G. Müller-Vogt

Phase separation in YBa2Cu3O7−δ single crystals near δ = 0

Abstract X-ray diffraction, resistivity, AC and DC magnetization data on high-quality single crystals of YBa 2 Cu 3 O 7−δ are presented. We demonstrate that for δ l ) diffraction lines are split into two, indicating that at these high oxygen concentrations the crystals are no longer single phase but actually consist of two (or more) different phases with slightly different c -axis parameters. In the two-phase region, the electrical resistivity and the AC and DC magnetic susceptibilities show a broadening of the superconducting transition. This broadening is thought to be due to the proximity effect or strains in two finely dispersed phases with slightly different transition temperatures.

The “90 K” plateau of oxygen deficient YBa2Cu3O7−δ single crystals

Low field magnetization, resistivity, and specific heat measurements were performed on large, high quality YBa2Cu3O7-δ single crystals in the range 0.05<δ<0.20, the so-called 90 K plateau. For each crystal the measurements were repeated in several states with a different oxygen concentration. Using the normal state resistivity as a relative measure for the oxygen concentration, it is demonstrated that with increasing δ, Tc goes through a small but well defined maximum near δ=0.12 whereas the jump in the specific heat at Tc, ΔCp/Tc, dramatically drops by a factor of three between δ=0.10 and 0.20.

Chemical reactions in the graphite tube for some carbide and oxide forming elements

Abstract The chemical reactions of V, Cr, Mo, Sn, Ge and Si in the graphite furnace are briefly summarized. A reaction diagram is presented which aids in the interpretation of the influence of the thermal program prior to atomization and of interfering elements on the absorption signal. It was found that the elements V, Cr and Mo form stable carbides in the graphite tube, which decompose into the elements during the atomization cycle. The atomic vapor of Si, Sn and Ge is produced mainly by reduction of their oxides to the metal. Losses of volatile suboxides of Sn and Ge and of nonatomizable SiC reduce the absorption signal of these elements.

Uniaxial pressure dependence of Tc of untwinned YBa2Cu3Ox single crystals for x=6.5–7

Abstract The dependence of Tc on uniaxial pressure dTc/dpi(i= a, b, c) is determined for untwinned YBa2Cu3Ox single crystals as a function of O-content using ultrahigh-resolution thermal-expansion data and the Ehrenfest relationship. The dTc/dpi values are found to be highly anisotropic and are likely due to several different effects. For uniaxial-pressure applied parallel to the c-axis, we find a strong O-content (or carrier density) dependence of the dTc/dpb values, which have opposite signs charge transfer model. This model does not, however, explain the large dTc/dpa and dTc/dpb values, which have opposite signs and do not show a strong O-content dependence except at x=6.7, where both dTc/dpa and dTc/dpb values increase sharply. The a, b effects are discussed in terms of the orthorhombic distortion.

Giant pressure effect in oxygen deficient YBa2Cu3Ox

Abstract The pressure effect on T c of polycrystalline and single crystalline YBa 2 Cu 3 O x investigated as a function of oxygen content x by ac-susceptibility measurements under helium pressure. In the overdoped region x > 6.93 the single crystals show a negative d T c /d p , as expected from the charge transfer model. For optimally doped samples with x = 6.93 we find d T c /d p = 0.4 K/GPa which points to pressure effects on T c aside from charge transfer. In the underdoped region x T c /d p values obtained from the experiment depend strongly on the storage temperature of the sample during the experiment. When the samples are stored at temperatures well below 240 K throughout the entire experiment including pressure application and pressure release, d T c /d p increases to approx. 7 K/GPa at x = 6.7 but with a further decrease of the oxygen content the d T c /d p drops to approx. 2 K/GPa at x = 6.4. These effects are intrinsic to the YBa 2 Cu 3 O x structure and can be explained by considering the anisotropic structure of YBa 2 Cu 3 O x . The decrease of the c -axis lattice parameter results in a charge transfer to the CuO 2 -planes mainly [1], whereas the compression of the a - and b -axis lattice parameter is known to produce different pressure effects which are responsible for the peak in d T c /d p at x = 6.7 [2]. When pressure is changed at room temperature oxygen ordering effects occur which cause a relaxation of T c to the equilibrium value T c ( p ) at this pressure with a time constant depending on the oxygen content x . A decrease x results in a peak effect in d T c /d p at x = 6.7 again, which is enhanced to approx. 12 K/GPa. If the oxygen content is decreased further, d T c /d p first drops to 5 K/GPa at x = 6.6, but the increases to values of more than 20 K/GPa for x T c increase (d T c /d p ) O due to pressure induced oxygen ordering via oxygen motion between unit cells.

