A Neutron Scattering Study of Superconducting Cuprates

One of the most intriguing phenomena in physics is the anomalous metallic state, such as high T_c superconductivity or colossal magnetoresistivity, that emerges when charge carriers are doped into a Mott insulator. A Mott insulator is a material that is insulating due to strong Coulomb re¬pulsions between electrons, even though it has partially filled electronic bands and band theory would predict it to be metallic. Virtual charge fluctuations in a Mott insulator generate a super-exchange interaction between the spins of unpaired electrons of neighboring ions, which leads to long-range antiferromagnetic order in many materials. Doping (adding charge carriers to) a Mott insulator induces phase transitions to metallic phases whose properties cannot be explained in terms of conventional metal physics. In the case of the layered perovskite La_2-xSr_xCu0₄, doping holes into the insulating parent compound La₂Cu0₄ rapidly weakens the static commensurate antiferromagnetic correlations that are replaced by incommensurate spin correlations with strong fluctuations. The incommensurate spin fluctuations exist over the entire superconducting region of 0.05 < x < 0.30, and disappear upon further doping when the system becomes nonsuperconducting. This indicates an intimate relationship between the incommensurate spin correlations and the superconductivity. One approach to describing the superconductivity by conventional superconducting theory, where the pairing occurs between weakly interacting quasi-particles, although the theory has to be pushed to the limits. In another approach, the competition between the kinetic energy of the holes, the Coulomb interactions between the holes and the magnetic interactions between spins leads to self-organized charge and spin inhomogeneities at the mesoscopic scale.
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Project Sponsored By: U.S. DOC- Natl. Institute Standards & Tech. (NIST)
Start Date: 8/1/2007 - End Date: 7/31/2009
Award Amount: $66,280.00