K. Fujinami

Strongly overdoped states and irreversibility-field characteristics of the Hg-1223 and Cu-1223:P superconducting cuprates

Essentially single-phase Hg-1223 and Cu-1223:P samples in wide hole-doping ranges were successfully synthesized by a high-pressure (5 GPa) technique utilizing internal and/or external oxidizing agents and by subsequent post-annealings performed in a thermobalance. The hole-doping level of the as-synthesized samples was found to be in the overdoped side for both of the phases by thermoelectric power measurements. values as low as 107 K and 67 K were obtained for the overdoped Hg-1223 and Cu-1223:P samples, respectively. The values of and were calculated from the measured M versus H data employing, respectively, the Bean model and two different criteria: (at ) and (as ). It was found that the heavier the doped hole concentration was, the more improved were the versus characteristics for both the Hg-1223 and the Cu-1223:P phases. This tendency was the same for versus characteristics calculated using either criterion. The improvement with increasing hole-doping level was especially profound in the case of Cu-1223:P: the versus characteristics of a Cu-1223:P sample with K, probably being close to the optimally doped state, were as poor as that reported for Bi-2212, while an overdoped Cu-1223:P sample with K was superior to an overdoped Hg-1223 sample with K. The mechanisms as well as the usefulness of the observed phenomena are discussed.

Effect of overdoping on the irreversibility field and critical current density of the HgBa2Ca2Cu3O8+δ superconductor

Abstract For HgBa 2 Ca 2 Cu 3 O 8+ δ which is of the pure Hg-1223 phase without substitutions in the cation sites, it is demonstrated that the larger the amount of oxygen excess ( δ ), the better the magnetic irreversibility field ( H irr ) vs. normalized temperature (1− T / T c ) characteristics. The best H irr vs. (1− T / T c ) characteristics in the present experiment was obtained for a strongly overdoped sample with T c =107 K which was synthesized at 950°C under a high pressure of 5 GPa using highly-oxidized BaCuO 2+ x ( x =0.13) as one of the starting materials. The relationship between the hole-doping level and the H irr vs. (1− T / T c ) characteristics is discussed.

Investigation of Different Hole-doping Routes in Cu(Ba,Sr)2(Yb1-xCax)Cu2O6+z

Abstract A variety of Cu(Ba 0.8 Sr 0.2 ) 2 (Yb 1− x Ca x )Cu 2 O 6+ z (Cu-1212) samples were successfully synthesized in the wide compositional ranges of 0≤ x ≤0.4 and 0≤ z ≤0.9. By controlling x and z , the conductances of the two different ways in hole-doping the Cu-1212 phase could be varied. Especially the x =0 and the z ∼0 sample series allowed us to investigate the calcium and oxygen doping schemes separately. Without Ca-doping (i.e., x =0) the Cu(Ba 0.8 Sr 0.2 ) 2 YbCu 2 O 6+ z phase becomes superconducting when z exceeds 0.45. With increasing oxygen excess, the tetragonal structure turns into orthorhombic, and T c increases rapidly. In the samples with z ∼0, on the other hand, the Cu–O z chains are unlikely to dope holes into the CuO 2 planes, and the distinct Ca-doping effects could be elucidated systematically. In the whole substitutional range of 0≤ x ≤0.4 the fully oxygen-depleted samples possess the tetragonal-type of Cu-1212 structure with the lattice constants a and c remaining quite independent of the value of x . Superconductivity appears at x ∼0.17, but with increasing Ca-doping T c was found to increase less efficiently than in the case of doping from the Cu–O z chains. For discussing the differences in the hole-doping efficiency, the bond-valence concept was utilized. Also, the possible role of the hole distribution in the in-plane Cu–O bond in determining the superconducting properties was considered.

