High-pressure study of the Raman modes in YBa2(Cu0.96Ni0.04)4O8

URL:http://link.aps.org/doi/10.1103/PhysRevB.60.4363 DOI:10.1103/PhysRevB.60.4363We present a study of the Raman phonons in YBa2(Cu0.96Ni0.04)4O8 under hydrostatic pressure in the superconducting phase. A comparison with our earlier work on the undoped YBa2Cu4O8 shows that the pressure coefficients of two vibrational modes involving oxygen atoms differ significantly from those of the corresponding modes in the undoped material. These are the O(1) chain mode which shifts 33% faster and the O(2)-O(3) in-phase mode which shifts 23% slower than the undoped counterparts. The other Raman modes in the Ni-doped sample shift in a manner similar to the undoped material. The observed behavior of the O(1) chain and O(2)-O(3) in-phase modes in YBa2(Cu0.96Ni0.04)4O8 under pressure and the softening of the Cu(1) Ag mode frequency with increasing Ni doping suggest that the Ni atoms substitute for the Cu atoms in the chain, which in turn decreases the compressibility in the vicinity of the Cu(Ni) chain atom.This work was supported by U.S. Department of Energy Grant No. DE-FG02-90ER45427 through the Midwest Superconductivity Consortium. D.J.P. thanks the U.S. Department of Education for support through Grant No.P200A50259

Photoluminescence of short-period GaAs/AlAs superlattices: A hydrostatic pressure and temperature study

URL:http://link.aps.org/doi/10.1103/PhysRevB.58.7222 DOI:10.1103/PhysRevB.58.7222The temperature and pressure dependence of type-I and -II transitions from photoluminescence (PL) spectra in a series of (GaAs)m/(AlAs)m superlattices show that the temperature dependence of energy bands can be described very well with a Bose-Einstein-type equation. From these measurements the parameters that describe the temperature dependence of excitonic transition energies and the corresponding broadening of the PL line are deduced. The pressure dependence of the PL linewidths of the type-I exciton as a function of pressure and temperature yield the intervalley deformation potential. Beyond the type-I-type-II crossover, the PL linewidth increases as a function of both pressure and temperature. The electron-phonon deformation potential for Γ-X scattering is found to be temperature dependent.We thank S. Satpathy and S. Zollner for valuable discussions. One of us ~H.R.C.! acknowledges support by the NSF under Grant No. DMR-9633107. M.C. thanks the U.S. Army for support through Grant No. DAAL03-92-0381. The work at Purdue University was supported by the National Science Foundation: Materials Research Science and Engineering Center Grant Nos. DMR 94-00415 and DMR 93-03186

Neutron diffraction studies on ABO₃ (A=La, Sr, B=Fe, Co, Ni, Cu, Mn, Ti) perovskite used in solid oxide fuel cell (SOFC) and double perovskite Ba₂YRu[0.85-subscript]Cu[0.15-subscript]O₆ superconductor

Title from PDF of title page (University of Missouri--Columbia, viewed on Feb 17, 2010).The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file.Dissertation advisor: Dr. William B. Yelon.Vita.Includes bibliographical references.Ph. D. University of Missouri--Columbia 2009.Dissertations, Academic -- University of Missouri--Columbia -- Physics.ABO₃(A=La, Sr, B=Fe, Co, Ni, Cu, Mn, Ti) perovskites are of great interest due to their mixed electronic and oxygen ion conductivity. They are candidates for the electrodes of SOFCs. This study investigates the effects of substitutions at A sites and/or B sites on the crystal and magnetic structure, oxygen vacancies, and the thermal expansion coefficients at different temperatures and gaseous environment. The oxygen vacancy concentration can relax the perovskite distortion and has a close relationship with the magnetic properties. La₀.₆Sr₀.₄Fe₀.₈Co₀.₂O₃₋[delta subscript], and La₀.₈Sr₀.₂Fe₀.₈Co₀.₂O₃₋[delta script] can be good candidates for the cathodes of SOFCs at intermediate temperature. The double perovskite Ba₂YRu₀.₈₅Cu₀.₁₅O₆ superconductor and a mixture of 5wt% YBa₂Cu₃O₇₋[delta subscript] and undoped Ba₂YRuO₆ were investigated with the aid of neutron diffraction. The 1:1 B site ordering is observed and long range antiferromagnetic ordering of the Ru sublattice with a type I magnetic structure appears when the temperature is below 38K. The decomposition of Cu-doped Ba₂YRuO₆ into undoped Ba₂YRuO₆ and YBa₂Cu₃O₇₋[deltas subscript] is not seen. YBa₂Cu₃O₇₋[delta subscript] is not stable at the temperature used to prepare the Cu-doped Ba₂YRuO₆ superconductor. These results confirm the presence of superconductivity without CuO₂ planes