, Argonne National Laboratory
[Host: Seunghun Lee]
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 Tc 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 Tcoh, characterizing the onset of a significant enhancement in electronic
lifetime. We have observed that both T* and Tcoh 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 Tc. 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 Tc
emerges. Our experimental finding that T* and Tcoh intersect is not compatible with the
theories invoking âsingle quantum criticalâ point near optimal doping, rather it is more
naturally consistent with the theories of superconductivity for doped Mott insulators.
Thursday, January 26, 2012
Physics Building, Room 204
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