Effect of external pressure on the magnetic properties of RRCoAsO (RR = La, Pr, Sm): a μ\muSR study

We report on a detailed investigation of the itinerant ferromagnets LaCoAsO, PrCoAsO and SmCoAsO performed by means of muon spin spectroscopy upon the application of external hydrostatic pressures $p$ up to $2.4$ GPa. These materials are shown to be magnetically hard in view of the weak dependence of both critical temperatures $T_{C}$ and internal fields at the muon site on $p$. In the cases $R$ = La and Sm, the behaviour of the internal field is substantially unaltered up to $p = 2.4$ GPa. A much richer phenomenology is detected in PrCoAsO instead, possibly associated with a strong $p$ dependence of the statistical population of the two different crystallographic sites for the muon. Surprisingly, results are notably different from what is observed in the case of the isostructural compounds $R$CoPO, where the full As/P substitution is already inducing a strong chemical pressure within the lattice but $p$ is still very effective in further affecting the magnetic properties.Comment: 8 pages, 9 figure

Fluid-mediated, brittle–ductile deformation at seismogenic depth – Part 2: Stress history and fluid pressure variations in a shear zone in a nuclear waste repository (Olkiluoto Island, Finland)

Abstract. The microstructural record of fault rocks active at the brittle–ductile transition zone (BDTZ) may retain information on the rheological parameters driving the switch in deformation mode and on the role of stress and fluid pressure in controlling different fault slip behaviours. In this study we analysed the deformation microstructures of the strike-slip fault zone BFZ045 in Olkiluoto (SW Finland), located in the site of a deep geological repository for nuclear waste. We combined microstructural analysis, electron backscatter diffraction (EBSD), and mineral chemistry data to reconstruct the variations in pressure, temperature, fluid pressure, and differential stress that mediated deformation and strain localization along BFZ045 across the BDTZ. BFZ045 exhibits a mixed ductile–brittle deformation, with a narrow (&lt;20 cm thick) brittle fault core with cataclasites and pseudotachylytes that overprint a wider (60–100 cm thick) quartz-rich mylonite. Mylonitic deformation took place at 400–500 ∘C and 3–4 kbar, typical of the greenschist facies metamorphism at the base of the seismogenic crust. We used the recrystallized grain size piezometry for quartz to document a progressive increase in differential stress, from ca. 50 to ca. 120 MPa, towards the shear zone centre during mylonitization and strain localization. Syn-kinematic quartz veins formed along the mylonitic foliation due to transiently high pore fluid pressure (up to lithostatic value). The overprint of the veins by dynamic recrystallization and mylonitic creep is further evidence of the occurrence of brittle events under overall ductile conditions. We propose a conceptual model in which the ductile–brittle deformation cycle was controlled by transient oscillations in fluid pressure and progressively higher differential stress, possibly occurring in a narrowing shear zone deforming towards the peak strength of the crust at the BDTZ. </jats:p

AC susceptibility investigation of vortex dynamics in nearly-optimally doped REFeAsO1−x_{1-x}Fx_{x} superconductors (RE = La, Ce, Sm)

Ac susceptibility and static magnetization measurements were performed in the nearly-optimally doped LaFeAsO$_{0.9}$F$_{0.1}$ and CeFeAsO$_{0.92}$F$_{0.08}$ superconductors, complementing earlier results on SmFeAsO$_{0.8}$F$_{0.2}$ [Phys. Rev. {\bf B 83}, 174514 (2011)]. The magnetic field -- temperature phase diagram of the mixed superconducting state is drawn for the three materials, displaying a sizeable reduction of the liquid phase upon increasing $T_{c}$ in the range of applied fields ($H \leq 5$ T). This result indicates that SmFeAsO$_{0.8}$F$_{0.2}$ is the most interesting compound among the investigated ones in view of possible applications. The field-dependence of the intra-grain depinning energy $U_{0}$ exhibits a common trend for all the samples with a typical crossover field value (2500 Oe $\lesssim H_{cr} \lesssim 5000$ Oe) separating regions where single and collective depinning processes are at work. Analysis of the data in terms of a simple two-fluid picture for slightly anisotropic materials allows to estimate the zero-temperature penetration depth $\lambda_{ab}(0)$ and the anisotropy parameter $\gamma$ for the three materials. Finally, a sizeable suppression of the superfluid density is deduced in a $s^{\pm}$ two-gap scenario

Common effect of chemical and external pressures on the magnetic properties of RECoPO (RE = La, Pr)

We report a detailed investigation of RECoPO (RE = La, Pr) and LaCoAsO materials performed by means of muon spin spectroscopy. Zero-field measurements show that the electrons localized on the Pr$^{3+}$ ions do not play any role in the static magnetic properties of the compounds. Magnetism at the local level is indeed fully dominated by the weakly-itinerant ferromagnetism from the Co sublattice only. The increase of the chemical pressure triggered by the different ionic radii of La$^{3+}$ and Pr$^{3+}$, on the other hand, plays a crucial role in enhancing the value of the magnetic critical temperature and can be mimicked by the application of external hydrostatic pressure up to 24 kbar. A sharp discontinuity in the local magnetic field at the muon site in LaCoPO at around 5 kbar suggests a sizeable modification in the band structure of the material upon increasing pressure. This scenario is qualitatively supported by \emph{ab-initio} density-functional theory calculations.Comment: 13 pages, 10 figure

