Magnetic and superconducting properties of FeAs-based high-T c superconductors with Gd

We report on successful synthesis under high pressure of a series of polycrystalline GdFeAsO1 − x F x high-T c superconductors with different oxygen deficiency x = 0.12−0.16 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 admixture of incidental phases. Critical temperature for all the materials was in the range 40 to 53 K. The temperature derivative of the critical field dH c2/dT is remarkably high, indicating potentially high value of the second critical field H c2 ∼ 130 T.