科研之友
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摘要
We compare the superconducting phase-diagram under high magnetic fields (up to H = 45 T) of Fe1+y Se0.4Te0.6 single crystals originally grown by the Bridgman-Stockbarger (BRST) technique, which were annealed to display narrow superconducting transitions and the optimal transition temperature T-c greater than or similar to 14 K, with the diagram for samples of similar stoichiometry grown by the traveling-solvent floating-zone technique as well as with the phase diagram reported for crystals grown by a self-flux method. We find that the so-annealed samples tend to display higher ratiosH(c2)/T-c, particularly for fields applied along the interplanar direction, where the upper critical field H-c2(T) exhibits a pronounced concave down curvature followed by saturation at lower temperatures T. This last observation is consistent with previous studies indicating that this system is Pauli limited. An analysis of our H-c2(T) data using a multiband theory suggests the emergence of the Fulde-Ferrel-Larkin-Ovchnikov state at low temperatures. A detailed structural x-ray analysis, reveals no impurity phases but an appreciable degree of mosaicity in as-grown BRST single crystals that remains unaffected by the annealing process. Energy-dispersive x-ray analysis showed that the annealed samples have a more homogeneous stoichiometric distribution of both Fe and Se with virtually the same content of interstitial Fe as the nonannealed ones. Thus we conclude that the excess of Fe, in contrast to structural disorder, contributes to the decrease of the superconducting upper-critical fields of this series. Finally, a scaling analysis of the fluctuation conductivity in the superconducting critical regime, suggests that the superconducting fluctuations have a two-dimensional character in this system.
- 出版日期2011-11-21