Chemical reactions of lead in graphite furnace atomic absorption spectrometry

The chemical reactions of lead in graphite furnace atomic absorption spectrometry (AAS) are determined experimentally by absorbance measurements, X-ray diffraction and molecular absorption. Lead nitrate is decomposed into lead oxide during the drying cycle, which is reduced to metallic lead by the carbon of the tube.Interference from chloride ions is due to the formation of volatile lead chloride. The formation of this compound can be prevented by the addition of the matrix modifiers ammonium dihydrogenphosphate and ammonium dichromate. These modifiers, which must be applied in great excess, form stable lead phosphates and chromates. The influence of the palladium nitrate modifier on the reactions of lead is also discussed.

Comparative study of oxygen diffusion in rare earth REBa2Cu3O7−δ single crystals (RE=Y, Er, Dy) with different impurity levels

We report on measurements of the chemical diffusion coefficients of rare earth cuprate single crystals REBa2Cu3O7−δ (RE=Y, Er, Dy) grown in crucibles made of Y2O3-stabilized ZrO2 and BaZrO3. The diffusion coefficients have been determined by in situ measurement of the electrical conductivity using a four point method during the oxygenation at various oxygen partial pressures and in the temperature range of 390°C–550°C. In this range the diffusion coefficient was determined between 3.7×10−9 cm2/s and 6.6×10−8 cm2/s independent of the rare earth atom or of the different impurity levels. For all crystals we show that the in- and out-diffusion process can be described with the same time constant at each annealing temperature. Within the accuracy of ±5% we measure for both processes the same value for the diffusion coefficient. The temperature dependence of the diffusion coefficient obeys an Arrhenius-law for all crystals, proving that the oxygen diffusion in the rare earth RE 123 superconductors is a purely thermally activated process. The values for the activation energy show a scattering up to 30% and vary from 0.76 to 1.09 eV. There is neither a dependence of the activation energy on the central rare earth atom nor on different impurity levels.

YBa2Cu3O7-δ-BaCuO2-CuO : investigations on the phase diagram and growth of single crystals. II

We report on phase studies in the pseudobinary YBa2Cu3O7−δ-BaCuO2 subsystem under oxygen atmosphere with special attention to the influence of the crucible material. We show that different crucibles made from Al2O3 and Y2O3-stabilized ZrO2 respectively lead to different results in the DSC experiments. Our phase diagram data are compared with literature data. With the experience obtained during these investigations, we performed crystal growth experiments in Al2O3 and in Y2O3-stabilized ZrO2 crucibles. The differences of the results are discussed.

YBa2Cu3O7−δ-BaCuO2-CuO: investigations on the phase diagram and growth of single crystals I. The system BaCuO2-CuOx

We report on the experimental determination of the phase diagram of the quasi-binary subsystem CuO-BaCuO2 with the aid of differential scanning calorimetry (DSC). Al2O3 crucible material shows severe corrosion and influence on the phase relationships. Crucibles made from zirconium oxide stabilized with yttrium oxide exhibit less chemical interaction. Due to the sluggish reactions within the system and the unavoidable corrosion, the parameters of the DSC runs must be carefully optimized. The oxygen content in the atmosphere changes the temperature of the liquids line and the eutectic solidification. Moreover, it influences the formation of monovalent copper oxide. BaCuO2 is shown to melt congruently. Reasons for differences between the given phase diagram and those reported in the literature are discussed.

Polarized reflectance spectra of a (001) surface of YBa2Cu3O7 before and after detwinning

Abstract We have performed reflection measurements on a (001) surface of a YBa 2 Cu 3 O 7 single crystal, using polarized light in the energy range between 50 meV and 6 eV. The crystal was grown with the typical twinned structure of this compound, showing a reflectance spectrum which was independent of the direction of polarization. The spectrum was characterized by free carrier absorption with a plasma edge near 1 eV. By applying uniaxial stress at 450°C the twinned structure was removed. The crystal surface was transformed into a single untwinned domain, resulting in a strong splitting of the plasma edge with respect to the direction of polarization. By fitting a Lorentz-Drude model to the polarized reflectance spectra of the single domain crystal, R ∥ a and R ∥ b , the tensor components of the dielectric function could be determined whereas the reflectance spectrum of the twinned surface represents an average of the components of both crystallographic directions.