Relation between the superconducting gap energy and the two-magnon Raman peak energy in Bi2Sr2Ca{1-x}YxCu2O{8+\delta}

The relation between the electronic excitation and the magnetic excitation for the superconductivity in Bi2Sr2Ca{1-x}YxCu2O{8+\delta} was investigated by wide-energy Raman spectroscopy. In the underdoping region the B1g scattering intensity is depleted below the two-magnon peak energy due to the "hot spots" effects. The depleted region decreases according to the decrease of the two-magnon peak energy, as the carrier concentration ncreases. This two-magnon peak energy also determines the B1g superconducting gap energy as $2\Delta \approx \alpha \hbar \omega_{\rm Two-Magnon} \approx J_{\rm effective}$ $(\alpha=0.34-0.41)$ from under to overdoping hole concentration.Comment: 10 pages, 4 figure

Resonant Two-Magnon Raman Scattering in Cuprate Antiferromagnetic Insulators

We present results of low-temperature two-magnon resonance Raman excitation profile measurements for single layer Sr_2CuO_2Cl_2 and bilayer YBa_2Cu_3O_{6 + \delta} antiferromagnets over the excitation region from 1.65 to 3.05 eV. These data reveal composite structure of the two-magnon line shape and strong nonmonotic dependence of the scattering intensity on excitation energy. We analyze these data using the triple resonance theory of Chubukov and Frenkel (Phys. Rev. Lett., 74, 3057 (1995)) and deduce information about magnetic interaction and band parameters in these materials.Comment: REVTeX, 4 pages + 2 PostScript (compressed) figure

Vibrational signature of broken chemical order in a GeS2 glass: a molecular dynamics simulation

Using density functional molecular dynamics simulations, we analyze the broken chemical order in a GeS$_2$ glass and its impact on the dynamical properties of the glass through the in-depth study of the vibrational eigenvectors. We find homopolar bonds and the frequencies of the corresponding modes are in agreement with experimental data. Localized S-S modes and 3-fold coordinated sulfur atoms are found to be at the origin of specific Raman peaks whose origin was not previously clear. Through the ring size statistics we find, during the glass formation, a conversion of 3-membered rings into larger units but also into 2-membered rings whose vibrational signature is in agreement with experiments.Comment: 11 pages, 8 figures; to appear in Phys. Rev.

Usefulness of the organ culture system in the in vitro diagnosis of coeliac disease: A multicentre study

Objective. Diagnosis of coeliac disease is based on the presence of villous atrophy which recovers following a gluten-free diet. The presence of circulating antiendomysial antibodies as well as their disappearance after a gluten-free diet supports the diagnosis. It has also been demonstrated that antiendomysial antibodies are detectable in supernatants of cultured intestinal biopsies from patients with coeliac disease. The objective of this study was to compare the histology and antiendomysial antibodies in culture supernatants of intestinal biopsies to validate the in vitro organ culture system as a future diagnostic tool for coeliac disease. Material and methods. Seventy-five antiendomysial serum-positive patients on a gluten-containing diet were evaluated. Patients underwent endoscopy with 5 biopsy fragments: 3 for histology, 1 cultured with and the other without gliadin-peptide activator. Antiendomysial antibodies were evaluated in all culture supernatants. Results. Sixty-eight patients had evidence of villous atrophy, while 73 out of 75 were positive to the organ culture system. The agreement rate between organ culture and histology results was 94%. Conclusions. As all the centres participating in the study obtained good agreement between organ culture and histology results, the new system could be considered a reliable tool for the diagnosis of coeliac disease. Nevertheless, it is possible to highlight cases with an organ culture-positive and -negative histology. This feature could be of considerable interest because, as the sensitivity of organ culture seems to be greater than the initial histology, the new system might be useful in uncertain cases where the risk of missing the diagnosis of coeliac disease is high

Raman scattering through a metal-insulator transition

The exact solution for nonresonant A1g and B1g Raman scattering is presented for the simplest model that has a correlated metal-insulator transition--the Falicov-Kimball model, by employing dynamical mean field theory. In the general case, the A1g response includes nonresonant, resonant, and mixed contributions, the B1g response includes nonresonant and resonant contributions (we prove the Shastry-Shraiman relation for the nonresonant B1g response) while the B2g response is purely resonant. Three main features are seen in the nonresonant B1g channel: (i) the rapid appearance of low-energy spectral weight at the expense of higher-energy weight; (b) the frequency range for this low-energy spectral weight is much larger than the onset temperature, where the response first appears; and (iii) the occurrence of an isosbestic point, which is a characteristic frequency where the Raman response is independent of temperature for low temperatures. Vertex corrections renormalize away all of these anomalous features in the nonresonant A1g channel. The calculated results compare favorably to the Raman response of a number of correlated systems on the insulating side of the quantum-critical point (ranging from Kondo insulators, to mixed-valence materials, to underdoped high-temperature superconductors). We also show why the nonresonant B1g Raman response is ``universal'' on the insulating side of the metal-insulator transition.Comment: 12 pages, 11 figures, ReVTe

Magnetic and Charge Correlations in La{2-x-y}Nd_ySr_xCuO_4: Raman Scattering Study

