Irreversibility-field enhancement by optimizing hole density distribution in the Hg-1212 superconductor

Abstract In this paper, enhancement of the irreversibility field characteristics for the n =2 member of the Hg-12( n −1) n homologous series by optimizing hole density distribution is reported. The hole density distribution in the high- T c layered copper oxides can be regarded as a source of anisotropy and also has been found to govern superconductivity properties such as transition temperature [M. Karppinen, H. Yamauchi, Philos. Mag. B, 79 (1999) 343–366], irreversibility field [H. Yamauchi, M. Karppinen, K. Fujinami, T. Ito, H. Suematsu, K. Matsuura, K. Isawa, Supercond. Sci. Technol., 11 (1998) 1006–1010], etc. In order to control the hole density distribution, solid solutions of HgBa 2 (Ca 1− x Y x )Cu 2 O 6+ δ (0≤ x ≤0.5) have been synthesized by an encapsulation technique and then post-annealed by a high-pressure oxidization (HPO) technique. The total amount of doped holes depends on both the amount of Y substitution for Ca ( x ) and that of excess oxygen in the HgO δ layer ( δ ). The former would reduce the hole density in the neighborhood of the CuO 2 planes and the latter would increase the amount of holes especially in the charge reservoir (CR) block. The experimental data have demonstrated that the best H irr -vs.-(1− T / T c ) characteristics is obtained by the excess oxygen doping solely, i.e. upon over-doping the samples. In such over-doped states, the T c 's tend to decrease. On the other hand, the samples simultaneously doped with small amounts of both yttrium and oxygen showed remarkably improved H irr characteristics without large suppression of the T c value.