Takahiro Akao

Development of Hg-containing superconductors: from precursors to materials prototypes

Here we present a brief overview of recent advances in the development of Hg-containing compounds for material applications. Included are the utilization of different synthetic routes for precursors powders preparation, modification of encapsulation technique, searching for promising compositions via substitutions, optimisation of superconducting properties and preparation of the materials prototypes by well-established technological processes.

Peak effect in critical current density induced by oxygen non-stoichiometry in cation-stoichiometric SmBa2Cu3O7-δ superconductor

Abstract Cation-stoichiometric SmBa 2 Cu 3 O 7− δ superconductor (Sm-123) samples were synthesized by a conventional (without any liquid phases involved) solid-state-reaction technique, followed by annealing in flowing O 2 gas at 350°C for 40–200 h. A “peak effect” was observed in the critical current density ( J c ) vs. applied magnetic field ( H ) characteristics measured at 77 K. Pronounced peak effect was observed in a sample annealed at 350°C for 40 h, while the peak was suppressed in a sample annealed at 350°C for 200 h. Since the average oxygen content determined by a coulometric titration method increased with decreasing J c value at the peak, certain oxygen-deficient regions induced by oxygen non-stoichiometry might have formed. They could work as the magnetic fluxon pinning centers, since transmission electron microscopy (TEM) observations revealed the presence of both the tetragonal phase and a twin-free orthorhombic phase with width of ca. 200 nm in the sample annealed in O 2 gas at 350°C for 40 h which would have contained a significant amount of oxygen deficiency. The presence of the oxygen-deficient Sm-123 phase especially close to the boundary between either or both of the orthorhombic and tetragonal phases and an ordinary orthorhombic phase containing twins could be working for magnetic flux pinning to yield the peak effect in the J c vs. H characteristics.

Single-crystal X-ray study of (Hg,Pb)-1223 superconductor at low temperature

Abstract The crystal structures of (Hg,Pb)-1223 superconductor have been examined as a function of temperature by single-crystal X-ray diffraction method. It was found that the c cell dimension and Cu(2)ue5f8O(3) distance of this crystal are characteristically shorter than those of end-member Hg-1223 because of the existence of Pb atoms in the Hg sites. The site-occupancy refinement gave a preference for Sr ions to be 0.74(2) in Ba sites and 0.09 in Ca sites. The occupancy of the O(4) sites determined in this study was 0.31(4). No significant structural change in (Hg,Pb)-1223 appeared on the temperature dependence of atomic coordinates and interatomic distances in the temperature range from 105 to 300 K, in the contrast to the out-of-plane characters of Hg-1212 superconductor.

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.

Hecto-Ångstrom modulation and peak effect in superconducting (Hg,Pb)(Ba,Sr)-1223

Abstract To elucidate the origin of the “peak effect” (PE) (or the so-called “fishtail” phenomenon) in the magnetization hysteresis loop of (Hg,Pb)(Ba,Sr)2Ca2Cu3O8+δ superconductors, the nanoscopic structure was studied for two different samples: one was prepared by an encapsulation technique, exhibiting PE and the other by a high-pressure technique, showing no PE. By a combination of transmission electron microscopy, electron diffraction and local compositional analysis, a hecto-angstrom-scale structural distortion accompanied by compositional modulation of both Hg and Pb along the 〈110〉 direction was found for the sample obtained by the encapsulation technique. The spatial inhomogeneity was considered to be a weak vortex pinning site, which may be responsible for PE. In contrast, no such inhomogeneity was detected in the high-pressure synthesized sample. The origin of the compositional modulation might be attributed to spinodal decomposition.

Synthesis of (M0.33Cu0.67)Sr2YCu2Oy: a novel layered cuprate with a block layer of (As0.33Cu0.67)O0.88

