High temperature superconductors, such as La_2-xSr_xCuO_x (Tc=40K) and YBa₂Cu₃O_7-x (Tc=90K), were discovered in 1987 and have been actively studied during the past 19 years. In spite of an intense, world-wide, research effort during this time, a complete understanding of the copper oxide (cuprate) materials is still lacking. Many fundamental questions are unanswered, particularly the mechanism by which high-Tc superconductivity occurs. More broadly, the cuprates are in a class of solids with strong electron-electron interactions. An understanding of such "strongly correlated" solids is perhaps the major unsolved problem of condensed matter physics. High-Tc superconductors also have significant potential for applications in technologies ranging from electric power generation and transmission to digital electronics. Many companies are working to develop these high-Tc superconductivity applications and considerable progress has been made since 1987. One example of this is the scanning SQUID microscope an instrument pioneered at the Maryland CSR and being developed by Neocera, Inc.

Some indications of the difficulty in understanding the high-Tc cuprates can be found in their generic "phase diagram" as shown in Fig 1. This figure shows the different electronic states found in the cuprates as a function of temperature and carrier concentration. The cuprates are quite anisotropic (two-dimensional) materials and the conduction band carriers are introduced into the copper oxide planes either by oxygen variation or element substitution on lattice sites between the planes. This "phase diagram" illustrates some of the major unsolved questions concernng the high-Tc cuprates. For example; what is the nature of the phase transition from antiferromagnetic insulator to metal/superconductor as carrier doping increases; what causes the "pseudogap" state and what is its relation, if any, to the superconducting state found at lower temperature; is there a transition from a non-Fermi liquid state to a Fermi liquid state as doping increases beyond that of the maximum Tc compostion and is this related to a quantum critical point (QCP).

At the Center for Superconductivity Research (CSR) we are presently doing experiments on several important aspects of high-Tc superconductivity research.

1. Electron-doped High-Tc Superconductors (Greene)

2.

3. Oxide interfaces, Andreev bound states and Spin Injection (Venky)

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Center for Superconductivity Research, University of Maryland, College Park, MD 20742-4111
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