Satoru Nakajima

Synthesis of bulk high Tc superconductors of TlBa2Can −1CunO2n + 3 (n = 2 − 5)

Abstract Bulk specimens of high Tc superconductors of TlBa2Can − 1CunO2n + 3 (n = 2 − 5) have been synthesized by heat treatment up to 10 hours. Crystal structures were analyzed by X-ray powder diffraction and high resolution electron microscopy, and superconducting properties were determined by resistivity and DC susceptibility measurements. Tc increases with increasing number of Cu layers up to n = 4, where it turns down.

Superconductivity of high-Tc TlBa2Ca1−xRExCu2O7 (RE=Nd, Gd or Y) system

Abstract T c and the unit cell dimensions of TlBa 2 Ca 1− x RE x Cu 2 O 7 (where RE is Nd, Gd or Y) have been investigated for a wide substitution range. The tetragonal unit cell dimensions c and a decreased and increased, respectively, with the substitution of Ca with RE. An as-prepared specimen of TlBa 2 CaCu 2 O 7 with T c =80 K was found to be in an over-doping state and T c increased to 100 K with increasing x up to x =0.2-0.35, where it began to decrease and became nonsuperconducting at x =0.4-0.5.

Thermogravimetric studies of the over-doping state of Tl2Ba2CuO6−δ due to charge transfer

Abstract Variation of the T c of Tl 2 Ba 2 CuO 6−δ (2201) was demonstrated under various synthesis conditions: annealing atmosphere and quenching temperature. The oxygen vacancy (δ) was determined precisely by thermogravimetric analysis in oxygen and nitrogen atmospheres. The nonsuperconducting phase (δ⋍0) in Tl 2 Ba 2 CuO 6−δ due to charge transfer Tl 3− t -(Cu-O) p was turned into a superconducting state by the introduction of a small amount of oxygen vacancies (δ>0). The highest T c value of 85 K was achieved with an optimum value of δ⋍0.15 by annealing at 600°C in a nitrogen atmosphere.

Iodometric determination of oxygen contents of the T1 single layer system and their relevance to superconductivity

Abstract The oxygen contents of oxygen-annealed T1 single layer systems were determined by iodometric titration. The oxygen contents, w , of T1Ba 2 Ca 1- x Y x Cu 2 O w increased with Y concentration, x , substituted for Ca. Oxygen vacancy, y , was estimated to be about. y =0.3 in oxygen-annealed T1Ba 2 Ca n -1 Cu n O 2 n +3- y ( n =2, 3, 4). The Cu valence of oxygen-annealed T1Ba 2 Ca n -1 Cu n O 2 n +3- y is calculated to be about 2.2+, if the hole is introduced only in the CuO 2 pyramidal sheets. Square planar CuO 2 planes of the T1 single layer system may not be active for superconductivity.

Oxygen K-edge fine structure of TlBa2Ca1-xYxCu2O7 studied by electron energy loss spectroscopy

Abstract The fine structures of the oxygen K-edge in TlBa2Ca1-xYxCu2O7 (x=0, 0.8 and 1.0) with single Tl-O laye rs were investigated by electron energy loss spectroscopy (EELS). The peak corresponding to the hole state near the Fermi energy decreases with the increase of the Y content. Although a small density of the hole state is observable in the compound Tl1Ba2Ca0.2Y0.8Cu2O7, it s hows the semiconducting character. It was pointed out that the decrease of the hole content mainly results in the transition from superconductor to semiconductor in the compound Tl1Ba2Ca1-xYxCu2O7.

