Topological excitations vs intergranular phase — coherence in a HTSC Y0.5Sm0.5Ba2Cu3O7 ceramics

This paper reports a detailed study of the electrical transport properties of good quality superconducting Y_0.5Sm_0.5Ba₂Cu₃O₇ ceramic samples. Analyses of the dependence of resistance on temperature, the relations current-voltage and the magnetoresistance allows to identify:a) the mean field critical temperature (T _CO =93.73±0.01 K);b) a power law behaviorVI atT _CO . The exponent jumps abruptly from 1 to 3, at a certain temperatureT _C =93.276±0.005 K;c) an exponential inverse root-square temperature dependence of the resistivity in theT _{C–T CO} temperature range. These features, typically observed when topological excitations sets in two dimensional systems, can also be interpreted as a signature of a percolation process and a transition towards intergranular phase coherence. We analyze and discuss the relevance of both models to account for the experimental data.     

新型聚芳醚酮的研究进展

文章主要介绍了聚芳醚酮的合成路线与性质,讨论了对其改性的方向和进展,如共聚共混改性,在聚芳醚酮的主链中引入大的侧基破坏其规整性,以及研制含氟的新型聚合物。     

Positron/positronium annihilation in nanocrystalline silicon thin films

Positron and positronium annihilation investigations were applied to nanocrystalline silicon (nc-Si) thin films, for the first time. The nc-Si thin films with average grain diameters of 3–5 nm show intense blue luminescence at room temperature. The nanometer-sized Si crystallites formed in amorphous Si (a-Si) matrix give rise to this luminescence. Very highS-parameters up to 0.62 were observed in the as-grown a-Si thin film suggesting positronium formation in the a-Si layer. The average lifetime of the positrons in the a-Si was determined to be about 520 ps. TheS-parameters dropped significantly to 0.53 by crystallization of the thin film at 800 °C for 10 seconds, which was almost the same to the value observed in bulk Si (100) substrate. Further crystallization from 60 seconds to 1 hour showed smaller change in theS-parameters than that from the a-Si to 10 seconds. The large change in theS-parameters due to the annealing might be caused by the formation of Si nanocrystallites in a-Si matrix suggesting that positron is a sensitive probe for structural investigations of the nc-Si materials.