High-pressure synthesis and physical properties of new iron (nickel)-based superconductors

We have utilized a high-pressure (HP) technique to synthesize a series of newly-discovered iron (nickel)-based superconductors. For the LnFeAsO-based superconductors (Ln = lanthanide), we show that the introduction of oxygen (O)-deficiency in the LnO layers, which is achievable only through HP process, is an effective way to dope electron carriers into the system, which results in yielding the superconducting transition temperature (Tc) comparable with those for F-substituted counterpart. The effect of O-deficiency, variation of Ln ions, and the external pressure on Tc are examined. All the experimental data indicate strong correlation between the crystal structure and the superconductivity of the oxypnictide superconductors. Upper critical field measurement on single crystalline sample of PrFeAsO1−y shows the superconducting anisotropy of 5, which is smaller than cuprates. We also demonstrate that HP technique is applicable for the so-called ‘122’ systems, by showing the results on polycrystalline (Ca, Na)Fe2As2, (Ba, K)Fe2As2, as well as single crystal BaNi2P2 samples.