Hydrogen-modified superconductors: A review

We present an overview of hydrogen-containing superconductors, including metallic, cuprate, pnictide and carbon-based materials. Particularly in BCS-like superconductors, hydrogen introduces new phonon modes and these may lead to better or worse superconducting properties, with anharmonicity of the H potential being a decisive factor. The availability of the deuterium isotope, in addition to protium, makes hydrogen an extremely useful extra probe of mechanisms of superconductivity. While hydrogen modification has not so far led to any confirmed remarkable improvement in already existing metallic superconductors, hydrogen plays an important role in the synthesis of novel iron-based superconductors and as a substitutional impurity appears to increase the transition temperature. Most recently, a new record for the superconducting transition temperature of 203 K has been claimed for a metallic hydride under high pressure, H2S. We survey elemental hydrides and superconductors, discuss techniques for introducing hydrogen into materials, then review and summarise published knowledge about hydrogen-containing superconductors.