J. W. Downey

Superconductivity in YBa2−xSrxCu3O7−δ

We report structure, resistivity, and Meissner effect measurements on YBa2−xSrxCu3O7−δ for 0<x<2.0. We find a region of solid solubility extending at least to x=1.0 and a monotonic depression of Tc with x. Using arguments based on structural changes with Sr doping, we speculate that the depression of Tc is due to the local distortion of the lattice in the neighborhood of the Sr site and the introduction of additional oxygen vacancies.

Atomic short-range order in oxygen deficient YBa2Cu3O7−σ

Abstract Low field magnetization measurements are presented for a set of oxygen deficient single crystals of YBa 2 Cu 3 O 7−σ . It is demonstrated that the superconducting transition temperature T c not only depends on the oxygen concentration but also on the degree of oxygen vacancy ordering of the samples. Various degrees of ordering were obtained by annealing crystals with reduced oxygen stoichiometries at various temperatures below 350°C and fast quenching them to cryogenic temperatures. Depending on oxygen stoichiometry and annealing temperature, changes in T c of up to 27 K are observed. The observed annealing and quenching behavior is explained in terms of the kinetics of vacancy short-range order in the Cu-O chains. One major conclusion is that the so-called “60 K plateau” in the plot of T c versus σ is completely absent when the crystals are in their most disordered state, i.e., with the smallest degree of vacancy short-range order.

New observation of a phase boundary in oxygen deficient YBa2Cu3O7−δ single crystals

Abstract We have measured the superconducting transition temperature Tcof oxygen deficient single crystals of YBa2Cu3Ox, where x = 7−δ in the range 6.5 T ∗ . We associate T ∗ with the onset of a transition from a vacancy ordered low temperature (ortho II and/or ortho III) phase to a high temperature ortho I phase. The nature of the vacancy ordering below T ∗ varies with x. We believe that, upon heating above T ∗ , a signigicant concentration of chain fragments develops in normally vacant chains of the ordered structure. When the crystals were quenched from temperatures above T ∗ , and subsequently permitted to anneal at 51°C, two relaxation processes were found. The two processes have relaxation times differing by about two orders of magnitude. The faster relaxation process is associated with changes in vacancy short-range disorder. The slower process is attributed to restoration of long-range chain vacancy order as misplaced fragments anneal out. T ∗ is determined for different oxygen concentrations and compared to theoretical phase diagram calculations. Electron diffraction measurements on well annealed single crystals with stoichiometries x = 6.50 and 6.65 show that different chain ordered phases (primarily ortho II and ortho III, respectively) exist at each of these compositions. Chain vacancy order is also observed at temperatures above T ∗ .

Scaling of the irreversibility line with superconducting transition temperature in oxygen deficient YBa2Cu3O7-δ

We report AC susceptibility and resistivity measurements of the vortex motion in single crystal YBa_2Cu_3O_(7−δ), with δ ~ 0.62. The superconducting transition temperature was varied from 7 K to 17 K by altering the oxygen vacancy ordering in the chains. We find that both the slope of the irreversibility line and its extrapolated zero temperature value scale linearly with T_c, providing strong evidence for the intrinsic nature of the irreversibility line. Moreover, the reduced magnetic field scale of oxygen deficient crystals allows us to obtain new results at previously inaccessible reduced temperatures, well below T_c.

Varied oxygen stoichiometry in RBa2Cu3O7−x ☆

Abstract Superconducting properties of RBa 2 Cu 3 O 7−x (R  Y, Gd and Nd) are studied as oxygen stoichiometries are varied. The region of x where samples are superconducting becomes narrower as the ionic radius of the R-ion increases. Samples most unstable to oxygen variation (i.e., Nd123) have the highest T c .

Preparation and characterization of the 110 K superconducting phase in the Bi-Sr-Ca-Cu-O system☆

Abstract Superconducting properties of the Bi2SrχCayCu3Oz system are studied in the composition range 1.2⩽χ⩽2.2 and 2⩽y2.4. Both resistivity and low field magnetization data are presented. Both types of measurements show the development of the 110 K phase from a low-Tc phase as the Sr content is reduced from 2.2 to 1.2. Best results are obtained for the nominal composition Bi2Sr1.6Ca2Cu3Oz. The very similar character of the resistive and inductive transition for two phase samples is attributed to interlayering of the two phases.

Comparison of Au-free versus Au-doped YBa2Cu3O7−δ single crystals: Overdoping and oxygen mobility

Abstract By systematically changing the oxygen concentration of YBa2Cu3O7−δ single crystals grown in Au-crucibles, we demonstrate that the incorporated Au-impurities have only a minor effect on the maximum in Tc due to overdoping. On the other hand, we demonstrate that the Au-impurities significantly increase the oxygen mobility in the basal planes of oxygen deficient YBa2Cu3O7−δ single crystals.

Anisotropic twin boundary pinning in YBa2Cu3Ox

Abstract The reduction in flux motion resistivity due to pinning at twin boundaries in single crystal YBa 2 Cu 3 O x is demonstrated. Pinning takes place only for magnetic fields aligned within a critical angle of the twin boundary plane. The critical angle varies with field direction and with field strength for H | c . The onset temperature of twin boundary pinning is established by its strong anisotropy, and shown to correspond with a feature in the temperature dependent resistivity for H | c .

Surface waves on the a, b and c faces of untwinned single crystals of YBa2C3O7-δ

Abstract Using Brillouin scattering we have measured the angular dependence of surface wave velocities on the three principal faces of untwinned single crystals of YBa 2 Cu 3 O 7−δ . The angular dependence of the velocities on each surface and a comparison between the three surfaces are in agreement with calculations based on available elastic constants. There is, however, an overall discrepancy between the absolute value of the calculated and measured velocities. We obtain the elastic constants C 44 , C 55 and C 66 and compare them with existing data. The temperature dependence, down to 20 K, is also reported for the surface velocities.

The effect of correlated and point defects on the vortex lattice melting transition in single-crystal YBa2Cu3O7-δ

Abstract The vortex melting transition T m in several untwinned and twinned crystals is measured resistively in fields up to 8T. A Lindemann criterion for vortex lattice melting is obtained in addition to a sharp hysteresis in the magnetoresistance at B m supporting a first-order phase transition. The anisotropy of twin boundary pinning and its reduction of the ‘kink’ in ϱ( T ) associated with the first-order melting transition is discussed in samples with very dilute twin boundaries. We also report on the direct suppression of the melting transition by intrinsic pinning for H ‖ ab and by electron-irradiation-induced point defects.