Evidence for a vortex-glass transition in superconducting Ba(Fe0.9_{0.9}Co0.1_{0.1})2_{2}As2_{2}

Measurements of magneto-resistivity and magnetic susceptibility were performed on single crystals of superconducting Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$ close to the conditions of optimal doping. The high quality of the investigated samples allows us to reveal a dynamic scaling behaviour associated with a vortex-glass phase transition in the limit of weak degree of quenched disorder. Accordingly, the dissipative component of the ac susceptibility is well reproduced within the framework of Havriliak-Negami relaxation, assuming a critical power-law divergence for the characteristic correlation time $\tau$ of the vortex dynamics. Remarkably, the random disorder introduced by the Fe$_{1-x}$Co$_{x}$ chemical substitution is found to act on the vortices as a much weaker quenched disorder than previously reported for cuprate superconductors such as, e.g., Y$_{1-x}$Pr$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$.Comment: 10 pages, 8 figure

Charge and nematic orders in AFe2As2 (A = Rb, Cs) superconductors

We discuss the results of 75As nuclear quadrupole resonance (NQR) and muon spin relaxation measurements in AFe2As2 (A = Cs, Rb) iron-based superconductors. We point out that the crossover detected in the nuclear spin-lattice relaxation rate 1/T1 (around 150 K in RbFe2As2 and around 75 K in CsFe2As2), from a high temperature nearly localized to a low temperature delocalized behavior, is associated with the onset of an inhomogeneous local charge distribution causing the broadening or even the splitting of the NQR spectra as well as an increase in the muon spin relaxation rate. We argue that this crossover, occurring at temperatures well above the phase transition to the nematic long-range order, is due to a charge disproportionation at the Fe sites induced by competing Hund’s and Coulomb couplings. In RbFe2As2 around 35 K, far below that crossover temperature, we observe a peak in the NQR 1/T1 which is possibly associated with the critical slowing down of electronic nematic fluctuations on approaching the transition to the nematic long-range order

Glassy Transition in the Vortex Lattice of Ba(Fe0.93Rh0.07)2As2 superconductor, probed by NMR and ac-susceptibility

By using Nuclear Magnetic Resonance and ac-susceptibility, the characteristic correlation times for the vortex dynamics, in an iron-based superconductor, have been derived. Upon cooling, the vortex dynamics displays a crossover consistent with a vortex glass transition. The correlation times, in the fast motions regime, merge onto a universal curve which is fit by the Vogel-Fulcher law, rather than by an Arrhenius law. Moreover, the pinning barrier shows a weak dependence on the magnetic field which can be heuristically justified within a fragile glass scenario. In addition, the glass freezing temperatures obtained by the two techniques merge onto the de Almeida-Thouless line. Finally the phase diagram for the mixed phase has been derived.Comment: 6 pages, 9 figure

Formas de uréia e doses de nitrogênio em cobertura na qualidade fisiológica de sementes de trigo.

A adubação nitrogenada, além de aumentar a produtividade, pode favorecer a qualidade fisiológica das sementes. O trigo no Paraná é geralmente cultivado após a cultura da soja, com isso a palha da soja e outros fatores do solo podem minimizar o efeito do nitrogênio no trigo. O objetivo do trabalho foi avaliar o efeito de doses de adubação nitrogenada em cobertura e de diferentes formas de ureia, na qualidade fisiológica de sementes de genótipos de trigo. Os experimentos foram conduzidos em Londrina e Ponta Grossa-PR. As áreas experimentais são manejadas no sistema de plantio direto com rotação de culturas, sendo a soja a cultura anterior. Foram avaliadas sementes de três genótipos de trigo (BRS 208, BRS Pardela e IWT 04008) cultivados sob quatro doses de nitrogênio em cobertura (0, 40, 80 e 120 kg ha-1), provenientes de três formas de ureia (convencional, com inibidor de urease e protegida). A adubação nitrogenada de cobertura foi realizada durante o estádio de perfilhamento, aos 20 dias após a emergência. Avaliou-se o teor de nitrogênio nas sementes, a massa de 1000 sementes, a germinação e o vigor (primeira contagem, frio, emergência, massa seca de plântulas, envelhecimento acelerado e condutividade elétrica). Os genótipos de trigo foram analisados conjuntamente para cada local. A linhagem IWT 04008 e a cultivar BRS Pardela apresentam sementes com qualidade fisiológica superior a cultivar BRS 208. Somente em Londrina, em consequência do acamamento, as doses de nitrogênio influenciaram negativamente a massa de 1000 grãos e a condutividade elétrica e, positivamente o teor de proteína. As formas de ureia e as doses de nitrogênio não alteram a qualidade fisiológica das sementes de diferentes genótipos de trigo

Análise de viabilidade econômico-financeira da rotação arroz-soja, com o sistema sulco-camalhão, em terras baixas do Rio Grande do Sul.

bitstream/item/224235/1/Circ-Tec-171.pd