High Temperature Superconductivity from a strange metal phase via multiple cross overs

Since the discovery of High Temperature Superconductor (HTSC) materials more than twenty years ago, there have been tremendous amount of efforts, both experimentally and theoretically, to understand the microscopic mechanism behind superconductivity in them. It is quite commonly believed that the enormity of the problem is predominantly due to the lack of a comprehensive knowledge of the underlying normal state (i.e. when the samples are not superconducting). We look into the evolution of the normal state electronic excitations with temperature and carrier concentration in BizSrzCaCuzOs+o (BISCO 2212) HTSC using angle resolved photoemission spectroscopy (ARPES). Unlike in conventional superconductors, where there is a single temperature scale Te (i.e. superconducting critical temperature below which superconductivity appears) separating the normal from the superconducting state, HTSCs are associated with two additional temperature scales. One is the so-called pseudogap scale T*, below which electronic excitations exhibit suppression of low energy spectral weight. The second one is the coherence scale Teoh, below which electronic states become long-lived. We observed that T* and Teoh change strongly with carrier concentration, and they cross each other near optimal doping (carrier concentration ~0.16). Hence, there is a region in the normal state around the optimal doping where the single particle excitations are gapped as well as coherent [Fig. 1]. Quite remarkably, this is the region from which superconductivity with highest Te emerges. Our experimental finding that the two crossover lines intersect is not consistent with a "single quantum critical" point near optimal doping, rather it is more naturally consistent with theories of superconductivity for doped Mott insulators.