Oxygen Doping Levels Research Articles

Effect of Bi–Sr replacement and oxygen doping on vortex-matter phase transitions in Bi 2+ xSr 2− xCaCu 2O 8+ δ

To investigate the effect of oxygen doping level with a wide range on vortex-matter phase transitions, magnetization for H‖ c was measured for Bi 2+ x Sr 2− x CaCu 2O 8+ δ single crystal with x=0.05–0.25 and various doping level δ. Optimum condition against δ for the superconducting transition temperature was found to depend on x, which is reasonably explained from the difference in valences between Bi and Sr. The second-peak of the low temperature magnetization curve was observed in all the samples, ranging from ∼120 Oe for the mostly underdoped sample to ∼1300 Oe for the most overdoped one. For the optimally-doped samples the second-peak was observed at ∼400 Oe almost independent of x. Vortex-melt transition field, H m, was well evaluated for the as-grown samples and for the slightly overdoped ones. The shift of the melting line, H m( T), with x and δ can be explained from the change in anisotropy and penetration depth. Near the superconducting transition temperature vortex transition is attributed to the transition due to the decoupling of pancake vortices, while at lower temperatures to the melting transition with both the electromagnetic- and the Josephson-coupling between vortices.

Disorder-driven intermediate state in the lattice melting transition ofBi2Sr2CaCu2O8+δsingle crystals

Scanning Hall probe microscopy has been used to study high quality single crystals of the ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ high-temperature superconductor over a wide range of temperatures at low fields. We observe pronounced stripes of high flux density which persist well above the melting line almost up to the critical temperature of the crystal. We show that these regions impose a disorder-driven intermediate state on the vortex lattice melting transition with a structure composed of alternating solid and liquid strips. We propose that the striping may reflect regions of slightly different oxygen doping level formed due to enhanced diffusion at the edges of atomic terraces during crystal growth. We are unable to resolve discrete vortices in the stripes above the vortex lattice melting line. We speculate either that the vortex liquid efficiently screens any pinned flux present or that spatial variations in the Gibbs free energy may be imposing a density modulation on the vortex liquid at these temperatures.

Phase Separation in La2CuO4+? by electrical transport measurements

The dc conductivity and the high-frequency capacitance of a La2CuO4+δ single crystal are determined by impedance spectroscopy in the frequency range 20 Hz to 1 MHz at liquid helium temperature. Using isochronal annealing, the effect of the phase separation on these quantitites is studied for E parallel and perpendicular to the CuO2 planes as well as for different oxygen doping levels,δ, of the sample. The results are consistent with the model of a diffusion-controlled formation of a metallic—and belowTc superconducting—percolation network in an insulating background.

Anisotropic Seebeck coefficient in Tl 2Ba 2CuO 6−δ single crystals

Anisotropic Seebeck coefficients have been carefully measured in Tl 2Ba 2CuO x single crystals with differing oxygen doping levels. In-plane Seebeck coefficients are small in magnitude (∼ 5 μV/K), negative at room temperature and have a decreasing magnitude as the temperature decreases. C-axis Seebeck coefficients are larger in magnitude (∼ 20 μV/K) and positive. The absolute magnitude of the Seebeck coefficients (all directions) increases as the oxygen (hole) doping levels increase in these overdoped materials. In samples with high transition temperatures, a crossover from negative to positive appears ∼ 15 K above the superconducting transition temperature. The good correlation between bulk compound data and appropriately weighted in-plane and c-axis crystal data is shown. The data are discussed in terms of single-band and multiband models but none are uniquely identified as appropriate for these complex materials.

Phase separation in La 2CuO 4+δ as probed by impedance spectroscopy

The dc-conductivity and the high-frequency capacity of a La 2CuO 4+δ single crystal are determined by impedance spectroscopy in the frequency range 20Hz – 1MHz at liquid helium temperature. Using isochronal annealing, the effect of the phase separation on these quantities is studied for E parallel and perpendicular to the CuO 2-planes as well as for different oxygen doping levels, δ, of the sample. The results are consistent with the model of a diffusion controlled formation of a metallic- and below T C superconducting - percolation network in an insulating background.

Oxygen stoichiometry and surface treatment effect on electromigration stability of high-Tc Tl2Ba2CuO6+x superconductor

Electromigration stability of Tl2Ba2CuO6+x ceramics is shown to decrease significantly when the material is treated in water vapor atmosphere. TheTc decrease in these samples is accompanied by a resistance increase, while the Seebeck coefficient,S, remains unchanged. The authors conclude that the main effect comes from grain-boundary degradation under the water vapor treatment. For initial samples, electromigration stability strongly depends on the sample oxygen doping level and increases for materials with higher oxygen content. The effect is assumed to be due to the filling of interstitials in Tl-O layers by oxygen atoms.

Optical absorption relaxation applied to SrTiO3 and ZrO2: An in-situ method to study trapping effects on chemical diffusion

Chemical diffusion coefficients have been reliably measured in the case of the electronic conductor SrTiO3 (Fe-doped) and the ionic conductor yttria-stabilized ZrO2 (Ni-doped) by using a contactless optical relaxation technique. A quantitative concept (proposed by one of us earlier) which allows the inclusion of internal trapping effects leads to a consistent analysis of D̃ in terms of temperature (T), oxygen partial pressure (P) and doping level (m). In both materials, SrTiO3 and ZrO2, the argument between experiment and theory is very good. In the first case, due to the availability of all the necessary parameters, no adjusting parameter has been used. In both cases, the analysis also leads to a reevaluation of literature data and is able to reconcile the apparent “scatter” of published results.

Nuclear relaxation behavior of the superconducting cuprates: Bi2Sr2CaCu2O8.

Nuclear-magnetic-resonance data are presented and analyzed for the high-${\mathit{T}}_{\mathit{c}}$ compound ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ for two oxygen doping levels. Both sample conditions lead to spin-gap behavior for the NMR shift, with a precursive downturn in the data at Tg${\mathit{T}}_{\mathit{c}}$. In addition, the relaxation times ${\mathit{T}}_{1}$ obey the relation (${\mathit{T}}_{1}$T${)}^{\mathrm{\ensuremath{-}}1}$\ensuremath{\propto}${\mathit{K}}^{\mathit{s}}$(T) at low temperatures (T\ensuremath{\lesssim}100 K), where ${\mathit{K}}^{\mathit{s}}$(T) is the spin paramagnetic shift. This relation, which is also obeyed by other superconductors, is argued to be related to the spin-gap effects and thus incompatible with a Fermi-liquid approach to the understanding of these systems.