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

We report on successful synthesis under high pressure of a series of polycrystalline GdFeAsO1 − xFx high-Tc 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 dHc2/dT is remarkably high, indicating potentially high value of the second critical field Hc2 ∼ 130 T.