Akihiko Fujiwara

Trial of intercalation of Br and Li into Bi2Sr2Can−1CunO2n+4 (n = 1, 2, 3)

Abstract Intercalation of Br and Li into Bi 2 Sr 2 Ca n−1 Cu n O 2n+4 ( n = 1, 2, 3) has been tried. For heavily Br-intercalated samples of n = 2 and 3 with x ≈ 1 in Bi 2 Sr 2 Ca n−1 Cu n O 2n+4 Br x , enlargement of the c-axis is detected, suggesting that intercalation compounds Bi 2 Sr 2 Ca n−1 Cu n O 2n+4 Br with Br atoms between double BiO layers are synthesized. For Li-intercalated samples, on the other hand, clear evidence to show the formation of intercalation compounds is not obtained. For lightly Br- and Li- intercalated samples of n = 1, 2, 3, changes of the electrical resistivity, the Hall coefficient and T c by the intercalation are observed, which are attributable to charge transfer between intercalants and CuO 2 planes in the host materials Bi 2 Sr 2 Ca n−1 Cu n O 2n+4 . That is, the intercalation of Br and Li is effective for hole and electron doping into CuO 2 planes, respectively. For n = 3, T c decreases by the light intercalation of both Br and Li, suggesting that Bi 2 Sr 2 Ca 2 Cu 3 O 10 has almost optimum hole concentration to attain the maximum of T c .

Two effects of iodine intercalation on Tc in Bi2Sr2Ca1−xYxCu2O8: Two-dimensionality and charge transfer☆

Abstract Effects of the stage-1 iodine-intercalation on T c have been investigated in Bi 2 Sr 2 Ca 1− x Y x Cu 2 O 8 . The iodine intercalation produces two large effects; one is enhancement of the two-dimensionality due to expansion of the crystal along the c -axis, and the other is increase of conductive hole carriers due to charge transfer between intercalated iodine atoms and CuO 2 sheets in the host material. For the stage-1 iodine-intercalation compounds, it has been found that the former effect decreases T c by about 10 K and that the latter effect shifts the x dependence of T c to larger x values by about 0.05 because of the increase of conductive hole carriers by about 0.025 per CuO 2 unit. It has been concluded that the decrease in T c through the I intercalation in Bi 2 Sr 2 CaCu 2 O 8 is due to both effects.

Synthesis and superconductivity of IBr-intercalated Bi2Sr2CaCu2O8

Abstract We have succeeded in synthesizing a new intercalation compound of Bi 2 Sr 2 CaCu 2 O 8 (IBr) 1 2 with the stage-1 structure. The increase in spacing between the BiO-BiO double layers through IBr intercalation is 2.1 A, which is a little larger than the 2.0 A in the case of Br intercalation alone and is smaller than the 3.5 A in the case of only iodine intercalation. The superconducting transition temperature T c of Bi 2 Sr 2 CaCu 2 O 8 (IBr) 1 2 decreases with increasing x for 0 ≦×⪅0.4 and is almost constant, 55 K, for x⪆0.4. This x dependence of T c is very similar to those of Bi 2 Sr 2 CaCu 2 O 8 Br x and Bi 2 Sr 2 CaCu 2 O 8 I x . This is discussed in relation to the changes of the two-dimensionality and hole concentration in the CuO 2 sheets through intercalation. A trial of ICl intercalation into Bi 2 Sr 2 CaCu 2 O 8 is also described.

Crystal structure and superconductivity of iodine-intercalated Bi2Sr2CaCu2O8Ix (0≦x≦1): Dependence on the iodine concentration

Abstract Variations of the crystal structure and superconductivity with iodine concentration have been investigated for iodine-intercalated sintered-samples of Bi 2 Sr 2 CaCu 2 O 8 I x with 0≦ x ≦1. It has been found from powder X-ray diffraction that samples with 0.4≲ x ≲0.7 are composed of mixed phases of the host compound Bi 2 Sr 2 CaCu 2 O 8 and the stage-1 iodine-intercalation compound Bi 2 Sr 2 CaCu 2 O 8 I. Diffraction peaks due to the stage-3 structure are vaguely observed for a sample with x =0.24. In spite of the mixed-phase structure for 0.4≲ x ≲0.7, the superconducting phase is single and of the bulk for all x values. The superconducting transition temperature decreases with increasing x for 0≦ x ≲0.4 and is almost constant for 0.4≲ x ≦1. A model of the x dependence of the spatial distribution of intercalated iodine atoms is proposed, and the x dependence of T c is discussed in relation to that of the charge transfer between iodine atoms and the CuO 2 sheets.

