R. Usami

Dependence of superconducting properties on the Cu-valence determined by iodometry in HgBa2CuO4+δ

Abstract We have synthesized nearly single-phase HgBa2CuO4+δ samples with various oxygen contents, and systematically investigated the relationship between the lattice parameters, the superconductivity transition temperature (Tc) and the oxygen content determined by iodometry. Furthermore, the nominal copper valence values were calculated from the oxygen content data by assuming constant valence values of +2.00 and −2.00 for mercury and oxygen, respectively. Both the a and c lattice parameters were found to decrease monotonically with the Cu valence over a wide range of +2.05–2.28. The obtained Tc versus Cu valence curve exhibits a “bell shaped” correlation with the maximum in Tc at the Cu valence of ∼ +2.18.

The oxygen-content dependence of the magnetization anisotropy of HgBa2CuO4 + δ

Abstract The anisotropy of magnetization of the HgBa 2 CuO 4 + δ superconductor has been found to depend on δ, the excess oxygen content. Nearly single-phase samples of HgBa 2 CuO 4 + δ in both over-doped and under-doped regions are successfully prepared by post-annealing the samples in different atmospheres. The magnetization for grain-aligned HgBa 2 CuO 4 + δ samples are measured in magnetic fields both parallel and perpendicular to the c -axis. In the field parallel to the c -axis, the excess oxygen in the HgBa 2 CuO 4 + δ sample has been found to work for raising the irreversibility line to a position higher than that for YBa 2 Cu 3 O 6.6 . It is also found that the presence of excess oxygen enhances the magnitude of the magnetization and lowers its field dependence in the field parallel to the c -axis. Thus the anisotropy of magnetization is lowered by the excess oxygen. It is likely that the excess oxygen in the HgBa 2 CuO 4 + δ phase makes the HgO δ layer more conductive along the c -axis so that the vortices may become quasi-three-dimensional.

Synthesis of HgBa2Ca3Cu4Oy under ambient pressure

Abstract The mercury-based oxide superconductor, HgBa 2 Ca 3 Cu 4 O y phases was synthesized by a controlled vapor/solid reaction technique using high-purity powders of HgO, BaO, CaO and CuO as starting materials. Two types of barsm, with different cation ratios of Hg:Ba:Ca:Cu=0:2:2:3 and 1:2:2:3, were heated in an evacuated quartz tube. The resulting HgBa 2 Ca 3 Cu 4 O y sample had a T c of 123 K and lattice parameters a=3.852(2) A and c = 19.01(4) A with a volume fraction of 83% of the total shielding signal. The formation condition was widely investigated, showing that the higher heating temperature and longer heating time lead to the formation of the HgBa 2 Ca n Cu n −1 O y phase with a higher number n of CuO 2 planes in a unit cell.

A New Homologous Series of Superconducting Cuprates with the 22(n-1)n Structure, (Hg1-xTlx)2Ba2Can-1CunOy (x∼0.4, n = 3, 4)

A new homologous series of superconducting cuprates containing a mercury-based bilayer has been synthesized using a high-pressure, high-temperature technique. It is found that Tl doping stabilizes a bilayer block with the rock-salt structure i.e. (Hg1-x Tlx )2–O2. (Hg1-x Tlx )2Ba2Ca2Cu3Oy and (Hg1-x Tlx )2Ba2Ca3Cu4Oy as-prepared samples with analized Tl contents of approximately 0.4 exhibited bulk superconductivity below 45 K and 114 K, respectively.

NMR and NQR study in HgBa2CuO4+δ

Abstract The temperature dependences of the Cu nuclear spin-lattice relaxation rate (1/ T 1 ) and the Knight shift at the planar Cu site in the as-sintered HgBa 2 CuO 4+δ with T c ≈ 84K have been measured by the nuclear quadrupole resonance(NQR) and the nuclear magnetic resonance(NMR) techniques. The as-sintered sample seems to be a lightly doped material because of the increase of T c by annealing in the oxygen atmosphere. From the broadness of the NQR spectrum and the distribution of the (1/ T 1 ), we conclude that the as-sintered sample has an inhomogeneity of the atomic location, especially oxygen. The temperature dependence of (1/ T 1 ) behaves as T n ( n ≈ 3−4) and there is no Hebel-Slichter peak below T c , which is re-confirmed the common feature in high- T c cuprate superconductor.

Microstructure of proton-irradiated HgBa2Ca(n−1)CuxOy (n = 2,3)

Abstract Irradiation with 4.0 MeV protons at a dose of 5 × 10 16 cm −2 was performed for a grain-aligned HgBa 2 Ca( n −1 Cu n O y n = 2,3 sample. Magnetization hysteresis loops in external fields up to 5.5 T applied along the c -axis for the irradiated sample became wider than those for the unirradiated sample, indicating introduction of pinning centers by the proton irradiation. X-ray diffraction and TEM studies indicated that the proton irradiation produced amorphous defects within irradiated grains. The shape of the amorphous defects are 16 A thick along the c -axis and 220 A long in average. It is likely that the amorphous defects due to the proton irradiation serve as pinning centers in the present HgBa 2 Ca ( n −1) Cu n O y n = 2,3 samples.

Irreversibility lines of HgBa2CuO4+δ with different oxygen contents

Abstract Single phase HgBa 2 CuO 4+δ samples were annealed in flowing oxygen gas or in flowing argon gas. Oxygen contents of both samples determined by iodometric titrations indicate that the samples annealed in oxygen gas and in argon gas were respectively in over-doped and in under-doped region. Both samples were respectively grain-aligned in magnetic fields. Hysteresis loops for grain-aligned over-doped and under-doped samples were measured in applied fields parallel and perpendicular to the c-axis. Region where irreversibility lines of over-doped sample was located was found to be higher than that of under-doped sample. Irreversibility line of over-doped sample was found to be higher than that of YBa 2 Cu 3 O 6.6 . Anisotropy of irreversibility lines of both samples were found to be smaller than that of Y-123 superconductors.