Showing papers by "David K. Christen published in 1993"

Correlations between the Hall coefficient and the superconducting transport properties of oxygen-deficient YBa2Cu3O7- delta epitaxial thin films.

Abstract: Strong correlations between the Hall coefficient ${\mathit{R}}_{\mathit{H}}$, the transition temperature ${\mathit{T}}_{\mathit{c}}$, and the critical current density ${\mathit{J}}_{\mathit{c}}$ were established in a series of epitaxial ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ thin films as a function of oxygen deficiency \ensuremath{\delta}. Steady increases in ${\mathit{R}}_{\mathit{H}}$ with \ensuremath{\delta} suggest that deoxygenation reduces the density of states which, according to BCS theory, should lead to corresponding decreases in ${\mathit{T}}_{\mathit{c}}$. In contrast, two well-known plateaus occurring at 90 K and 60 K were observed in ${\mathit{T}}_{\mathit{c}}$ vs \ensuremath{\delta}. Others have ascribed these plateaus to either electronic phenomena or phase separations. We find that in the 90-K plateau, the critical current density ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\delta},H=0) decreases with \ensuremath{\delta} and extrapolates toward zero at the edge of the plateau, while the relative-field dependence of ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\delta},H) and the flux-creep pinning energies are independent of \ensuremath{\delta}. These observations suggest that the phase-separation scenario occurs on the 90-K plateau. However, electronic origins cannot be ruled out at present due to difficulties in determining the equilibrium superconducting properties of oxygen-deficient ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ films.

Observation of free flux flow at high dissipation levels in YBa 2 Cu 3 O 7 − δ epitaxial films

Abstract: The total resistivity (\ensuremath{\rho}=E/J not dE/dJ) was measured in epitaxial ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ films up to high dissipation levels using a pulsed current source In the reversible region, \ensuremath{\rho}(J) has an ssS shape; it is Ohmic at low J (thermally activated free-flux motion), goes through a nonlinear transition region (depinning), and becomes Ohmic again at the highest J (free flux flow, ie, purely viscous motion) The free-flux-flow resistivity ${\mathrm{\ensuremath{\rho}}}_{\mathrm{fff}}$ obeys ${\mathrm{\ensuremath{\rho}}}_{\mathrm{fff}}$/${\mathrm{\ensuremath{\rho}}}_{\mathit{n}}$\ensuremath{\simeq}H/${\mathit{H}}_{\mathit{c}2}$(T), with ${\mathit{dH}}_{\mathit{c}2}$/dT\ensuremath{\approxeq}2 I/K

Vortex fluctuations, magnetic penetration depth, and Hc2 in Hg- and Tl-based high-Tc superconductors.

Abstract: The presence of strong vortex fluctuation effects in two families of layered high-temperature superconductors (${\mathrm{HgBa}}_{2}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$ and ${\mathrm{Tl}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10+\mathrm{\ensuremath{\delta}}}$) is established from experimental studies of the equilibrium magnetization in the mixed state using the theory of Bulaevskii et al. Detailed information is obtained within the framework of this theory on the London penetration depth ${\ensuremath{\lambda}}_{\mathit{a}\mathit{b}}$, upper critical field ${\mathit{H}}_{\mathit{c}2}$, and Ginzburg-Landau parameter ${\mathrm{\ensuremath{\kappa}}}_{\mathit{a}\mathit{b}}$ (with magnetic field perpendicular to the copper oxide planes) of these materials.

In situ growth of epitaxial Bi2Sr2CaCu2O8−x and Bi2Sr2CuO6−x films by pulsed laser ablation

Abstract: Pulsed‐laser ablation has been used to grow epitaxial films of Bi2Sr2CaCu2O8−x (Bi‐2212) and Bi2Sr2CuO6−x (Bi‐2201) on (001) MgO with entirely in situ processing. The films’ layer‐stacking sequence, microstructure, and superconducting Tc are highly sensitive to the growth temperature and deposition rate. Pure Bi‐2212 films exist over only a narrow temperature range. Pure Bi‐2201 films appear at higher growth temperature, while lower growth temperature enhances the Bi‐2223 phase, though it is mixed with Bi‐2212. The phase homogeneity, in‐plane connections among grains, and superconducting Tc of Bi‐2212 grown at a given temperature are greatly improved by reducing the deposition rate. Epitaxial Bi‐2212 films grown at 740 °C and 0.5 Hz (∼0.05 nm/s) have Tc0 (R=0)=71 K, with Jc∼8×105 A/cm2 at 50 K and Jc∼5×106 A/cm2 at 4.2 K.

Strong evidence for vortex-glass-collective-pinning theory in YBa2Cu3O7 superconductors.

Abstract: Magnetization studies on a melt textured-growth sample of YBa[sub 2]Cu[sub 3]O[sub 7[minus][delta]] have tested several theoretical models to see which of them best describes the relaxation of supercurrents in this high-[ital T][sub [ital c]] superconductor. By combining conventional flux-creep experiments with measurements of magnetization versus magnetic-field sweep rate, we are able to detect the decay of magnetization in both its intermediate and very early stages, thereby expanding the observational window up to 5--6 decades of time. Among four recent models tested, only vortex-glass--collective-pinning theory consistently describes both stages. The resulting values for the characteristic exponent [mu] strongly depend on temperature and magnetic field. Values for the effective hopping time for vortices, [ital t][sub [ital e][ital f][ital f]], are [similar to]0.1 s.

Enhanced transport critical current at high fields after heavy ion irradiation of textured TlBa2Ca2Cu3Oz thick films

Abstract: Spray pyrolyzed T1(1223) films deposited on polycrystalline YSZ substrates are characterized before and after heavy ion irradiation. A factor of 2 decrease in zero field critical current is observed. However, significantly improved critical current is found at fields above 1 T, where intragranular effects dominate. The irreversibility line at 5 T is shifted by ∼20 K to higher temperatures. Scaling of the data before and after irradiation and at different temperatures is modeled by an expression which interpolates between single vortex pinning and collective creep.

Anomalous transport and structural properties of Sr1−xCuO2−δ thin films

Abstract: Transport and structural properties of Sr 1− x CuO 2−δ thin films grown by pulsed laser deposition support the contention that the tetragonal phase is capable of accommodating a significant density of alkaline-earth deficiencies up to x ≤0.3. Resistivity measurements indicate a significant change in the carrier density of the CuO 2 planes as Sr vacancies are introduced. In addition, an enigmatic anomaly in the resistivity at 185 K is observed for Sr 0.85 CuO 2−δ thin films. Magnetic measurements on these samples indicate that, although a significant drop in resistivity at 185 K is observed, it is not due to a superconducting transition. Hall measurements, as well as changes in the resistivity with film growth conditions, suggest that the majority carriers in these Sr 1− x CuO 2−δ thin films are electrons even with the Sr-vacancies present.

Vortex dynamics in YBaCuO single crystals with point- and line-like defects-flux creep studies

Abstract: Studies of vortex dynamics and pinning were conducted in irradiated Y1Ba2Cu3O7 single crystals by observing the decay of supercurrents with time. The materials contained either point-like defects from proton irradiation or line-like defects from irradiation with energetic heavy ions. Analysis of the first case gives clear support for collective creep theory. With linear defects, the experiments give evidence for thermal smearing of the vortex pinning potential at higher temperatures, as predicted theoretically.