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| VOLUME 90 (2009) | ISSUE 5 | PAGE 429 |
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Magnetic and Superconducting Properties of FeAs-based High-Tc Superconductors with Gd
E. P. Khlybov
 , O. E. Omelyanovsky*∇, A. Zaleski , A. V. Sadakov , D. R. Gizatulin , L. F. Kulikova+, I. E. Kostuleva, V. M. Pudalov*∇+Institute for High Pressure Physics, RAS, Troitsk, Moscow region, 142190 Russia *P.N. Lebedev Physical Institute, RAS, Moscow, 119991 Russia ∇P.N. Lebedev Research Center in Physics, Moscow, 119991 Russia Institute of Low Temperatures and Structure Research PAN, Wroclaw, 50-950 Poland Moscow Institute for Physics and Technology, Russia International Laboratory of High magnetic fields and Low Temperatures, Wroclaw, 53-421 Poland
PACS: 74.25.Ha, 74.62.Bf, 74.70.Dd
Abstract
We report on successful synthesis under high pressure of a series of polycristalline GdFeAs O1-xFx high-Tc superconductors with different oxigen deficiency  and also with no fluorine.
We have found that the high-pressure synthesis technique is crucial for
obtaining the single-phase superconducting materials: by sythesizing the
same compounds with no pressure in ampoules, we obtained non-superconducting
materials with an admixure of incidental phases. Critical temperature for all
the materials was in the range 40 to 53 K. The temperature derivative of the
critical field dHc2/dT is remarkably high, indicating potentially high
value of the second critical field  T.
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