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

Magnetic properties of NaV2O5, a one-dimensional spin 1/2 antiferromagnet with finite chains

We have performed measurements of the magnetic susceptibility of NaV$_2$O$_5$ between 2 and 400 K. The high temperature part is typical of spin 1/2 chains with a nearest--neighbour antiferromagnetic exchange integral $J$ of 529 K. We develop a model for the susceptibility of a system with finite chains to account for the low temperature part of the data, which cannot be fitted by a standard Curie-Weiss term. These results suggest that the next nearest--neighbour exchange integral $J_2$ in CaV$_4$O$_9$ should be of the order of 500 K because, like $J$ in NaV$_2$O$_5$, it corresponds to corner sharing VO$_5$ square pyramids.Comment: An early version of the manuscript was mistakenly submitted. Although relatively minor, the changes concern the list of authors, the main text, the references and the figure captions. 10 pages of latex, 2 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

Chemical physics: The standing of a mature discipline

It is always promising and enticing to start a new editorial task in the scientific arena and the launch of the Chemistry Central Journal is no exception. The different thematic sections making up this journal are quite representative of the whole chemistry enterprise. However, one of them has a special relevance. In fact, Chemical Physics (CP) is the most general and it embodies a wide diversity of issues. Of particular importance at the launch of this groundbreaking new journal is the confidence of the Section Editor in BioMed Central (owners of Chemistry Central) as publishers, and from Chemistry Central to its Editorial Board. I feel deeply grateful for this new assignment and I hope to be able to perform a thorough job in editing this section. Below, I make my request to you as potential authors and reviewers

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

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