Nearly-free electron superconductor Ag$_5$Pb$_2$O$_6$

Superconductivity in the silver lead oxide Ag$_5$Pb$_2$O$_6$ has been discovered below 52~mK [1,2]. Although its $T_c$ is one of the lowest among the known oxide superconductors, this oxide is interesting from the viewpoint that it is the first superconductor with a nearly-free-electron Fermi surface. This fact is revealed by our quantum oscillation study [3] as well as recent band-calculation studies, which concluded that the system possesses one near-spherical Fermi surface with a small electron- mass enhancement. We will present its type-I superconducting properties, as well as the properties of the normal state where the resistivity varies nearly as $T^2$ up to room temperature [1]. \newline [1] S. Yonezawa and Y. Maeno, Phys.~Rev.~B 70, 184523 (2004). [2] S. Yonezawa and Y. Maeno, Phys.~Rev.~B 72, 180504(R) (2005). [3] M. Sutherland et al., Phys.~Rev.~Lett. 96, 097008 (2006).