Two aspects in connection with the magnetic properties of La_{2-x-y}Nd_ySr_xCuO_4 single crystals are discussed. The first is related to long wavelength magnetic excitations in x = 0, 0.01, and 0.03 La_{2-x}Sr_xCuO_4 detwinned crystals as a function of doping, temperature and magnetic field. Two magnetic modes were observed within the AF region of the phase diagram. The one at lower energies was identified with the spin-wave gap induced by the antisymmetric DM interaction and its anisotropic properties in magnetic field could be well explained using a canonical form of the spin Hamiltonian. A new finding was a magnetic field induced mode whose dynamics allowed us to discover a spin ordered state outside the AF order which was shown to persist in a 9 T field as high as 100 K above the N\'eel temperature T_N for x = 0.01. For these single magnon excitations we map out the Raman selection rules in magnetic fields and demonstrate that their temperature dependent spectral weight is peaked at the N\'eel temperature. The second aspect is related to phononic and magnetic Raman scattering in La_{2-x-y}Nd_ySr_xCuO_4 with three doping concentrations: x = 1/8, y = 0; x = 1/8, y = 0.4; and x = 0.01, y = 0. We observed that around 1/8 Sr doping and independent of Nd concentration there exists substantial disorder in the tilt pattern of the CuO_6 octahedra in both the orthorhombic and tetragonal phases which persist down to 10 K and are coupled to bond disorder in the cation layers. The weak magnitude of existing charge/spin modulations in the Nd doped structure did not allow us to detect specific Raman signatures on lattice dynamics or two-magnon scattering around 2200 cm-1.Comment: 26 pages, 22 figure

Magnetic Raman Scattering in Two-Dimensional Spin-1/2 Heisenberg Antiferromagnets: Spectral Shape Anomaly and Magnetostrictive Effects

We calculate the Raman spectrum of the two-dimensional (2D) spin-1/2 Heisenberg antiferromagnet by exact diagonalization and quantum Monte Carlo techniques on clusters of up to 144 sites and, on a 16-site cluster, by considering the phonon-magnon interaction which leads to random fluctuations of the exchange integral. Results are in good agreement with experiments on various high-T_c precursors, such as La_2CuO_4 and YBa_2Cu_3O_{6.2}. In particular, our calculations reproduce the broad lineshape of the two-magnon peak, the asymmetry about its maximum, the existence of spectral weight at high energies, and the observation of nominally forbidden A_{1g} scattering.Comment: 12 pages, REVTEX, 1 postscript figur

Efeito da implantação de uma floresta mista sobre a população de microrganismos celulolíticos em solos do semi-árido mineiro.

Na recuperacao e manutencao da fertilidade dos solos degradados, a decomposicao apresenta-se particularmente importante sendo a razao celulose/N um dos principais fatores que influenciam a efetividade desses processo. Neste trabalho avaliou-se a ocorrencia e a dinamica da populacao de microorganismos celuloliticos nos solos de diferentes modelos de reflorestamento de uma area degradada do Projeto Jaiba/MG. Esses modelos incluiram especies arboreas de eucalipto, leguminosas e nao leguminosas nativas. Observaram-se diferencas significativas no numero de celuloliticos entre as estacoes e os locais estudados. Nas areas impactadas ocorreu elevacao menos intensa dos microrganismos evidenciando o efeito limitante do impacto sobre essa populacao microbiana. Nos modelos de plantio avaliados o numero de celuloliticos diferiu significativamente, sendo o mais elevado no modelo representado pelo plantio do maior numero de especies. em todos os experimentos, a maior populacao de celuloliticos ocorreu nos consorcios entre leguminosas e outras especies vegetais nativas e a menor nas areas de plantio de eucalipto, sugerindo uma acao limitante dessa planta sobre o crescimento desses microrganismos. Esses resultados evidenciaram o papel dos celuloliticos como bioindicadores da qualidade do solo e da resposta as diferentes praticas de manejo

Charge and spin excitations of insulating lamellar copper oxides

A consistent description of low-energy charge and spin responses of the insulating Sr_2CuO_2Cl_2 lamellar system is found in the framework of a one-band Hubbard model which besides $U$ includes hoppings up to 3^{rd} nearest-neighbors. By combining mean-field calculations, exact diagonalization (ED) results, and Quantum Monte Carlo simulations (QMC), we analyze both charge and spin degrees of freedom responses as observed by optical conductivity, ARPES, Raman and inelastic neutron scattering experiments. Within this effective model, long-range hopping processes flatten the quasiparticle band around $(0,\pi)$. We calculate also the non-resonant A_{1g} and B_{1g} Raman profiles and show that the latter is composed by two main features, which are attributed to 2- and 4-magnon scattering.Comment: 6 pages, 3 figures, To be published in PRB (july

Phase Separation Models for Cuprate Stripe Arrays

An electronic phase separation model provides a natural explanation for a large variety of experimental results in the cuprates, including evidence for both stripes and larger domains, and a termination of the phase separation in the slightly overdoped regime, when the average hole density equals that on the charged stripes. Several models are presented for charged stripes, showing how density waves, superconductivity, and strong correlations compete with quantum size effects (QSEs) in narrow stripes. The energy bands associated with the charged stripes develop in the middle of the Mott gap, and the splitting of these bands can be understood by considering the QSE on a single ladder.Comment: significant revisions: includes island phase, 16 eps figures, revte