Kazuyuki Isawa

Encapsulation method for the synthesis of nearly single-phase superconducting HgBa2Ca2Cu3O8+δ with Tc⩾ 135 K

Nearly single-phase polycrystalline samples of HgBa2Ca2Cu3O8+δ were successfully synthesized under relatively simple and unprecedented synthesis conditions, i.e. at a “low” temperature (<700°C) and for a “long” period of time (5–168h), using mono-oxide powders of Hg, Ba, Ca, and Cu as starting materials. A pressed powder mixture was heated at 665°C for ≈75 h in an evacuated fused quartz tube, and then post-annealed in flowing O2 gas to exhibit a Meissner signal below 135.4 K. Zero resistance of this sample was observed at 133.9 K.

The effect of Pb doping in HgBa2Ca2Cu3O8+δ superconductor

Abstract Polycrystalline samples with the starting compositions of (Hg 0.8 Pb 0.2 ) x Ba 2 Ca 2 Cu 3 O 8+δ (1.0⩽ x ⩽2.5) were prepared under various synthetic conditions. Both powder X-ray-diffraction analysis and energy dispersive X-ray spectroscopy revealed that the Pb ions were successfully doped into the Hg-1223 phase. High-quality Pb-doped Hg-1223 samples for the starting composition with x =1.5 were obtained when heated at 675°C only for 1 h. The resistivity (ϱ) of such a sample was low, being ∼8×10 3 Ω cm at 300 K, and the ϱ versus T curve was linearly extrapolated to zero resistance at 0 K, being similar to the cases of high-quality high- T c cuprates. The zero resistance due to superconductivity started at 122.2 K. When the sample was reduced in flowing argon gas, it exhibited a Meissner signal below 132 K and its Meissner volume fraction exceeded 20% at 5 K. According to thermoelectric power measurements, the carrier density in the sample was reversibly dependent on the oxygen partial pressure, and the sample post-annealed in flowing oxygen gas was likely overdoped.

Composition, crystal structure and superconducting properties of HgBaCuO and HgBaCaCuO superconductors

Abstract The composition, crystal structure and superconducting properties of HgBaCuO and HgBaCaCuO superconductors were investigated. Samples with various nominal compositions were prepared from the mono-oxides of Hg, Ba, Ca and Cu using various sintering processes. The samples were examined by X-ray powder diffraction and energy dispersive X-ray analysis (EDX). The HgBaCuO samples were found to be nearly single phases of HgBa2CuO4+δ (Hg-1201), but the HgBa CaCuO samples contained HgBa2Ca2Cu3O8+δ (Hg-1223), HgBa2CaCu2O6+δ (Hg-1212) and other phases. The fraction of the higher-Tc phase in the HgBaCaCuO sample varied in accordance with the nominal composition. High-resolution transmission electron microscopic (HRTEM) studies revealed the existence of Hg-1201 (tetragonal cell with a=3.85 and c=9.5 A ), Hg-1212 ( c=12.6 A ). and Hg-1223 structures ( c=15.7 A ). Most of the HgBaCuO samples exhibit 95 K-classs superconductivity, and the Hg BaCaCuO samples showed 130 K-class superconductivity. The highest Meissner onset temperature, Tcm determined from DC susceptibility was 134 K. The same sample showed superconductivity with a ≈ 16% diamagnetic volume fraction at 5 K.

Annealing effects on the Meissner signals from HgBaCaCuO superconductors

Abstract As-sintered samples of the HgBaCaCuO superconductors were annealed in flowing argon, flowing oxygen and high-pressure oxygen gases. The superconducting critical temperature, T c , of the sample increased after annealing in either flowing oxygen or a high-pressure oxygen gas, while it decreased after annealing in flowing argon. These results indicated that the as-sintered samples were in slightly “under-doped” states.

Pb‐doping effect on irreversibility fields of HgBa2Ca2Cu3O8+δ superconductors

We prepared nearly single‐phase polycrystalline samples of HgBa2Ca2Cu3O8+δ(Hg‐1223) without and with Pb doping, i.e., Pb‐free Hg‐1223 and (Hg0.8Pb0.2)‐1223, by means of a sample encapsulation technique. According to quantitative energy dispersive x‐ray spectroscopy analyses and iodometric oxygen titration, the Pb‐free Hg‐1223 and the (Hg0.8Pb0.2)‐1223 sample were identified to be Hg0.97Ba2.00Ca2.04Cu3.00O8.33 and (Hg0.79Pb0.29)Ba2.00Ca1.98Cu2.94O8.42, respectively. The highest Meissner‐signal onset temperatures for the Pb‐free Hg‐1223 (after post‐annealing in O2 gas) and the (Hg,Pb)‐1223 (as‐sintered) sample were observed at 135.4 and 132.0 K, respectively. The difference in the irreversibility fields (Hirr) for both samples was very small at temperatures above 80 K, while, at temperatures below 70 K, the Hirr versus T curve for the (Hg,Pb)‐1223 sample shifted toward the higher field side. Thus, Pb doping was effective for the improvement of magnetic property of Hg‐1223 superconductors, at least, at tempe...