Abstract Conditions for forming a single phase of (M 0.33 Cu 0.67 )Sr 2 YCu 2 O y [(M, Cu)-1212] were examined. When the valence of M ions ranged from divalent to heptavalent, the (M, Cu)-1212 phase was formed. A wide tolerance of the M valence in (M 0.33 Cu 0.67 )Sr 2 YCu 2 O y is originated from the oxygen vacancy in the (M 0.33 Cu 0.67 )O z layer. On the other hand, the ionic radius of M was closely correlated with the formation of (M, Cu)-1212, showing that the ionic radius of the M ion is an important factor in the preparation of the 1212 phase. Among these compounds, (As 0.33 Cu 0.67 )Sr 2 YCu 2 O y was newly prepared. The (As 0.33 Cu 0.67 )Sr 2 YCu 2 O 6.88 has a tetragonal unit cell with a =3.830 A and c =11.47 A. Annealing the sample in an oxygen atmosphere led to an increase in the Cu valence as large as 2.38. However, resistivity measurement showed that (As 0.33 Cu 0.67 )Sr 2 YCu 2 O 6.88 was not superconducting but semiconducting. Nonsuperconductivity of (As 0.33 Cu 0.67 )Sr 2 YCu 2 O 6.88 was attributed to the fact that the large number of O ions were located in O(3) site, which resulted in localization of holes. In this case, about 50% of oxygen ions in the (As, Cu)O 0.88 layer is located at O(3) site, leading to localization of holes. If the number of oxygen ions in the O(4) site is increased, superconductivity can be expected.

Preparation of a new layered cuprate (As0.33Cu0.67)Sr2YCu2Oy and its physical properties

(As 0.33 Cu 0.67 )Sr 2 YCu 2 O y was newly prepared. (As 0.33 Cu 0.67)Sr2 YCu 2 O 6.88 has a tetragonal unit cell with a = 3.829(6), c = 11.47(1)A. Annealing the sample in oxygen led to an increase in the Cu valence as large as 2.38. However, resistivity measurements show that (As 0.33 Cu 0.67 )Sr 2 YCu 2 O 6.88 is not superconducting but semiconducting.

Preparation of (As0.33Cu0.67)Sr2YCu2O6.88 and its physical properties

(As0.33Cu0.67)Sr2YCu2Oy was newly prepared. The (As0.33Cu0.67)Sr2YCu2O6.88 has a tetragonal unit cell with a = 3.829(6), c = 11.47(1)A. Annealing the sample in an oxygen atmosphere led to an increase in the Cu valence as large as 2.38. However, resistivity measurement showed that (As0.33Cu0.67)Sr2YCu2O6.88 was not superconducting but semiconducting. Nonsuperconductivity of (As0.33Cu0.67)Sr2YCu2O6.88 was attributed to the fact that the large number of O ions was located in O(3) site, which resulted in localization of holes. In this case, about70% of oxygen ions in the (As, Cu)O0.88 layer is located at O(3) site, leading to localization of holes. If the number of oxygen ions in the O(4) site is increased, superconductivity can be expected.

Single Crystal Study of Enchanced Flux Pinning in (Hg,Pb)(Ba,Sr)2Ca2Cu3O8+δ and (Hg,Pb)Sr2Ca2Cu3O8+δ

Here we report the strong enhancement effects of the cation substitutions on irreversibility (Hirr-T/Tc) and fishtail (Hs.p.-T/Tc) line positions of Hg-1223 deduced from magnetization measurements of single crystal samples. Such an enhancement was attributed to reduced electronic anisotropy caused mainly by substitutions of Ba by Sr and enhanced flux pinning properties due to the possible inhomogeneity in the Hg/Pb distribution in the studied single crystals. The high values of Tc [>110 K] and Jc(77K,1T) [>3×104A/cm2] in addition to the peak-effect phenomenon [1.0<Hs.p<3.0.T at 80<T<40 K] have made the studied compounds promising candidates for high temperature and high magnetic field applications.

Peak Effect Caused by Compositional Modulation in Superconducting (Hg,Pb)(Ba,Sr)-1223 Single Crystal

We have discovered fish-tail phenomenon / peak effect in a (Hg,Pb)(Ba,Sr)2Ca2Cu3O8+δ[(Hg,Pb)(Ba,Sr)-1223] single crystal. Microscopic analyses and observations were performed to elucidate the origin of peak effect in terms of the micro-structure of the (Hg,Pb)(Ba,Sr)-1223 single crystal. In a [001] ED pattern of the crystal, streaks were observed along <110> at each spot. Distortions were seen along <110> in HRTE-micrograph of the (001) plane, and a compositional modulation was clearly detected with respect to the Hg/Pb ratio along [110] with an average periodicity of ∼18 nm. Such a spinodal-wave-like inhomogeneity in the local composition may yield mesoscopically distributed weak pinning centers which is most likely responsible for the appearance of peak effect.