Yasukage Oda

Two Crystal Phases in the Superconducting Ba-Y-Cu-O System and Their Reactivity to Water

Crystals with a stoichiometry either Ba2YCu3-xO7-y or Ba4YCu2O8-y are a high tc superconductor. The former is well known. With respect to the latter the crystal structure was determined to be tetragonal with lattice parameters a=5.78 A and c=7.99 A. It was found that both of them reacted with water. The former was slowly decomposed by warmed water at about 35°C, while the latter was destroyed at room temperature in an instant.

NMR Observation in Nd1.85Ce0.15CuO4-y

An anomalous Cu NMR signal with extremely small e 2 q Q has been observed in the oxide superconductor Nd 1.85 Ce 0.15 CuO 4- y . The Knight shift decreased significantly as temperature decreased below T c . The temperature-independent T 1 of 63 Cu suggests that the relaxation is dominated by the fluctuation of the Nd-moment, and no magnetic order is achieved for this content of Ce.

Crystal Structure of Superconductor Ba2YCu2.89O6.80

Crystal structure of Ba2YCu2.89O6.80 sigle crystal was determined by the X-ray diffraction method at room temperature. The space group is Pmmm with lattice parameters a = 3.8699(8) A, b = 3.8667(8) A and c = 11.688(2) A. A specific structure was found in the c-plane sandwiched between two barium layers. The Cu-O bond perpendicular to the plane 1.828(6) A is shorter than the normal bond 1.95 A. Within the plane oxygen atoms show an extremely large thermal vibration.

NQR and NMR studies of Cu in the superconducting La2CuO4+δ

Abstract In the superconducting La2CuO4+δ obtained by annealing in high pressure mixture gas of oxygen and argon, nuclear quadrupole resonance (NQR) signals of 63Cu and 65Cu have been observed around 33.1 and 36.0 MHz together with the antiferromagnetic Cu NMR signal around 70–100 MHz. The ratio of the resonance frequency of 63Cu and 65Cu is the same as that of the quadrupole moment of each Cu nucleus, indicating no internal magnetic field (paramagnetic) at Cu site. The nuclear spin-lattice relaxation time, T1, of Cu NQR indicates that the paramagnetic phase is in the superconducting state at low temperatures. The NQR intensity is roughly proportional to the partial pressure of the oxygen gas. The NQR result suggests that the volume fraction of paramagnetic phase in La2CuO4+δ increases with increasing oxygen content.

57Fe mössbauer study of high-T c Y−Ba−Cu oxide superconductors

Abstract57Fe Mössbauer measurements have been performed for Fe-doped YBa2 (Cu1-x Fex)3O7-v (x=0.005–0.10) superconducting oxides. Fe atoms mainly substitute at Cul chain sites and the fraction of Fe at Cu2 plane sites is discussed thermodynamically. The spin direction of Fe at Cu2 was determined to be normal to thec-axis using an oxygen deficient specimen having a strong texture along thec-axis. A magnetically broadened spectrum at 4.2 K for Fe at Cul is compared with a simulated one which takes the distribution of hyperfine fields into account.

Superconducting properties of La2CuO4+δ annealed in high pressure oxygen gas

Abstract Electrical and magnetic properties of (La1−xSrx)2CuO4+δ annealed in high pressure oxygen gas up to 400 bar are studied. The superconducting La2CuO4+δ includes two separate phases: one is the superconducting phase with extra oxygen, and the other is the well known antiferromagnetic phase with Neel temperature TN at 240–300 K depending on the annealing atmosphere. The superconducting phase in the undoped sample becomes unstable by Sr doping. In the high pressure oxygen annealed La2CuO4+δ, a temperature dependent paramagnetism is observed even below TN, and spontaneous vortices are considered to be induced in the zero magnetic field cooling.

Hole doping effect in (La1-xMx)2CuO4+δ(M = Sr, Ca) annealed in high pressure oxygen gas

Electrical properties of (La1−xMx)2CuO4+δ (M = r, Ca) annealed in high pressure oxygen gas up to 400 bar are systematically studied in the composition range between x = 0 and x = 0.075. The increase of hole concentration which is supposed to arise from the incorporated excess oxygen by oxygen annealing is found especially in low doping level x < 0.03. This is consistent with the dependence of incorporated excess oxygen content Δδ on the dopant concentration x. A model to explain the plateau in the temperature dependence of resistivity of the undoped La2CuO4+δ is discussed.

New superconducting phases in La2CuO4+δ annealed in high- pressure oxygen gas

Abstract The superconducting property of La 2 CuO 4+ δ annealed in high-pressure oxygen gas has been studied through resistivity and susceptibility measurements. Three different superconducting onsets have been found, which depend on the cooling process. When the sample was quenched down to 77 K from above 250 K, one superconducting onset was observed at 38 K. On the other hand, when the sample was slowly cooled down to 77 K from 300 K or quenched from below 200 K, two superconducting onsets were observed at 42 and 48 K. These three onset temperatures are definite, and do not depend on the oxygen pressure of annealing. The three onsets are concluded to be ascribed to three superconducting phases. The two of them are considered to occur through a phase separation at temperatures between 200 and 250 K in the process of slow cooling.

Clamp Type High Pressure Apparatus Using Small Bridgman Anvil at Low Temperature

High pressure apparatus at liquid helium temperature is built. It is a high pressure cell of clamp type using small Bridgman anvil (4 mm face) geometry. The pressure is applied to a sample using a standard hydraulic press and clamped by means of three bolts at room temperature, then the cell is cooled down to low temperature. The pressure is calibrated by means of the phase transition of Bi, Tl and Sn at room temperature, and pressure dependence of superconductive transition temperature of tin at low temperature which is measured by means of the electrical resistance and a.c. mutual inductance bridge.

195Pt NMR study of the heavy fermion superconductor UPt3

Abstract In the superconducting state, T -1 1 of 195 Pt has no enhancement just below T c and varies as T 3 at low temperatures. This indicates the existence of an anisotropic superconducting energy gap which vanishes at lines on the Fermi surface. The Knight shift data show that the conduction electron spin susceptibility does not decrease below T c .