Magnetic and Superconducting Properties of FeAs-based High-Tc Superconductors with Gd

We report on successful synthesis under high pressure of a series of polycristalline GdFeAs O$_{1-x}$F$_x$ high-$T_c$ superconductors with different oxigen deficiency $x=0.12\div 0.16$ 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 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 $dH_{c2}/dT$ is remarkably high, indicating potentially high value of the second critical field $H_{c2}\sim 130$ T.