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College Park, Maryland      June 6 - 10 , 2004

WP62: Effect of the metal-insulator transition on oxygen phonons in Ca-doped YBa2Cu3O6

S. Chang (Ames Laboratory), R. J. McQueeney (Ames Laboratory; Iowa State University), P. Dai (Oak Ridge National Laboratory; University of Tennessee, Knoxville), F. Dogan (University of Missouri - Rolla), F. R. Trouw (Los Alamos National Laboratory)

High-temperature superconductors are based on antiferromagnetic insulating materials caused by strong electronic correlations. Doping charge carriers (holes) into this system creates a two-dimensional correlated metallic state in the CuO2 plane that becomes superconducting at low temperatures. The development of the metallic state with hole doping and the metallic state itself still remain poorly understood. Inelastic neutron scattering measurements of the lattice dynamics show evidence of strong and unusual electron-lattice coupling in many high-Tc compounds with the primary effect being the large softening of oxygen optical phonons in the energy range from 55-80 meV. Here we present the phonon density-of-states as a function of hole doping in YBa2Cu3O6 obtained from inelastic neutron scattering experiments. The holes were introduced via the systematic replacement of Y by Ca (Y1-xCaxBa2Cu3O6), where the Ca concentrations were chosen to straddle the metal-insulator transition (MIT) at x~0.2. Changes in the oxygen phonon modes at higher energies arising from electron-phonon coupling (55-80 meV) are well separated from any introduced Ca modes (<40 meV), similar to the unambiguous experiments LSCO where holes are introduced by Sr doping. In addition, Ca doping has the advantage of avoiding new oxygen phonon states, as happens when holes are introduced by increasing the oxygen content. We see clear changes in the phonon density-of-states when doping across the MIT, which we attribute to coupling of holes to oxygen phonons.

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