Neutron scattering studies of superconducting YBCO and relaxor ferroelectrics lead(magnesium-niobium)oxide and lead(zinc-niobium)oxide

Neutron scattering is an important tool in studying the structure, lattice vibrations, and spin dynamics in materials. Neutron elastic and inelastic scattering was used in this work to investigate the fluctuations and the structure in the YBa2CuO6+x superconductor as well as the relaxor ferroelectrics Pb(Mg1/3Nb2/3)O 3 and Pb(Zn1/3Nb2/3)O3. The spin fluctuations in the YBa2Cu3O6+x superconductor were studies for x in the underdoped region with x = 0.5 (Tc = 59 K). YBa2Cu 3O6.5 has good structural properties with well ordered oxygen chains. This provides a unique opportunity to study the spin fluctuations in a high-temperature superconductor in the presence of minimal structural disorder. The spin response in the YBa2Cu3O6.5 superconductor has been investigated using both time-of-flight and triple-axis spectrometers up to energy transfers of about 110 meV. The spin spectrum in YBa2Cu3O6.5 is dominated by a strong resonance at 33 meV. The low-energy scattering is characterized by one-dimensional incommensurate scattering with incommensurate peaks displaced along [100] direction. The high-energy fluctuations in the range from ho = 40--100 meV are qualitatively similar to the linear spin-waves measured in the insulator.; The lattice dynamics and structural properties were measured in the ferroelectrics Pb(Mg1/3Nb2/3)O3 and Pb(Zn1/3Nb 2/3)O3. Relaxor ferroelectrics have attracted much attention recently due to their exceptional piezoelectric properties. Despite the previous belief that these two materials are very different, the lattice dynamics are found to be nearly identical. The main difference between the two materials is the presence of a large near-surface region in Pb(Zn1/3Nb 2/3)O3 which undergoes a well defined structural transition. The relaxor transition can be interpreted in terms of a random field model where the appropriate universality class is the three-dimensional Heisenberg model with cubic anisotropy. The random field model can be further applied to reconcile previous apparent conflicting interpretations of the lattice dynamics.