S. Pekker

Transport and vibrational spectra of oxygen doped Y Ba2Cu3O6+δ

Abstract Y Ba 2 Cu 3 O 6+δ has been prepared for various valuesof δ from the fully oxidized orthorhombic and from the fully reduced tetragonal phase. The conductivity and T c were found to decrease rapidly with decreasing δ without any plateaus in T c . Simulataneously strong changes in Raman and IR spectra were observed. One particular line at 580 cm −1 disappeared completely with decreasing δ and is thus assigned to the Cu ue5f8 O vibration in the linear CuO chains.

Ordering of oxygen into chains and distribution of hole density in YBa2Cu3O1+x

Abstract A series of Y 0.999 Gd 0.001 Ba 2 Cu 3 O 6+ x compounds is studied by Gd 3+ EPR in the full range of oxygen content. The fine structu re is modified by the hole distribution in the Cu(2)O 2 layers mirroring oxygen chains in the Cu(1)O x layers. The fine structure of 5 differently occupied first-neighbour Cu(1)O x oxygen chain configurations surrounding the Gd 3+ ions is resolved. Formation of chain begins at a threshold oxygen content x 0 =0.14. For x >0.14 oxygen in Cu(1)O x planes is ordered mainly into chains, the amount of disordered oxygen decreases with increasing x . The EPR intensities are in quantitative agreement with a model calculation of 2-dimensionally ordered oxygen chains.

Orientation distribution of magnetic field oriented YBa2Cu3O6 powders

Abstract The orientation of the crystallites in YBa 2 Cu 3 O 6 powders is studied by X-ray diffraction. It is found that a conventional powder diffractogram might be misleading in evaluating the degree of orientation. The mapping of single crystal type reflections has been carried out to obtain a more detailed picture of the orientation of crystallites. Experimental results are presented for the (0, 0, 5) reflection of Y 0.98 Gd 0.02 Ba 2 Cu 3 O 6.02 oriented powder samples with various crystallite sizes.

EPR in Gd-doped YBa2Cu3O6+y (0⩽y⩽1): local method to determine oxygen ordering

Abstract The Gd 3+ 9GHz EPR spectra of a series of 10 magnetic field oriented Y 0.999 Gd 0.001 Ba 2 Cu 3 O 6+ y (0⩽ y ⩽1) compounds show 7 well resolved fine structure sets corresponding to various configurations of oxygen in the Cu(1) planes. The oxygen contents are confirmed by X-ray diffraction lattice constants. The data suggest that for y ⩾0.3 oxygen on the O(1) sites forms alternating occupied and vacant chains. The chains themselves are at least partially ordered. The relation between oxygen order at intermediate y and the superconducting transition temperature is briefly discussed.

Superconducting phases and influence of oxygen content in the Bi2Sr2CuOy-CaCuO2 system☆

Abstract Superconducting and other major phases in the Bi2Sr2CuO6-CaCuO2 system were studied over a wide range of compositions by electrical resistivity and powder X-ray diffraction measurements. In the range from Bi2Sr2CuO6 to Bi2Sr2CaCu2O8 a subsolidus compatibility join is suggested between the two superconducting phases. At higher nominal CaCuO2 contents four major phases are observed simultaneously: Bi2Sr2CaCu2Oy, Bi2Sr2Ca2Cu3Oy, Ca2CuO3 and CuO. It is argued that the Tc = 110 K phase (2223) is metastable at room temperature. Its maximum occurrence is found at an off-stoichiometric composition of 2256. Significant effects of quenching and oxygen content on the superconducting properties of the Tc = 110 K phase are discussed.

Electronic properties of La-Cu oxides

Abstract Two anomalies are found in the temperature dependence of the d.c. electric conductivity of variously prepared La-Cu oxides: a shoulder around the temperature of T ⋍ 240 K, associated with a magnetic phase transition in La 2 CuO 4 and a decrease of resistance below 40 K associated with a yet to be identified superconducting La-Cu-O phase.

Structure and electronic properties of La-Sr-Cu oxide ceramics

Abstract The dc electrical conductivity of a series of La 2−x Sr x CuO 4 compounds has been investigated. At room temperature the compounds exhibit a continuous change from the orthorombic to the tetragonal structure as x is increased from zero to x=0.15. Superconductivity appears in the tetragonal phase. The resistivity maximum, preceeding the superconducting transition varies in correlation with the Sr concentration. Application of hydrostatic pressure suppresses the resistivity maximum.

Oxygen induced phase changes in YBa2Cu3O6+δ. transport, structural and spectroscopic evidence

YBa2Cu3O6+? was prepared for various values of ? by equilibrating pieces of fully oxidized orthorhombic and fully reduced tetragonal material at 600?C. Tc was found to decrease rapidly with decreasing ? and reached zero comptely wihin the orthorhombic phase. Strong changes in frequency and intensity of Raman and IR spectra were observed and could be correlated to the structural changes. In particular two IR-active modes at 580cm-1 and at 630cm-1 were observed which allowed to observe directly the doping process by the O(1)-oxygen. Also, the ratio of two special Raman lines was found to be a critical function of the oxygen concentration. From the experiments an assignmnt of vibrational modes was obtained in very good agreement with a recent normal coordinate calculation.

EFFECT OF THE EQUILIBRATING TEMPERATURE ON THE CRITICAL TEMPERATURE OF CERAMIC YBa2Cu3Oy

The transition temperature of YBa2Cu3Oy ceramic samples were studied as a function of oxygen content by DC electrical resistivity measurements. Two series of samples had been prepared by equilibrating pieces from the fully oxidized orthorombic and fully reduced tetragonal phase at low (series I 420 °C) and high (series II 600 °c) temperatures. The Tc vs. oxygen content dependence was found to show a continuous change in series II and a 55 K plateau around y = 6.6–6.7 in series I. A current dependence was observed in series I in the range y = 6.5–6.6 where room temperature X-ray data may indicate the occurence of more than one orthorombic phases.

NUCLEAR MAGNETIC RESONANCE STUDY OF 205Tl IN MULTIPHASE Tl-Ba-Ca-Cu OXIDE SUPERCONDUCTORS

Several batches of Tl-Ba-Ca-Cu oxide superconductors have been synthesized and characterized by resistivity, magnetic susceptibility and X-ray diffraction measurements. The 205Tl NMR line snifts (K), the spin-lattice and spin-spin relaxation times have been measured at room temperature. The temperature dependence of the spin-lattice relaxation rate is also reported. The resonance around K = 0.25 % has a composite line shape indicating the presence of two Tl sites. The two sites are tentatively assigned to thallium atoms in the (2223) and (2212) Tl-Ba-Ca-Cu phases.