Tetsuo Fukase

Correlation between Tc and hole concentration in the cation-substituted Bi2Sr2CaCu2O8+δ system

Abstract Correlation between superconducting T c and hole concentration was investigated in the Bi 2 Sr 2 CaCu 2 O 8+δ system. The hole concentration was varied by substituting cations with different valences for Sr or Ca. In order to increase the hole concentration, some samples were annealed at 430°C in 250 bar of oxygen pressure. We find that T c increases, takes a maximum at the hole concentration corresponding to 0.2–0.3 holes per CuO 2 unit, and then decreases, with increasing hole concentration. This correlation is markedly similar to those in the La 2− x Sr x CuO 4 and YBa 2 Cu 3 O 7− δ systems.

Growth and very anisotropic upper critical field of single-crystal Bi2Sr2CaCu2Oy

Flaky single crystals of a new high-Tc oxide Bi2Sr2CaCu2Oy with dimensions of the order of 1×1×0.1 mm3 have been prepared using a CuO flux method. The electrical resistivity shows a sharp superconducting transition at 80.6 K. The upper critical field Hc2 was determined to be very anisotropic. The values of -dHc2/dT for H//c-axis and H⊥c-axis are 0.39 T/K and 7.3 T/K, respectively. The ratio of -dHc2⊥/dT to -dHc2///dT is 19. The value of Hc2⊥(0) for H⊥c-axis is estimated to be as large as 400 T. The Ginzburg-Landau coherence length along the c-axis at 0 K ξ//(0) is 2.1 A, suggesting quasi-two-dimensional superconductivity.

Charge Ordering and Sound Velocity Anomaly in La1-XSrXMnO3(X≥0.5)

The electrical resistivity, magnetization, dilatation and sound velocity have been measured for La 1- X Sr X MnO 3 (0.48≤ X ≤0.90) polycrystals. Observed anomalies in the sound velocity and the dilatation strongly suggest the occurrence of the charge ordering within the Sr concentration range 0.48≤ X ≤0.82. A phase diagram of the charge-ordered state in La 1- X Sr X MnO 3 as a function of temperature T and the Sr concentration X is proposed.

63/65Cu/139La-NMR Study on Antiferromagnetic Ordering in High-Tc Oxides La2-xSrxCuO4 (\(x\cong 0.115\)) and La2-xBaxCuO4 (\(x\cong 0.125\))

La-based high- T C oxides La 2- x Ba x CuO 4 (LBCO) and La 2- x Sr x CuO 4 (LSCO) show a local minimum in the curve of T C versus x around 0.125 for LBCO and around 0.115 for LSCO. In order to investigate the origin of this local minimum, temperature dependence of 63/65 Cu/ 139 La-NMR/NQR spectra has been studied on spray-dry-made samples with various Ba/Sr-concentrations. Existence of the antiferromagnetic ordering was confirmed by a significant broadening in the resonance lines at low temperatures below 35 K for LBCO (\(x\cong 0.125\)) and 32 K for LSCO (\(x\cong 0.115\)). By the analysis on the anisotropy in the broadening, the direction of the ordered field was found to be within the CuO plane. The hyperfine field and the Neel temperature showed a maximum at the Ba/Sr-concentration around which T C is most suppressed, suggesting the crucial interplay between the antiferromagnetic ordering and the suppression in T C .

Ultrasonic studies of structural phase transitions and superconductivity in La2−xBaxCuO4−δ and La2−xSrxCuO4−δ

Attenuation and velocity of 7 MHz longitudinal ultrasound in polycrystalline ceramic La 2−x Ba x CuO 4−δ and La 2−x Ba x CuO 4−δ have been measured in the temperature range from 300 K to 1.5 K. On cooling a pronounced softening of the lattice is observed for 0.09 ≤ × ≤ 0.20 just above the transition temperature T dl from the tetragonal phase to the orthorhombic phase. A large attenuation peak is observed at T dl . The further low-temperature transformation from the orthorhombic phase to the tetragonal low-temperature phase is observed, with partial lattice stiffening and a small attenuation peak at T d2 , not only in La 2−x Ba x CuO 4−δ but also in La 2−x Sr x CuO 4−δ The mechanism of the unusual T C reduction around x = 0.12 is proposed on the basis of the distortion of CuO 2 basal planes from a square to a rhombus below T d2 .

Anomalous Splitting of 1H-NMR Spectra in λ-(BEDT-TSF)2FeCl4

For an organic π–d metal λ-(BEDT-TSF) 2 FeCl 4 , where BEDT-TSF is an abbreviation of bis(ethylenedithio)tetraselenafulvalene, we have measured 1 H-NMR to clarify the quite anomalous ferroelectric-...

Sound Velocity Anomaly Associated with Polaron Ordering in La1-xsrXMnO3.

The sound velocity v s ( T ) and the electrical resistivity ρ( T ) of La 1- X Sr X MnO 3 (0.11≤ X ≤0.17) have been measured and anomalies in v s ( T ), which are related to the polaron-ordered phase centered at X =1/8, have been observed. The phase diagram between the polaron-ordering temperature T P and X has been determined from the v s anomalies.

Weak Localization in Graphite

Conductivity and magnetoconductivity have been measured for various kinds of graphite crystals at low temperatures down to 0.03 K. Anomalous behaviours have been found in polycrystalline (but well oriented) specimens of graphite. At low temperatures below 0.1 K, the conductivity shows T 1/2 dependence in and without magnetic field, Anisotropic magnetoconductivity is observed. It varies as H 1/2 , with the sign which changes from negative to positive with increasing magnetic field strength. These behaviors are fairly well explained in terms of localization and electron-electron interaction in the three-dimensional weakly localized regime. The inelastic scattering time τ e shows a little weaker temperature dependence than T -1 .

Anomalous Lattice Softening at X=0.19 and 0.82 in La1-XCaXMnO3

The longitudinal sound velocity v s ( T ) has been measured for La 1- X Ca X MnO 3 crystals, and anomalous softenings of v s ( T ) have been observed at the peculiar Ca concentrations of X =0.19 and X =0.82. For the X =0.19 sample, in which the ferromagnetic (FM) metallic phase competes with the FM insulating phase with a polaron ordering, the anomalous v s ( T ) softening occurs immediately below the FM transition temperature T c . For the X =0.82 sample, in which the charge-ordered (CO) phase competes with the canted antiferromagnetic (CAF) phase, the deep local minimum of v s ( T ) occurs around the CO transition temperature T CO . These anomalous v s ( T ) softenings result from the giant lattice fluctuations which originate from phase competitions and suggest strong charge-lattice and spin-lattice coupling in this system.

Growth of YBa2Cu3O9−δ single crystals from the high temperature solution

YBa2Cu3O9-δ single crystals in the shape of a needle and a square plate were grown from a CuO-rich solution at a high temperature. The maximum size of the platy crystals with superconductivity was about 1.5×1×0.2 mm3. The details of the growth procedures of the single crystals are reported here.