Anisotropic superconducting properties of high-Tc Tl-Ba-Ca-Cu-O single crystal

Abstract The electrical resistance was measured on a Tl-Ba-Ca-Cu-O single crystal. From X-ray diffraction the composition of the sample is expected to be Tl:Ba:Ca:Cu=1:2:1:2. The superconducting transition temperature T c is 106 K. The upper critical field H c2 shows anisotropy, that is, (-d H c2 /d T ) T c for H ⊥ c -axis and for H ∥ c -axis are 1.9 T/K and 0.2 T/K, respectively. The ratio of the former to the latter is 9.5. The low resistance part in the transition shows a thermally activated temperature dependence. The activation energy U 0 estimated from its temperature dependence is also anisotropic. The U 0 values for H ⊥ c -axis and H ∥ c -axis are about 4000 K and 1200 K at 0.5 T, respectively, and depend on applied magnetic fields. The anisotropy of the superconducting properties is intermediate in magnitude for this compound compared with the other high- T c oxides such as YBa 2 Ca 3 O 7−δ and Bi 2 Sr 2 CaCu 2 O y .

Structural changes of 2201-, 2212- and 2223-type Tl-oxide superconductors at temperatures below 630 K

Abstract Two similar sequential transitions at about 420 and 550 K are found by means of thermal analysis, DTA, in the 2201-, 2212- and 2223-type Tl-oxides. The lattice parameters of the oxides are measured in the temperature range from room temperature to 630 K by means of X-ray powder diffraction measurement during both heating and cooling processes. The c -axis lengths of the three oxides show similar reversible thermal expansion with discontinuities at about 420 and 500 K. The former change at about 420 K is weak in the 2201- and 2212-type oxides but is relatively obvious in the 2223-type oxide. In situ X-ray single-crystal diffraction measurement was performed for the 2223-type oxide to elucidate the possible structural change at about 420 K. No structural change was detected but some temperature dependent displacement of the O 3 (the apical oxygen) and O 4 (the Tl-O layer oxygen) was noticed.

Oxygen K-edge fine structure of Tl2Ba2Can−1CuxO2n+4 studied by electron energy loss spectroscopy

Abstract The fine structures of the oxygen K-edge in Tl 2 Ba 2 Ca n −1 Cu n O 2 n +4 ( n =1−3) with double Tl-O layers were investigated by electron energy loss spectroscopy (EELS). A peak at 528 eV becomes smaller with the increase of the number of Cu-O layers, n , in Tl 2 Ba 2 Ca n −1 Cu n O 2 n +4 , indicating that the average hole content in a Cu-O bonding decreases with the increase of n . Reducing the hole content in Tl 2 Ba 2 Cu 1 O 6 by replacing a part of Ba(+2) with La(+3), the peak at 528 eV becomes smaller and T c of the compounds increases up to 55 K. It was shown that holes of Tl 2 Ba 2 Cu 1 O 6 are overdoped. The present EELS results were compared with the previous results of Tl 1 Ba 2 Ca n −1 Cu n O 2 n +3 with single Tl-O layers.

Oxygen K-Edge Fine Structure of Tl1Ba2Can-1CunO2n+3 Studied by Electron Energy Loss Spectroscopy

The fine structures of the oxygen K-edge in the Tl1Ba2Can-1CunO2n+3 with single Tl-O layers were investigated by electron energy loss spectroscopy (EELS). The peak corresponding to the hole state near the Fermi energy was sensitive to the number of Cu-O layers and the direction of momentum excitation. It was shown that the hole content in Tl1Ba2Can-1CunO2n+3 decreases with the increase of n (n=2–4) and that most holes exist in the O 2px,y orbitals of 2-dimensional Cu-O planes.

Local Structure and Superconductivity in Tl2Ba2CuOy Studied by Extended X-Ray Absorption Fine Structure

Local structure of Tl2Ba2CuOy (T12201) has been investigated by extended X-ray absorption fine structure (EXAFS) on the Cu K-edge. The EXAFS spectra of the high-Tc sample with Tc=70 K shows the splitting of in-plane Cu-O bondlength. The splitting increases with decreasing temperature well above Tc and abruptly decreases at Tc. On the other hand, no splitting was observed in that of non- superconductor samples. This fact demonstrates the existence of the local distortion in the CuO2 plane well above Tc relating with the superconductivity. No apparent anomalies are observed in the apical-oxygen contribution in the vicinity of Tc.