  The 1111-type iron arsenide hydride CaFe1-xCoxAsH was synthesized by
high-pressure solid-state reaction, and its electronic structure and
superconducting properties were investigated. Bulk superconductivity was
observed at x = 0.07-0.26. A maximum superconducting critical temperature Tcmax
of 23 K was observed at x = 0.07. These values are in agreement with those of
CaFe1-xCoxAsF. The calculated Fermi surface of CaFeAsH has a small
three-dimensional 3D hole pocket around the Gamma point. This is a result of
weak covalent bonding between the As 4p and H 1s orbitals. No such covalency
exists in CaFeAsF, because the energy level of the F 2p orbital is sufficiently
deep to inhibit overlap with the As 4p orbital. The similar superconducting
properties of CaFe1-xCoxAsH and CaFe1-xCoxAsF are explained on the nesting
scenario. The small 3D hole pocket of CaFe1-xCoxAsH does not significantly
contribute to electron excitation. These findings encourage exploration of
hydrogen-containing 1111-type iron-based materials with lower anisotropies and
higher Tc applicable to superconducting wires and tapes.
