Colloquia

High Temperature Superconductors (HTSCs) were discovered more than 25 years ago. However, a microscopic theory of them is yet to be realized. In order to identify the mechanism behind superconductivity in these systems, we must understand the normal state from which superconductivity emerges. From our detailed Angle Resolved Photoemission Spectroscopy (ARPES) measurements on Bi₂Sr₂CaCu₂O₈+_δ (BISCO 2212) HTSCs we have found that unlike conventional superconductors, where there is a single temperature scale T_c 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 states are partially gapped, while the second one is the coherence scale T_coh, characterizing the onset of a significant enhancement in electronic lifetime. We have observed that both T* and T_coh change strongly with carrier concentration and they cross each other near optimal doping, i.e. the carrier concentration at which an HTSC attains its maximum T_c. Furthermore, there is an unusual phase in the normal state where the electronic excitations are gapped as well as coherent. Quite remarkably, this is the phase from which the superconductivity with maximum T_c emerges. Our experimental finding that T* and T_coh intersect is not compatible with the theories invoking “single quantum critical” point near optimal doping, rather it is more naturally consistent with the theories of superconductivity for doped Mott insulators.