W. Holm

Structure and properties of Ba6 − xLnxNb10O30, Ln = La, Ce and Nd compounds

Ba{sub 6{minus}x}Ln{sub x}Nb{sub 10}O{sub 30} compounds, Ln = La, Ce and Nd, with a tetragonal tungsten bronze-type structure have been synthesized and were characterized by X-ray powder diffraction and EDS analysis. The homogeneity regions of the obtained compounds were found to be 0 {le} x {le} 2 for La, and 0 {le} x {le} 1.5 for Ce and Nd. The crystal structures of Ba{sub 5}LaNb{sub 10}O{sub 30} and Ba{sub 4}La{sub 2}Nb{sub 10}O{sub 30} were refined using X-ray powder diffraction data. Refinement indicated the presence of vacancies in the Ba/La positions. This observation was supported by oxidation studies of the La-substituted compounds. The temperature dependence of the normalized resistivity of the compounds showed a nonmetallic behavior. The increase in the R/R{sub 273} ratio with decreasing temperature is less distinct for the phases with the higher concentration of rare-earth cation.

Studies on Ba2Nb5−xTixO9 (0≤x≤1.75)

Abstract The structure of Ba 2 Nb 5 O 9 can be described as an intergrowth between perovskite- (BaNbO 3 ) and NbO-type slabs. The substitution of Ti for Nb according to the formula Ba 2 Nb 5− x Ti x O 9 (0.00≤ x ≤1.75) has been investigated. The decrease in unit cell parameters as a function of overall composition determined by microanalysis shows the maximum value of x to be around 0.9. At higher x values increasing amounts of BaNb 1− x Ti x O 3 and Nb were found. Rietveld refinements of the structure using X-ray powder diffraction data show Ti to prefer Nb positions in the perovskite slab (NbO 6 octahedra) and the Nb positions in the NbO slab closest to the perovskite slabs. This latter Nb is five-fold coordinated by oxygen atoms, forming a square pyramid. The third Nb position, in the middle of the NbO slab with a square planar coordination of oxygen atoms, remains undoped. The electrical resistivity and magnetic susceptibility of the x = 0.0, 0.25, 0.50 and 0.75 samples were measured. All samples were found to be metallic conductors and temperature-independent paramagnetic.

Density of states fluctuations in the magnetoresistivity of YBa2Cu3O7−δ

Abstract The effects of fluctuations in the density of states (DOS) have been studied. A regularisation method was used to calculate the otherwise divergent DOS term. It was found that the DOS term can explain an observed change of sign in the c-axis magnetoconductivity but that its effect on in-plane magnetoconductivity is less significant.

Fluctuation magnetoconductivity in Fe doped YBa2Cu3O7−δ. Sign change for B and I parallel to the planes

Abstract The magnetoconductivity in Fe doped YBa 2 Cu 3 O 7−δ single crystals has been measured in the temperature region above T c where superconducting fluctuations are known to be important. For small Fe concentrations and B¦c we obtain excellent agreement with accepted fluctuation theories. However, at higher Fe concentrations this description seems to break down. For B ⊥ c we observe an anomalous dependence on the relative orientation of current and field, both applied along the ab -planes. For B¦I the magnetoconductivity is negative, as expected. For B ⊥ I we observe a cross-over to a positive magnetoconductivity above about T c + 10 K.

Density of states fluctuations in the c-axis magnetoconductivity of YBa2Cu3O7−δ

The magnetoresistivity up to 12 T of YBa2Cu3O7−δ has been measured with B//I//c fromTc+3 K toTc+40 K. A change of sign was observed, from positive nearTc to negative at temperatures above approximatelyTc+10 K. This previously not reported change of sign can be explained by recent fluctuation theories that include fluctuations in the normal density of states (DOS).

Study of magnetoresistance in oriented YBa2Cu3O7−δ thin film

The magnetoresistance of a c-axis oriented YBa2Cu3O7-delta thin film has been measured in magnetic fields, B, up to 12 T. The magnetic field was oriented both perpendicularly and parallel to the ba ...

Negative magnetoresistance in an YBa2Cu3O7−δ thin film☆

Abstract Negative magnetoresistance was observed in a thin YBa2Cu3O7−δ film which could be correlated with the presence of magnetic impurities. After empirical compensation for this effect the fluctuation conductivity was well described by the Aronov-Hikami-Larkin theory.

Magnetoresistivity of untwinned single crystals of YBa2Cu3O7-δ

Abstract The magnetoconductivity of untwinned single crystals has been measured for in-plane current and various orientations of the magnetic field. Some results and preliminary analyses in terms of fluctuation theories are presented.

C-axis Transport in YBa2Cu3O7-δ

For a number of years experimental results on the c-axis resistivity of YBa2Cu3O7-δ were strongly contradictory both regarding the magnitude of the resistivity, p, and the sign of the normal state temperature derivative, dp/dT [1]. Further studies have shown however, that both these properties are extremely sensitive to oxygen concentration, and in fully oxygenated samples also the c-axis resistivity is metallic [2]. In recent years an increasing number of studies have been devoted to experimental investigations of c-axis transport. The layered structure of the high temperature superconductors is a key element in understanding many of their unusual properties and c-axis transport studies can help clarify central issues such as coupling between charge carriers in different planes and the nature of c-axis charge transport, the role of the chains in 123 compounds, and the importance of the anisotropy.

Transition from integral to pancake like vortices in single crystal YBa2Cu3O7−δ

Abstract Single crystals of YBa2Cu3O7−δ were studied in a six terminal voltage contact configuration and magnetic fields up to 8 T applied parallel and perpendicularly to the basal plane. Sample thickness dependent phase diagrams were obtained for the magnetic field dependent temperature Tbr, where vortices lose their coherence over the c-axis length of the sample.