Microscopic phase separation in the overdoped region of high-Tc cuprate superconductors

We propose a phenomenological model for high-Tc superconductors (HTSC) assuming: (1) a microscopic phase separation between superconducting and normal-metal areas in the overdoped region; and (2) existence of a homogeneous superconducting phase only below the pseudo-gap T∗ line, which shows a sharp reduction towards T∗∼0 at a mildly overdoped critical concentration xc. This model explains anomalous doping and temperature dependences of ns/m∗ (superconducting carrier density/effective mass) observed in several overdoped HTSC systems. We point out an analogy to superfluid 4He/3He films, and discuss an energetic origin of microscopic phase separation.