Critical Currents of Overdoped Co-evaporated YBCO Coated Conductors
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1001-M14-04
Critical Currents of Overdoped Co-evaporated YBCO Coated Conductors Jens H‰nisch, Jonathan Storer, Chris Sheehan, Yates Coulter, and Vladimir Matias Superconductivity Technology Center, Los Alamos National Laboratory, Los Alamos, NM, 87545 ABSTRACT Coated conductor samples, prepared by reactive co-evaporation, are investigated with respect to the hole-doping dependence of the critical current density. The samples are annealed in an atmosphere of variable oxygen content after which critical currents, critical temperature and the c-axis lattice spacing are measured. The lattice spacing increases with decreasing oxygen content, consistent with literature data. These co-evaporated samples show hole overdoped behavior with respect to the maximum Tc. The achievable range of hole doping in these samples seems to depend on surface coverage. Both self-field and in-field Jc at 75.5 K have a maximum in the overdoped region but at less than maximum oxygen content. The reason for the overdoping of these samples is discussed briefly in terms of Y-Ba disorder. INTRODUCTION The superconducting properties of high-temperature superconductors depend strongly on the amount of hole doping in the CuOx planes [1]. The hole doping level, p, is primarily dependent on the amount of oxygen in YBa2Cu3O7-δ, usually written as oxygen deficiency δ from the optimal doping. In the Y-Ba-Cu-O system, YBa2Cu3O7 (YBCO) is very close to optimum doping and Tc goes to zero at an oxygen level of around δ = 0.6. However, the hole doping level can also be influenced by elemental substitutions and deviations from strict 1:2:3 stoichiometry. A prominent example is Ca doping, which has been investigated in connection with possible improvement of grain boundaries in high-Tc superconductors [2]. To optimize the performance of coated conductors, it is important to know under which conditions a certain oxygen doping level can be achieved and how the critical temperature Tc and the critical current density Jc in a given magnetic field depend on the oxygen content. Recent studies by Strickland et al. [3] showed that Jc monotonically decreases with increasing δ for coated conductor samples deposited by metal-organic decomposition (MOD). The maximum doping level for those samples is only slightly in the overdoped region. Postannealed e-beam evaporated samples [4] at low O2 pressure show an initial increase in Jc at 85 K for the lowest δ values and slight overdoped behavior as well. Reactively co-evaporated films using a similar process have also been reported as overdoped [5]. We present here data on several coated-conductor samples made by a process where the films are deposited by alternating between the deposition zone (low pressure) and an oxygen pocket (high pressure). Similar processes have been implemented for coated conductors by other groups [6,7].
SAMPLE PREPARATION The coated conductor samples were prepared by reactive co-evaporation [8] on IBAD templates from elemental sources. The individual rates are controlled by atomic absorption. Ba is i
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