High-Temperature Superconductors

J. D. Jorgensen, H. Shaked, O Chmaissem, B. Dabrowski; in collaboration with D. G. Hinks and J.F. Mitchell, Emerging Materials Group, M. R. Norman, Condensed Matter Theory Group, and J. C. Campuzano, Synchrotron Radiation Studies Group

Our work on high-temperature superconductors focuses on the relationship among chemical composition, crystal structure, and superconducting properties.  At the present time, we are investigating what features of the chemistry and structure, beyond the use of defects to control the carrier concentration, are critical to superconductivity.  This work has led to a hypothesis for the ideal structure for a high-temperature superconductor and has revealed new aspects of the chemistry that are a reflection of the novel underlying physics of these materials.

Structural Anomalies at the Maximum Tc.  A characteristic feature of the high-temperature layered copper-oxide superconductors is the existence of a chemical composition that gives a maximum superconducting transition temperature, Tc, bounded below and above by the so-called under-doped and over-doped regimes.  This behavior is thought to be universal for the high-Tc superconductors.  In practice, there are only a few high-Tc compounds where the composition can be extended from the under-doped regime through the maximum Tc into the over-doped regime.  This has made it difficult to search for correlations between structural properties and Tc that extend over the entire doping range. We have studied the correlations between structure and Tc in a compound with the 123 structure in which both the under-doped and over-doped regimes can be accessed.  We observed a remarkable scaling between Tc and the buckling of the CuO2 planes; both go through a maximum at the same oxygen composition (and hence doping level) (see Figure).  This scaling implies a common origin for the behavior of Tc and the buckling.  Previous work has led to the conclusion that increased CuO2 plane buckling lowers Tc for a fixed chemical composition.  Thus, the observation of a maximum in the buckling at the maximum Tc indicates that, as the composition (and hence doping) is changed to increase Tc, there is a structural response that competes with superconductivity.

Coexistence of Superconductivity and Ferromagnetism in Layered High-Temperature Superconductors.  We have worked on layered compounds that allow the investigation of the relationship between superconductivity and magnetism, with the following results.  We published low-temperature structural information for Sr2RuO4 (one-layer compound that superconducts below 1 K), correcting errors in a previous publication that claimed structural anomalies.  We published a structure for Sr3Ru2O7 (two-layer compound that shows ferromagnetism, but no superconductivity) correcting errors in the literature concerning subtle distortions and their relationship to the magnetic ordering.  We then published the first structural information for the newly discovered ferromagnetic superconductor RuSr2GdCu2O8.  We showed: (1) that predictions of the magnetic structure were not correct; (2) that a structural response in the CuO2 planes at the Ru magnetic ordering temperature was evidence for overlap of the Ru and Cu electronic structures; (3) that dramatic effects of annealing on the superconducting properties probably result from elimination of domain walls that (for a reason not yet understood) suppress superconductivity.