T_{SP}several new peaks and a new broad band appear. The new peak at 62 cm^{-1}
originates from the SP-gap excitation. The new peak at 128 cm^{-1} and the new
broad band between 130 and 400 cm^{-1} come from two magnetic excitations. The
new peaks at 102, 646 and 944 cm^{-1} are assigned to the folded phonon modes
and their Na^+-ion deficiency dependence shows that the defect of Na^+ ion
suppresses the SPtransition. The polarized Raman spectra below T_{SP} suggest
that the possible crystal symmetry is C_s^2(Pn) or C_1^1(P1). The asymmetric
lineshape of the 531-cm^{-1} peak superimposed on the electronic Raman band
from the d-d transition around 600 cm^{-1} is interpreted in terms of the Fano
resonance between the electronic continuous band and the phonon with a finite
lifetime. The defects of the Na^+ ions reduce the Fano effect because the life
time of the phonon and the phonon-continuum interaction are decreased.Comment: 24 pages, 10 Postscript figures,uses jpsj.sty and epsf.sty. in press
in J. Phys. Soc. Jp
Unprecedented Zeolite-Like Framework Topology Constructed from Cages with 3-Rings in a Barium Oxonitridophosphate
The Electrochemical Oxidation : a New Way for Preparing Highly Oxidized Ferrites
The electrochemical oxidation is depicted as a novel and powerful route for preparing highly oxidized ferrites. The reaction is achieved at room temperature, in air, in alkaline solution (1M KOH or NaOH). This process has been used for preparing various new compounds and especially perovskite-type ferrites such as SrMO3 ( M = Fe or/and Co ) or Sr2LaFe3O9-ε. The characterization of the samples shows that they remain well crystallized after the electrochemical treatment and bulky oxidized. The reaction can be described as an intercalation process of oxygen atoms into the host lattice of the starting material. For instance, the chemical analysis as well as Mössbauer spectroscopy data of SrFeO3 or of Sr2FeCoO6 reveal iron cations to be exclusively tetravalent. On the other hand, Sr2LaFe3O9-ε exhibits at decreasing temperature a disproportionation of the mixed valence of iron. Potentiostatic and galvanostatic experiments have shown that the amount of intercalated oxygen can be controlled and that the process is reversible. A mechanism involving the formation of O- species that can diffuse inside the bulk of the material, is proposed