Bogdan M. Dabrowski

Pressure-induced charge transfer and dTc/dP in YBa2Cu3O7−x

Abstract Subtle pressure-induced structural changes in YBa2Cu3O6.93 and YBa2Cu3O6.60 have been measured by neutron powder diffraction for samples in a hydrostatic helium-gas pressure cell. Small, but significant, differences in the compression of particular Cu-O bonds (notably Cu(2)-O(4)) are observed. However, when the charges on the two copper sites are calculated, requiring overall charge conservation versus pressure, it is found that the net pressure-induced charge transfer of holes from Cu(1) to Cu(2) is essentially the same for both systems. We conclude that the much smaller value of dTc/dP for YBa2Cu3O6.93 results from the fact that, in the 90 K superconductor, the Tc has already reached its optimum value and the introduction of additional hole carriers cannot further increase Tc.

Oxygen vacancy ordering and superconductivity in YBa2Cu3O7−x

Abstract The superconducting properties of YBa 2 Cu 3 O 7−x have been shown to depend markedly on the overall oxygen stoichiometry and on the ordering of oxygen vacancies on the available sites. YBa 2 Cu 3 O 7−x has been studied in situ by high-temperature neutron powder diffraction in controlled oxygen atmospheres in order to learn the structural properties of the system in thermodynamic equilibrium. Additionally, metastable, oxygen-deficient samples produced by quenching into liquid nitrogen from high temperature and various oxygen partial pressures have been studied at room temperature or low temperature in order to correlate the structural and superconducting properties. Such experiments have shown that a systematic relationship exists between the superconducting transition temperature, T c , the number of oxygen vacancies, and the degree of oxygen vacancy ordering. This brief paper reviews our work in these areas.

Solubility and superconductivity in RE(Ba2-xREx)Cu3O7+δ systems (RE=Nd, Sm, Eu, Gd, Dy)

Solid solutions of RE(Ba2-xREx)Cu3O7+ delta (RE=Nd, Sm, Eu, Gd, Dy) for x=0 to x=0.5 have been investigated. X-ray and resistivity measurements show that there exists a solid solution region, through which the structure changes from orthorhombic to tetragonal and the superconducting properties are depressed. The solubility limits depend strongly on the size of the rare-earth ion, with the smallest (Dy) showing no appreciable solubility. The superconducting transition temperature versus x for all of the rare-earth-ion substitutions falls on a universal curve, indicating that the Ba sites are extremely ionic and magnetically isolated.

Phase separation in Nd2_xCexCuO4 evidence for superconductivity at a single composition

Abstract A series of samples with bulk compositions Nd 2-χ Ce χ CuO 4 , 0≤χ≤0.25, have been examined by pulsed neutron and high-resolution synchrotron X-ray diffraction. The diffraction data show that single phase material is formed only for the undoped compound, Nd 2 CuO 4 , and (possibly) for the optimum superconducting composition, χ≈0.165. Other samples appear to be a mixture of this superconducting composition and non-superconducting material of other compositions. The phase separation cannot be explained in the context of an equilibrium phase diagram and, thus, must result from non-equilibrium behavior during synthesis.

Tunneling spectroscopy in superconducting Ba1−xKxBiO3: Direct evidence for phonon-mediated coupling

Abstract Tunneling measurements have been performed on bulk samples of Ba1−xKxBiO3 (x=0.375) using the natural, surface oxide as the tunnel barrier and an In thin film counterelectrode. High bias conductance and second derivative measurements show reproducible features in the energy range 10 to 65 meV which are characteristic of phonon effect as seen in conventional superconductors. The structures appear at energies which correspond to peaks in the phonon density of states as measured by inelastic neutron scattering and Raman spectroscopy. Estimates of the electron-phonon coupling strength, λ, and the average phonon frequency, , from the tunneling data indicate that phonon mediated pairing is responsible for superconductivity in this compound. Strong lifetime broadening effects are observed in the gap-region conductance of these junctions indicating gapless superconductivity for samples with concentration, x=0.375.

Pressure-induced structural changes in La1.85Sr0.15CuO4

Abstract The structure of La 1.85 Sr 0.15 CuO 4 has been studied versus pressure to 0.53 GPa at room temperature using neutron powder diffraction. Contrary to a recent report [1], we observe no anomalous change in the Cu-O(2) (apical) bond length in the pressure range studied. However, the pressure-induced shortening of this bond is larger than the cell compression along the same direction, suggesting that pressure-induced charge transfer may provide an explanation for the effect of pressure on T c .

Synthesis of high‐density Ba1−xKxBiO3 superconducting samples

A synthesis procedure for producing high‐density Ba1−xKxBiO3 has been developed. A novel melt‐processing technique is described which yields strong, near 100% dense superconducting material. This material is much more amenable to experimental study than the powder samples produced by the previous synthesis methods.

Variable-range hopping conduction in Ba1−xKxBiO3−y system

Abstract Normal state resistivity of the Ba1−xKxBiO3−y system is reported for various K concentrations. In the concentration range where the material is superconducting, phonon-assisted variable-range hopping [exp(T0/T) 1 4 ] is observed between 160 and 300 K.

La2CuO4+δ and La2NiO4+δ: Phase separation resulting from excess oxygen defects.☆

Abstract The structural, superconducting and magnetic properties of La 2 CuO 4+δ and La 2 NiO 4+δ were studied by neutron powder diffraction, thermogravimetric analysis, resistivity and magnetic susceptibility measurements. La 2 CuO 4+δ was found to phase separate around 320K into antiferromagnetic La 2 CuO 4 and superconducting La 2 CuO 4.08 . La 2 NiO 4+δ forms a compound isostructural to La 2 CuO 4+δ , for 0.02≤δwere studied by the system separates into two phases with different defect concentrations and structures. The structure of the interstitial oxygen defect was determined for La 2 NiO 4.18 .

Oxygen content and the synthesis of Ba1−xKxBiO3−y

Abstract In situ thermogravimetric analysis during the synthesis of Ba 1− x K x BiO 3− y shows the various processes occurring as a function of temperature and the gas composition of the ambient atmosphere. This allows the synthesis procedure to be optimized. Single-phase material can be produced with the metal-ion stoichiometry of the starting oxide mixture.