Optimization of the synthesis and tuning the oxygen content of the CuBa2Ca2Cu3O8+δ (Cu-1223:P) superconductor

Abstract The high-pressure (5 GPa) synthesis of the superconducting CuBa 2 Ca 2 Cu 3 O 8+ δ (Cu-1223:P) phase was optimized in terms of temperature, the starting metal composition and the amount of AgO used as an external oxygen source. The most phase-pure Cu-1223:P samples were obtained from the 2BaCuO 2+ x –Ca 2 CuO 3 –CuO starting stoichiometry when performing the synthesis at 1050–1100°C and using 200 mol% of AgO. The as-synthesized Cu-1223:P samples with T c =67 K were found to be strongly overdoped by a thermoelectric power measurement. With post-annealings carried out in a thermobalance in a flowing Ar gas at temperatures ranging from 300 to 420°C the hole-doping state in the as-synthesized Cu-1223:P material could be tuned in the same continuous manner observed for the YBa 2 Cu 3 O 6+ δ (Cu-1212:P) compound, even though all the obtained Cu-1223:P samples were, in contrast to so far obtained YBa 2 Cu 3 O 6+ δ samples, in an overdoped state. The decrease in the hole concentration in the Cu-1223:P phase with increasing annealing temperature was accompanied with increases in T c and in both lattice parameters of the tetragonal unit cell. The highest obtained T c was 118 K for the sample annealed at 400°C. Above 400°C in Ar the Cu-1223:P phase was found to decompose. From the observed overall weight loss upon the Ar-annealing up to decomposition temperature the amount of removable oxygen in the Cu-1223:P phase could be established at Δ δ ≈0.4.

Growth, Characterization and Application of Single-Crystal La2-xSrxCuO4 Having a Gradient in Sr Concentration

We succeeded in growing sizable single crystals of La2-xSrxCuO4 (LSCO) having a gradient in the Sr concentration along the axis of crystal growth. These crystals were completely characterized and the gradient of x (x ~0.03/cm), extending from x = 0.00 to 0.30, was determined from the positional dependences of local lattice constants and by inductively coupled plasma analysis. In accordance with the gradient of x, a gradient of hole concentration was also confirmed from the positional dependences of resistivity and superconducting transition temperatures. A real space boundary between the superconducting and the insulating phases was first observed as the boundary of the vortex state by scanning magnetic fields along the gradient of x using a scanning superconducting quantum interference device (SQUID) microscope. Precise measurement of the diamagnetic susceptibility of LSCO as a function of doping yielded a phase diagram that is consistent with one obtained from samples with constant x. We used these crystals for the phonon measurement with a synchrotron X-ray beam and succeeded in determining the doping dependence of the dispersion relation by changing the position of the incident beam spot on the crystals with the same experimental setup.

Substitution effects of Ca for Sr in Pb-based layered copper oxides, (Pb(1+x)/2Cu(1−x)/2Sr2(Y1−xCax)Cu2O7+δ

Abstract (Pb/Cu)-1212 cuprates in which Sr sites are partially substituted by Ca, (Pb (1+x)/2 Cu (1−x)/2) (Sr 1−y Ca y ) 2 (Y 1−x Ca x )Cu 2 O 7+δ , are synthesized and characterized. While single-phase samples are obtained only in the range of 0≦ ×≦ 0.35 for y=0, partial substitution of Ca for Sr stabilizes the (Pb/Cu)-1212 compounds which contain Ca in the Y sites with x>0.35. For x = 0.5, nearly single-phase samples are obtained for y = 0.05 and 0.1. Superconductivity transition temperature as high as 80 K is measured for the sample with x = 0.5 and y = 0.1.

151Eu Mössbauer study of the CuEuO2+δ delafossite

Abstract Eu-based delafossite samples CuEuO 2+ δ , with δ =0.00 and 0.67 were synthesized for 151 Eu Mossbauer investigations. The value of the quadrupole coupling constant for the as-synthesized stoichiometric ( δ =0.00) and the oxygen-loaded ( δ =0.67) samples was eQ g V zz =9.4(2) and 6.1(2) mm/s, respectively. The asymmetry parameter η was 0.44(4) and 0.76(4), respectively. Lattice point-charge calculations reproduced the positive eQ g V zz values and the increase of the asymmetry parameter η upon oxygen loading.

Thermoelectric power of electron-doped superconductor, Pr1−xCexLaCuO4+δ (0⩽x⩽0.2)

Abstract Superconducting properties were measured for polycrystalline nearly single-phase samples of Pr1−xCexLaCuO4+δ (PLCCO) (0⩽x⩽0.2). Thermoelectric power (S) was found to be negative for all the samples in the temperature range between 50 and 310 K. This evidence clearly indicates that the dominant charge carriers of Pr1−xCexLaCuO4+δ are electrons.