Ever since high-temperature superconductivity was discovered in copper oxide or “cuprate” materials in 1986, researchers have been searching for similar behaviour in other metal oxides. Now Takayoshi Sasaki from the Japan Science and Technology Corporation and colleagues from the National Institute for Materials Science in Tsukuba have found a cobalt oxide compound that becomes a superconductor when water is added to it. There is a strong similarity between the cuprate superconductors and cobalt oxide materials, which suggests that the fundamental physics in both might be the same (K Takada et al. 2003 Nature 422 53).
Superconductors are compounds that lose their electrical resistance below a certain “transition temperature”. High-temperature cuprate superconductors consist of layers of copper and oxygen, separated by metal atoms such as yttrium and barium. The supercurrent flows through the copper oxide layers. Attempts to find superconductivity in other transition-metal oxides have been unsuccessful and researchers believe that the copper oxide layer itself might be essential for superconductivity.
Sasaki and co-workers used layers of cobalt oxide, separated by sodium, to make a superconductor. This was achieved with an oxidation process which involved the incorporation of water molecules into the structure (see figure). X-ray diffraction on this material revealed a marked increase in the spacing between two cobalt oxide layers, from around 10 Å before the oxidation process to around 20 Å after. This increase occurs to accommodate the “guest” water molecules.
The researchers found a sharp decrease in the magnetic susceptibility – the ease with which a material can be magnetized by an external field – at about 5 Kelvin. The large susceptibility value suggests that the material undergoes a superconducting transition at this temperature. The electrical resistivity of the material also decreases sharply around the same temperature, which adds further evidence for a superconducting transition.
The two-dimensional nature of the copper oxide layers is thought to be important in this class of superconductors. Similarly, the large separation of the cobalt oxide layers used in the Japanese experiment seems to be crucial in the superconducting behaviour of this compound. The main difference between the two systems is that cobalt ions form a triangular lattice as opposed to the square lattice seen in the cuprates.
The group now hopes to look at how the spacing between the layers, and their composition, influences the superconducting properties of these materials.
