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Study on enhancement of [iopmath latex="$T_{c}$"] Tc [/iopmath] ( [iopmath latex="${\geq}130$"] 130 [/iopmath] K) in TlBa2Ca2Cu3O [iopmath latex="$_{y}$"] y [/iopmath] superconductors

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A TlBa2Ca2Cu3O [iopmath latex="$_{y}$"] y [/iopmath] (Tl-1223) superconductor with an ideal composition has a much higher [iopmath latex="$T_{c}$"] Tc [/iopmath] than the established value ( [iopmath latex="${\sim}120$"] ~120 [/iopmath]  K). The intrinsic [iopmath latex="$T_{c}$"] Tc [/iopmath] arises by suppression of Tl substitution for Ba and Ca sites, which inherently occurs in samples prepared by conventional methods. The substitution of Tl was suppressed by preserving the samples in a heavily over-doped state ( [iopmath latex="$T_{c} \sim 100$"] Tc~100 [/iopmath]  K) which was realized by a high-pressure (solid and oxygen) synthesis method. In addition, we found several synthesis conditions for which we could obtain high- [iopmath latex="$T_{c}$"] Tc [/iopmath] samples with good reproducibility. Residual carbon in the precursor was found to be harmful for superconductivity, so that it had to be reduced as much as possible before the high-pressure synthesis. The synthesis temperature and starting concentration of Tl, which also affect [iopmath latex="$T_{c}$"] Tc [/iopmath] , had to be controlled properly to enhance [iopmath latex="$T_{c}$"] Tc [/iopmath] . Samples show that [iopmath latex="$T_{c} \geq 130$"] Tc130 [/iopmath]  K by post-annealing in a reducing atmosphere at 650-700 °C for 0.5 h to 12 h. The sample prepared with the starting composition of Tl [iopmath latex="$_{0.75}$"] 0.75 [/iopmath] Ba2Ca2Cu [iopmath latex="$_{3.25}$"] 3.25 [/iopmath] O [iopmath latex="$_{y}$"] y [/iopmath] had an almost ideal grain composition and showed a [iopmath latex="$T_{c}$"] Tc [/iopmath] of 133.5 K. This [iopmath latex="$T_{c}$"] Tc [/iopmath] is comparable with that of HgBa2Ca2Cu3O [iopmath latex="$_{y}$"] y [/iopmath] (Hg-1223) which is known to have the highest [iopmath latex="$T_{c}$"] Tc [/iopmath] in superconductors.

Document Type: Miscellaneous

Affiliations: 1: National Institute of Advanced Industrial Science and Technology (AIST) and CREST, AIST Central 2, Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan 2: Science University of Tokyo and CREST, 2461 Yamazaki, Noda, Chiba 278-8510, Japan

Publication date: July 1, 2001


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