Magnetic, phonon, and structural scattering studies of the electron-doped high-temperature superconductor neodymium cerium copper oxide

This thesis describes single-crystal neutron and x-ray scattering experiments on the electron-doped high-temperature superconductor Nd 2-xCexCuO 4+delta (NCCO). First, it reports on the evolution with doping of the instantaneous antiferromagnetic spin correlations in the paramagnetic phase, the Neel temperature, and the ordered moment. The spin correlations of as-grown, non-superconducting NCCO (0 ≤ x ≤ 0.18) can be effectively described by quantum Monte Carlo calculations for the nearest-neighbor spin-1/2 square-lattice Heisenberg antiferromagnet with random site dilution. However, quantum fluctuations have a stronger effect on the ordered moment, which decreases more rapidly than for the quenched-disorder model. On the other hand, the doping dependence of the spin correlations and ordered moment are found to be in qualitative agreement with recent calculations for the Hubbard Model. Second, it is demonstrated that it is possible to use inelastic x-ray scattering to measure phonon dispersions in complex oxide materials. Specifically, NCCO is found to exhibit phonon dispersion anomalies of the highest longitudinal optical modes that are similar to those previously observed for hole-doped cuprates. These anomalies are attributed to a strong electron-phonon coupling. Finally, it is discovered that the oxygen-reduction procedure necessary to create a superconducting phase causes NCCO to partially decompose to quasi-two-dimensional (Nd,Ce)2O3 which precipitates epitaxially on the CuO 2 planes inside the NCCO matrix. This secondary phase has a significant paramagnetic response to a magnetic field, and the detailed results presented here invalidate recent reports of a magnetic-field-induced quantum phase transition of NCCO from a superconducting to a long-range-ordered antiferromagnetic state.