Nearly free electrons in the layered oxide superconductor Ag5Pb2O6
  We present first measurements of quantum oscillations in the layered oxide superconductor   Ag5Pb2O6. From a detailed angular and temperature dependent study of the dHvA effect we determine the electronic structure and demonstrate that the electron masses are very light, m^* is approximately equalt to 1.2 m_e. The Fermi surface we observe is essentially that expected of nearly-free electrons - establishing   Ag5Pb2O6 as the first known example of a monovalent, nearly-free electron superconductor at ambient pressure. 
Mike Sutherland, P.D.A. Mann, Christoph Bergemann, Shingo Yonezawa,   Yoshiteru Maeno