EAGER: Local Structure and Dynamics in Tc-Enhanced Overdoped Cuprates: Beyond BCS?

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Non-technical Abstract:Superconductivity is a property of certain materials in which they lose electrical resistance, giving them the capability of carrying electric currents with none of the losses that occur in normal metal electrical conductors. This enables electromagnets such as the ones used for MRI scans to operate for weeks or even months without having to replenish them, and superconducting wires would save the large fraction of the original energy that is lost in transmission. The problem is that superconductivity that can used in these applications only occurs at very low temperatures, requiring expensive and complicated refrigeration equipment. This is, however, superconductivity as we have known it since it?s discovery just over 100 years ago. Our team has found that a certain class of copper-oxide superconductors may follow a different set of rules, and in particular one compound shows behaviors that contradict the existing theory. A second type of superconductivity could provide insights or even the means to extend superconductivity to higher temperatures to save energy by substantially increasing the efficiency of electrical devices.Technical Abstract:A review of the cuprate literature reveals the surprising fact that the phase diagram is "universal" because the YBa(2-x)Srx(CuO2)2(Cu(1-y)My)O(7+delta) class of "overdoped" cuprates has been neglected. When prepared by the High Pressure Oxygen (HPO) method using chlorate or peroxide their transition temperatures increase continuously within their 50-80 K range even as their O stoichiometry is increased to give an excess charge on the CuO2 coppers well past the 0.27 value where the superconductivity ends in conventional materials. Since the HPO method only produces fine powders and not crystals only a few techniques are amenable to their study, including the X-ray Absorption Fine Structure measurements that are the principal capability of this project team was that the Cu-neighbor atom pairs exhibit highly anharmonic behavior correlated with the superconductivity in the c direction, including a non-degenerate Cu2-apical O double well potential. Renormalization of this potential causes the energy difference between its two components to track the gap energy to the Tc-30 K lower limit of the team's measurements. Even more radical are results from Sr2CuO3.3, a La2CuO4+delta structural analog. The static structure contains only partial CuO2 planes, in contrast to the fully ordered ones in all other cuprates. These are replaced by more complex, three-dimensional Cu-O structures. In addition, the superconducting transition occurs concomitantly with a structural transformation that is a shift of a set of these O ions that is sufficiently large to modify the Fermi surface by an energy far larger than that of the gap. This would be a substantial expansion of conventional BCS theory, or possibly even indictative of a novel pairing and condensation mechanism. Crucial issues to be addressed by this EAGER project would be a direct comparison with conventional cuprates, extension to lower temperatures, and measurements at the complementary Sr edge.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.