Professor Paolo G. Radaelli OSI

Abstract:

High real-space resolution atomic pair distribution functions (PDFs) have been obtained from ZnSe(1-x)Te(x) using neutron powder diffraction. Distinct Zn-Se and Zn-Te nearest neighbor (nn) bonds, differing in length by delta_r= 0.14Angstroms, are resolved in the measured PDF allowing the evolution with composition of the individual bond-lengths to be studied. The local bond-lengths change much more slowly with doping than the average bond-length obtained crystallographically. The nn bond-length distributions are constant with doping but higher-neighbor pair distributions broaden significantly indicating that most of the strain from the alloying is accommodated by bond-bending forces in the alloy. The PDFs of alloys across the whole doping range are well fit using a model based on the Kirkwood potential. The resulting PDFs give excellent agreement with the measured PDFs over the entire alloy range with no adjustable parameters.

Abstract:

Mesoscopic (500-2000 Angstrom) and microscopic (5-20 Angstrom) phase segregation with temperature and magnetic field was studied in the model manganite Pr0.7Ca0.3MnO3 by high-resolution neutron diffraction and inelastic neutron scattering. Intra-granular strain-driven mesoscopic segregation between two insulating phases, one of which is charge ordered (CO), sets in below the CO temperature in zero field. The CO phase orders antiferromagnetically, while the other insulating phase shows spin-glass behavior. After field-induced metallization, the CO phase coexists with a ferromagnetic metallic phase.