Superconductivity of cobalt oxide hydrate, Nax(H3O)zCoO2 · yH2O

• The experimental results of the Nax(H3O)zCoO2·yCoO2·yH2O superconductivity was reviewed.
• The superconductivity is unconventional.
• Contradictory interpretations and results of physical properties of the substance are discussed in details.

Nax(H3O)zCoO2·yCoO2·yH2O has a highly two-dimensional triangular lattice of Co ions and exhibits the superconducting transition at TC=4.7TC=4.7 K. It is classified as an extreme type-II unconventional superconductor and the superconducting gap function has a line-nodal structure. The superconducting phase diagram shows that the superconductivity appears on the verge of a magnetic phase. In addition to the phase diagram, microscopic experiments on the normal state and the magnetic phase suggest that the superconductivity is induced by magnetic fluctuations. However, despite intensive study on Nax(H3O)zCoO2·yCoO2·yH2O and related compounds, there remain some contradictory interpretations and experimental results regarding the Fermi surface topology and superconducting pairing symmetry. They are summarized and discussed in details.