On the search for new superconducting systems: new discoveries since MgB2

The new superconductors discovered after MgB2 are discussed. Two serious claims have been made so far. The first claim concerns a graphite-sulfur composite undergoing a transition at 35 K. Since the magnetic transition clearly has the characteristics of a superconducting shielding effect, it would be worthwhile to investigate the possibility of increasing the sample volume responsible for this transition. However, there is no zero-resistance transition and the authors suggest that the superconductivity could be due to a surface effect. Carbon in this sample has the same structure as boron in MgB2. The second claim is more modest, but it is really a superconduting transition at 1 K. The sample is a single crystal of Cd2Re2O7. The Re cations could be in a mixed valence state, but the charges do not seem to localize by lowering the temperature. The importance of these results is the discovery of a new oxide structure such as the pyrochlore, which can host a superconducting state.