Student Colloquium: Junjing Zhao
"Emergence of High Temperature Superconductivity from a strange metal phase via two crossovers"
J. Zhaoa, b, U. Chatterjeeb, D. Aia, b, and J. Campuzano, b
Department of Physics, University of Illinois at Chicago, Chicago, IL 60607
Materials Science Division, Argonne National Lab, Argonne, IL 60439
Abstract: High Temperature Superconductors (HTSCs) were discovered more than 25 years ago, however, a microscopic theory is yet to be realized for this unique class of materials. In order to identify the mechanism behind superconductivity in the HTSCs, we must understand the normal state from which it emerges. From our detailed Angle Resolved Photoemission Spectroscopy (ARPES) measurements on Bi2Sr2CaCu2O8+δ (Bi2212) HTSCs we find that unlike in conventional superconductors, where there is a single temperature scale Tc separating the normal from the superconducting state, HTSCs are associated with two additional temperature scales. One is the pseudogap scale T*, below which electronic excitations exhibit an energy gap. The second is the coherence scale Tcoh, below which electronic excitations become long-lived. We observe that both of these temperature scales change strongly with carrier concentrations and they cross each other near optimal doping, i.e., the carrier concentration at which HTSCs has the maximum Tc. 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 Tc emerges. Our experimental finding that the two crossover lines T* and Tcoh intersect is not compatible with the theories invoking “single quantum critical” point near optimal doping, rather it is more naturally consistent with theories of superconductivity for doped Mott insulators.