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High temperature superconductivity is often found in the vicinity of antiferromagnetism. This is also true in LaFeAsO1−xFx (x ≤ 0.2) and many other iron-based superconductors, which leads to proposals that superconductivity is mediated by fluctuations associated with the nearby magnetism. Here we report the discovery of a new superconductivity dome without low-energy magnetic fluctuations in LaFeAsO1−xFx with 0.25≤ x ≤0.75, where the maximal critical temperature Tc at xopt = 0.5∼0.55 is even higher than that at x ≤ 0.2. By nuclear magnetic resonance and Transmission Electron Microscopy, we show that a C4 rotation symmetry-breaking structural transition takes place for x > 0.5 above Tc. Our results point to a new paradigm of high temperature superconductivity.

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A study was made of the thermal properties of low pressure chemical vapor deposition (LPCVD) silicon thin films with amorphous and polycrystalline microstructures, produced by varying the substrate temperature. Thermal diffusivity measurements were conducted using a thermal wave technique. The thermal diffusivity of the polycrystalline films was found to be about three times that of the amorphous films, but about one eighth that of bulk silicon single crystals. There was also an indication that the diffusivity increased with deposition temperature above the transition temperature from the amorphous to the polycrystalline state. The relationships between the thermal properties and microstructural features, such as grain size and grain boundary, are discussed.

Articles you may be interested inLow temperature thermal diffusivity measurements of transparent solids and gases by the mirage technique AIP Conf. Low temperature thermal diffusivity measurements of gases by the mirage technique Rev. Sci. Instrum. 70, 98 (1999); 10.1063/1.1149548Low thermal diffusivity measurements of thin films using mirage technique A method to determine anisotropic thermal diffusivities parallel to the surface of a solid sample is presented. The measurement method is based on the mirage effect and the data are analyzed using multiparameter least-squares regression fitting. The specimens studied were of medium and low diffusivity: rare earths, ceramics, and polymers. Before this study, the low-diffusivity limit for the applicability of the mirage method was about 0.02 cm2/s, and therefore this technique has not been suitable for polymeric materials. In this work the limit has been reduced to 5 X 10V4 cm2/s by improving the measurement apparatus and the data analysis method. The thermal diffusivities obtained agree with the values obtained using the flash method within f 20%. The agreement with published values is good, taking into account that for ceramics and polymers the literature gives only the correct order of magnitude due to the differences in the manufacturing process. Even though the sensitivity of the measurement decreases with the decreasing diffusivity, the thermal diffusivity of bulk polymers can still be determined within an accuracy of 10%. For polymer foils the method has been found to be reliable when the foil thickness exceeds 100 pm.