Probing Two Particle Correlations by Angle Resolved Photoemission

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Non-technicalHigh temperature superconductors are fast becoming important for the country's energy needs. They are expected to have their greatest impact in devices which help stabilize the national power grid, where demand is expected to double by 2010, or in delivering sufficient power to cities undergoing re-urbanization as a result of increasing energy costs. Although some commercial applications are now available, further improvement in materials is imperative. Several approaches are necessary for this improvement, starting with the discovery of new materials, understanding the origin of superconductivity, the increase in superconducting transition temperatures, and the improvement of 2nd generation conductors. It is in the understanding of these complex new materials that this proposal will contribute to this enterprise, by developing new experimental techniques. First is the direct detection of pairing of electrons, the basic process that leads to superconductivity. The work will address the long-debated question of whether electrons form transient pairs at temperatures significantly higher than those where superconductivity sets in, in a strange state called the pseudogap. If this pairing can be directly observed, then one could find ways to increase the critical temperature of superconductors by manipulating the proper quantities that control the pairing. The second experiment will study how this pairing, and its destruction, are affected by electrical currents applied to the material. It is believed that in less than perfect materials, which naturally occur in large scale manufacturing, electric currents affect superconductivity by changing the phase between paired electrons, rather than by the self-generation of magnetic fields. In addition, this work will continue our excellent track record of training minorities for success in academia and industry.TechnicalThis project aims to determine whether the electrons in the high temperature superconductors pair above the superconducting transition temperature, in the ill-understood pseudogap phase. It has long been debated whether the pseudogap state has its origin in the pairing of electrons in individual pairs, which all become coherent in phase as the pairs condense into a macroscopic superconducting state as the temperature is lowered. There are other proposals for the origin of the pseudogap which are unrelated to superconductivity. So far there is no direct proof of this pairing, or its absence. This work proposes a novel experimental technique, where a pair of electrons is detected as they absorb a single photon, which breaks the pair and causes them to be simultaneously emitted by the photoelectric effect. The project will also examine the state of electrons under an applied current. In less than optimally doped high temperature superconductors, superconductivity might be destroyed by the alteration of the phase between pairs by the applied current, rather than the usual effect of vortices from self-generated magnetic fields. The students and post-doctoral fellows are exposed to work at the frontier of condensed matter physics, as this work involves an extensive national and international collaboration between groups that grow samples, carry out a variety of experiments on the same samples, and apply theoretical work to the development of new methods of data analysis.