Critical Current Enhancement in Single-Crystal Ba 2 Ycu 3 O 7
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CRITICAL CURRENT ENHANCEMENT IN SINGLE-CRYSTAL Ba 2 YCu 3O 7 R. B. van Dover, E. M. Gyorgy, L. F. Schneemeyer, A. E. White, S. Glarum, R. J.Felder, and J. V. Waszczak AT&T Bell Labs, Murray Hill, NJ 07974
ABSTRACT The critical current density, Jc, of single crystals of Ba 2 YCu 30 7 is comparable to that of melttextured-growth materials as well as to the intragrain J, of bulk polycrystalline Ba 2 YCu 3 0 7 . Typical values at 77 K and 9 kOe are near 5x 103 A/cm 2 , and are presumably limited by weak pinning. We have obtained a significant enhancement of this pinning-limited Jc by three techniques: 1) a short reannealing procedure, 2) fast neutron irradiation, and 3) proton irradiation. Interestingly, the combination of the first technique followed by either irradiation is not as effective as the iradiation alone. That is, the best candidates for irradiation are not necessarily those with the highest initial J,. INTRODUCTION The concept of deliberately introducing defects into superconductors in order to increase pinning has played a key role in materials development of the technologically useful superconductors, such as NbTi and Nb 3 Sn, prompting similar approaches to the high-T0 superconductors [1-3]. Materials issues have proven ubiquitous in the study of high-T, superconductors, and the study of critical currents and flux motion is no exception. To put it simply, not all starting materials are the same. This study, for example, shows that even in single crystals of Ba 2 YCu 3 0 7 we can reproducibly obtain either low or moderate critical currents, depending on the thermal history of the sample. While the ruinous effect of flux creep at 77 K in Ba 2 YCu 3 O7-single-crystal as well as polycrystalline ceramic-has by now been adequately documented, it also has been clear that the weak pinning in nominally defect-free grains can be increased dramatically by deliberate introduction of defects, e. g., by fast neutron irradiation[2,4-6]. We have recently shown[7] that neutron-irradiated single crystals can exhibit critical currents as high as 6x 105 A/cm 2 at 77 K and 9 kOe. In this paper we comment on the reproducibility of the critical current, the effect of variations in the initial Ba 2 YCu 3 07 samples, and the extension to proton irradiation. EXPERIMENTAL The technique used to prepare single crystals of Ba 2 YCu 3 0 7 was described previously[8]. All crystals were annealed in 02 for 1 week at 500 * C, and for 5 months at 400 * C (no properties are known to change if the second step is reduced to 3 weeks), a protocol we will refer to as the "initial anneal." Some crystals were then heated to 650 * C for 20 minutes, followed by 450 * C for 1 week, a protocol we will call "reannealed." Many other protocols are possible. The most obvious distinction between the initial anneal and reannealed crystals is that their T,'s are consistently 89 and 91 K respectively. No difference in terms of structure (X-ray diffraction, TEM) has been found to date, suggesting that the difference in T, may be due to subtle changes in the
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