Stoichiometry and copper valence in the Ba1–yCuO2+δ system

The cation stoichiometry and the tunability of oxygen content in the Ba 1–y CuO 2+δ phase were investigated by means of various methods of chemical analysis. Samples with different nominal metal stoichiometries, y nom ranging from 0 to 0.1, were synthesized. Single-phase samples were obtained for y nom ≈0.1, i.e. with about 10 mol% barium deficiency. The possibility to stabilize the Ba 1–y CuO 2+δ phase with different oxygen stoichiometries was studied thermogravimetrically. Under an argon atmosphere below 400 °C a part of the excess oxygen of the fully oxygenated Ba 0.9 CuO 2.01 sample was found to be released in two steps leading to stoichiometries of Ba 0.9 CuO 1.97 and Ba 0.9 CuO 1.94 . On the other hand, above 400 °C it was possible to continuously tune the oxygen stoichiometry at least down to Ba 0.9 CuO 1.85 (700 °C). A decrease in the copper valence with decreasing oxygen content led to an increase in the effective magnetic moment obtained from the magnetic susceptibility measurements performed for two Ba 0.9 CuO 2+δ samples with different oxygen stoichiometries.

Ca doping effect on superconductivity in (Yb1−xCax)(Ba0.8Sr0.2)2Cu3O6+z

(Yb 1-x Ca x )(Ba 0.8 Sr 0.2 ) 2 Cu 3 O 6+ samples with widely ranged values of x and z were successfully synthesized. The as-synthesized samples were annealed in flowing Ar gas in a thermobalance. Totally oxygen-deficient samples whose oxygen contents were 6 + z ∼ 6.0 were prepared by annealing the as-synthesized samples at 800°C in flowing Ar gas. For the samples with the oxygen content, 6 + z ∼ 6.0, no superconductivity occurred for 0 ≤ x < 0.20, but for 0.20≤ x≤ 0.35 T c increased as x increased.

Synthesis and Characterization of Strongly Over-doped HgBa2Ca2Cu3O8+δ Samples

Strongly over-doped HgBa2Ca2Cu3O8+δ samples with a T c as low as 97 K were successfully obtained using highly oxidized BaCuO2+e (e ≈ 0.13) and BaO2 powders as starting materials for the high-pressure synthesis at 5 GPa and 950 °C. The over-doped state was confirmed by observing a negative value for the Seebeck coefficient at temperatures above T c in a thermoelectric power measurement. Synthesis from less oxidized BaCuO2+e (e ≈ 0.06) yielded material with a lower doping level. By subsequent annealings in O2 at 300 °C and in Ar at 400 °C optimized (T c = 134 K) and under-doped(T c = 118 K) samples, respectively, were obtained. According to the magnetization experiments, the heavier the doped hole concentration was, the better was the H irr -versus-(T/T c ) characteristics.

Hole Distribution and Magnetic-Field Irreversibility in Cu(Ba0.8Sr0.2)2(Yb1-xCax)Cu2O6+z

Mutual correlations of (1) the irreversibility field (H irr ), (2) changes in the local atomic positions and (3) the concentration and distribution of holes over the crystal of the Cu-1212:P (or “123”) phase were investigated. A series of samples, Cu(Ba0.8Sr0.2)2(Yb1-xCax)Cu2O6+z, with a variety of doping levels (0.1 ≤ x ≤ 0.35 , 0.55 ≤ z ≤ 0.96) was employed. The hole distributions among the different oxygen sites were calculated from the O K-edge XANES results previously obtained for the same samples. Moreover, the gradual changes of the local crystallographic structure and in the BVS’s of the Cu and O atoms in the CuO2 plane were followed utilizing the neutron-diffraction data previously measured for the samples with increasing Ca substitution and/or O doping levels. It was concluded that the H irr characteristics of Cu-1212:P is controlled by (i) the hole concentration in the infinite-layer block, (ii) the thickness of the charge-reservoir block, and (iii) the hole concentration in the charge-reservoir block.

Hole Doping Mechanism and Structural Changes in Cu(Ba0.8Sr0.2)2(Yb1-xCax)Cu2O6+z

The effectiveness of different ways to introduce holes into the CuO2 planes was compared in the Cu(Ba0.8Sr0. 2)2(Yb1-xCax)Cu2O6+z (Cu-1212) system in the wide substitution ranges of 0 ≤ x ≤ 0.4 and 0 ≤ z ≤ 0.9, both in respect to the capability to increase T c and in terms of the hole production as estimated from neutron diffraction data via bond-valence calculation. Oxygen doping was found to increase the hole concentration less efficiently, and further, at a certain hole concentration value higher T c values were obtained with calcium doping. The two different doping ways exhibited also different T c - versus- Cu-O bond length relations. As a conclusion, the possible role of the hole distribution in the in-plane Cu-O bond in determining the superconducting properties was recognized.