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(1995)Journal ArticleNormal-state reentrant behavior is observed in the resistive transition for Bi-2212/2223 intergrowth single crystals. It is found that for a small driving current of 50 μA the resistivity for the intergrowth system exhibits a superconducting transition with zero values at 105 K, which returns to the resistive state at a lower temperature. With a further temperature decrease, the system again becomes superconducting at 92 K. In the case of large driving currents, the reentrant behavior is not found and only a two-step resistive transition is observed. The normal-state reentrant behavior is interpreted using a model where the Josephson coupling between the CuO2 trilayers is weakened by thermally activated resistance between these layers. The two-step transition can be explained by the proximity effect along the ab plane. Our results suggest that due to thermally activated resistance between the superconducting layers in the layered superconductors like Bi-Sr-Ca-Cu-O, the three- to two-dimensional crossover of superconducting behavior may be induced at temperatures much lower than Tc by applying magnetic or electric fields.
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(1995)Journal ArticleAn anomalously broad foot structure is observed in the zero-field resistive transition for the ab plane of Bi2Sr2CaCu2O8+y single crystals, which are found to have a high density of dislocations. However, this anomalous behavior is not apparent for resistive measurements along the c axis. The foot structure can be treated as a superposition of a long resistance tail and a dissipation peak, in which the long resistance tail obeys the theory of KT scaling behavior of resistivity. The dissipation peak is related to the fluxon transition in Josephson-coupled layered superconductors as detailed by Horovitz [Phys. Rev. Lett. 72, 1569 (1994)].
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(1995)Journal ArticleSr-based 123 compounds R1-xCaxSr2Cu2.7Mo0.3Oy with R = Pr, Nd, and Y are successfully stabilised by partial substitution of Mo for Cu. A tetragonal-orthorhomic-tetragonal transition is observed for R = Y and Nd, which is induced by different oxygen content in the samples. Tc of the samples is sensitive to the oxygen content other than the structure. For Pr-contained samples the resistivity decreases systematically with increasing the Ca dopant and oxygen content. The results show that the hybridisation of the conduction electrons with the 4f band of Pr in Sr-based 123 compounds is weaker than that in Ba-based 123 compounds.
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(1996)Journal ArticleThe effects of contact area as well as the weight fraction of Y2BaCuO5 (211) substrate on the melt textured growth (MTG) of YBa2Cu3O7- (123) crystals are investigated. It is found that the 123 grain size and its orientation have been greatly improved by a well designed 211 substrate with maximized number of contact points. The size of the 123 single domain has been extended up to 2.7 cm with no porosity and cracks by applying 50% of contact area and 35% of absorbed liquid weight into 211 substrate. Magnetization and levitation measurements show that the MTG 123 crystals have a high Jc and a good performance in high magnetic fields.
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(1998)Journal ArticleTo identify structural features important for low temperature activity in archaeal proteins, elongation factor 2 (EF-2) genes (aef2) were sequenced from psychrophilic, mesophilic and thermophilic methanogens. Scatter plots were used to compare evolutionary distances for EF-2 amino acid sequences vs. 16S-rRNA sequences from methanogens growing at diverse temperatures. The absence of a temperature bias for the rate of protein vs. nucleic acid evolution demonstrated the importance of comparing closely related proteins in order to identify changes indicative of thermal adaptation. Three-dimensional modelling of the new EF-2 sequences enabled the identification of amino acid residues that may be important for conferring low temperature activity and included greater structural flexibility produced by fewer salt bridges, less packed hydrophobic cores and the reduction of proline residues in loop structures.
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