Breakdown by a magnetic field of the superconducting fluctuations in the normal state: A simple phenomenological explanation

We first summarize our recent observations, through magnetization measurements in different low-Tc superconductors, of a rather sharp disappearance of the superconducting fluctuations in the normal state when the magnetic field approaches Hc2(0), the upper critical field extrapolated to T=0K. We propose that a crude phenomenological description of the observed effects may be obtained if the quantum limits associated with the uncertainty principle are introduced in the Gaussian-Ginzburg-Landau description of the fluctuation-induced magnetization.Comment: LaTeX, 8 pages, including 2 eps figures. Proceedings of SNS'04, Sitges, Spai

Effects of hydrocarbons on the physiology and growth of Ulva sp. (Chlorophyta)

Se realizaron experimentos de laboratorio para investigar las respuestas fisiológicas de una especie de Ulva. Los discos de Ulva fueron expuestos a 16 °C hasta 12 días, a emulsiones en agua de mar con cada una de las cuatro fracciones de petróleo estándar (P1, d 0,7, bp 80-110 °C; P2, d 0,73, bp 100-140 °C; P3, d 0,76, bp 140-160 °C; P4, d 0,76, bp 180-220 °C), en cada caso a una concentración de 500, 1 000, 2 000, 4 000 y 8 000 ppm ( ᵛ/ᵥ ). Las variables fisiológicas determinadas fueron tasa de fotosíntesis, tasa de respiración, eficiencia fotoquímica (Fv/Fmáx), disipación no fotoquímica (qN), concentración de clorofila a y tasa de desarrollo. Se usó un análisis de probit para determinar los tiempos y concentraciones de inhibición del 50 % de las respuestas fisiológicas. Nuestros resultados revelaron que el mejor indicador del estrés debido a la exposición de hidrocarburos de petróleo fue la tasa de fotosíntesis. La fracción P1 (baja densidad, bajo punto de ebullición) fue el más tóxico, con una concentración de 871 ppm para la inhibición del 50 % de la tasa de fotosíntesis en 72 horas.Laboratory experiments were performed to investigate the physiological responses of an Ulva species to oil pollution. Ulva thalli were exposed at 16 °C for up to 12 days to emulsions in seawater of one of four standard petroleum fractions (P1, d 0.7, bp 80-110 °C; P2, d 0.73, bp 100- 140 °C; P3, d 0.76, bp 140-160 °C; P4, d 0.76, bp 180-220 °C), in each case at a concentration of 500, 1 000, 2 000, 4 000 or 8 000 ppm ( ᵛ/ᵥ ). The physiological variables determined were: photosynthesis rate, respiration rate, photochemical efficiency (Fv/Fmax), non-photochemical quenching (qN), chlorophyll a concentration, and growth rate. A probit analysis was used to determine 50 %-inhibition time and concentrations of the physiological responses. Our results showed that the best indicator of stress due to exposure to petroleum hydrocarbons was photosynthesis rate. The fraction P1 (low density, low boiling point) was the most toxic, with the 72- hour IC50 for rate of photosynthesis being 871 ppm.Instituto Español de Oceanografí

Chlorophyll fluorescence analysis and cadmiumcopper bioaccumulation in Ulva rigida (C. Agardh)

The objective of the present study was to determine the usefulness of the alga Ulva rigida (C. Agardh) in monitoring dissolved cadmium and copper in seawater, as well as the physiological stress caused by both metals. Discs from the alga (d = 19 mm) were cultured in different Cd and Cu growth media during a 12-day period. The concentrations established were 0.1, 0.3, 0.9, 2.7 mg metal 1⁻¹, as well as the corresponding controls. Chlorophyll fluorescence readings were performed on days 1, 2, 5, 9 and 12. Discs were frozen for later AAS analysis (Perkin-Elmer). Treatments with higher Cd concentration (0.3, 0.9, 2.7 mg l⁻¹) presented clear signs of disturbing photosystem II, since optimum quantum yields (Fv/Fm) were lower than the control responses. Similarly, Cu treatments diminished the alga's photosynthetic efficiency. Cu and Cd internal levels in U. rigida increased successively during the experiments, which also were proportional to the treatments. These levels were transformed into contamination factors (CF), calculated by dividing each concentration by its respective control and adjusting with Michaelis- Menten kinetics.Mediante este estudio se pretende determinar la utilidad del alga Ulva rigida (C. Agardh) para monitorizar Cd y Cu disueltos en agua de mar, así como el estrés fisiológico causado por ambos metales. Para ello, discos de dicha alga (d = 19 mm) se colocaron en diferentes soluciones de Cd y Cu, durante un periodo de 12 días, siendo las concentraciones seleccionadas: 0,1; 0,3; 0,9; 2,7 mg metal l⁻¹ de solución, así como los respectivos controles. Las lecturas de fluorescencia de clorofila se efectuaron los días 1, 2, 5, 9 y se analizó el contenido metálico de los discos mediante espectrofotometría de absorción atómica con llama (AAS). Los tratamientos de Cu y los de mayor concentración de Cd (0,3; 0,9; 2,7 mg l⁻¹) presentaron claras señales de perturbación del fotosistema II, con reducciones claras del rendimiento cuántico óptimo (Fv/Fm). De igual forma, los tratamientos de Cu redujeron la eficiencia fotosintética del alga. Los niveles corporales de Cd y Cu en U. rigida registraron incrementos sucesivos durante el periodo experimental y fueron proporcionales a los tratamientos. Las cinéticas de carga fueron modelizadas mediante ajuste a Michaelis-Menten.Instituto Español de Oceanografí

Comment on "High Field Studies of Superconducting Fluctuations in High-Tc Cuprates. Evidence for a Small Gap distinct from the Large Pseudogap"

By using high magnetic field data to estimate the background conductivity, Rullier-Albenque and coworkers have recently published [Phys.Rev.B 84, 014522 (2011)] experimental evidence that the in-plane paraconductivity in cuprates is almost independent of doping. In this Comment we also show that, in contrast with their claims, these useful data may be explained at a quantitative level in terms of the Gaussian-Ginzburg-Landau approach for layered superconductors, extended by Carballeira and coworkers to high reduced-temperatures by introducing a total-energy cutoff [Phys.Rev.B 63, 144515 (2001)]. When combined, these two conclusions further suggest that the paraconductivity in cuprates is conventional, i.e., associated with fluctuating superconducting pairs above the mean-field critical temperature.Comment: 9 pages, 1 figur