Superconducting energy gap in Bi2Sr2CaCu2O8 observed by high-resolution photoemission spectroscopy

Abstract A high-energy resolved low-temperature photoemission spectrometer of angle-integrated mode was constructed to study the detailed electronic structure near the Fermi level of novel materials, in particular, cuprate high-Tc superconductors. Photoemission measurement with this spectrometer on a Bi2Sr2CaCu2O8 polycrystal clearly showed a transfer of spectral weight from the vicinity of the Fermi level to the higher binding-energy region below the superconducting transition temperature, indicating opening of the superconducting gap. Numerical simulation for both normal and superconducting states showed that the superconducting gap parameter (Δ) of 18 meV with dx 2 −y 2 symmetry gives a most reasonable fitting.

Intercalation of Br and Li in Bi2Sr2Can−1CunO2n+4

Abstract Effects of intercalation of Br and Li into Bi2Sr2Can−1CunO2n+4 (n = 1, 2, 3) have been investigated. For Bi-intercalated samples of n = 2 and 3, intercalation compounds Bi2Sr2Can−1CunO2n+4Br with enlarged unit cells along the c-axis are synthesized. For Li-intercalated samples, it is not clear whether Li atoms act as intercalants or oxygen getters. For lightly Br- and Li-intercalated samples, the electrical resistivity, the Hall coefficient and Tc change depending on the quantity of intercalants, which is attributable to charge transfer between intercalants and CuO2 planes. For n = 3, Tc decreases by the intercalation of both Br and Li, suggesting that Bi2Sr2Ca2Cu3O10 has almost optimum hole concentration to attain the maximum of Tc. In conclusion, the intercalation is effective for controlling the carrier concentration in CuO2 planes.

Effects of extra oxygen on the physical properties in the Pb3201 phase of (Pb2Cu)Sr0.9La1.1CuO6+δ prepared by the polymerized complex method

Abstract We have investigated the effects of extra oxygen on the crystal structure, transport properties and superconductivity in the Pb3201 phase of (Pb 2 Cu)Sr 9 La 1.1 CuO 6+δ prepared by the polymerized complex method. A structural phase transition from orthorhombic to tetragonal phase is observed with increasing δ. The sample is orthorhombic for 0 ≲ δ ≲ 0.4, tetragonal for 1.7 ≲ δ ≲ 2 and a mixture of the orthorhombic and tetragonal phasse for 0.4 ≲ δ ≲ 1.7. In the orthorhombic phase of δ ≲ 0.3−0.4, T c increases with increasing δ, owing to the increase of holes in the CuO 2 sheets from the extra oxygen. In the tetragonal phase, on the other hand, the superconductivity disappears on account of the small hole concentration in the CuO 2 sheets, owing to the increase of the Pb valence from +2 to +4.

Iodine and bromine intercalation into the Bi-2222 phase of Bi2Sr2(Gd0.82Ce0.18)2Cu2O10+σ

Abstract We have succeeded in synthesizing a new intercalation compound Bi2Sr2(Gd0.82Ce0.18)2Cu2O10+δI with stage-1 structure. The increase in spacing between the BiO-BiO double layers through the iodine intercalation is 3.7 A. The electrical resistivity and superconductivity have been investigated for iodine-intercalated samples of Bi2Sr2(Gd0.82Ce0.18)2Cu2O10+δIx with 0≦x≦1 and bromine-intercalated ones of Bi2Sr2(Gd0.82Ce0.18)2Cu2O10+δBrx with 0≦x⪅0.2. It has been found that non- superconducting assintered samples of Bi2Sr2(Gd0.82Ce0.18)2Cu2O10+δ become superconducting at low temperatures below about 20 K through the light bromine intercalation. This suggests that the role of annealing under high pressures of oxygen in the appearance of superconductivity for the Bi-2222 phase is to supply the CuO2 sheets with enough hole carriers.

Effects of iodine intercalation into Bi-based copper oxide superconductors

We have investigated the stage-1 iodine-intercalation compounds, whose host materials are Bi2Sr2Ca1−xYxCu2O8 (Bi-2212 phase) and Bi1.6Pb0.5Sr1.9−xLaxCu1.05O6 (Bi-2201 phase) with variousx values. In the Bi-2212 phase, it has been found that the change inTc through the stage-1 iodine intercalation is due to two effects; a change of the two-dimensionality and an increase in the carrier number. The former effect decreasesTc by about 10 K independent of the carrier number of the host sample. The latter effect shifts thex dependence ofTc to largerx values by about 0.08, taking account of oxygen release from the host samples during the intercalation process. This result is clear evidence for the occurrence of both the change of two-dimensionality and the increase in carrier number about 0.04 per CuO2 unit due to the charge transfer through the stage-1 iodine intercalation. In the Bi-2201 phase,Tc decreases by about 10 K through the stage-1 iodine intercalation.

Crystal structure, thermoelectric power and superconductivity in La1.6 − xNd0.4SrxCuO4

Abstract The correlation between the crystal structure, thermoelectric power and superconductivity has been investigated in the tetragonal low-temperature (TLT) phase of La1.6 − xNd0.4SrxCuO4 with 0.10 ⩽ × ⩽ 0.25. The crystal structure has been studied by means of high-resolution powder neutron diffraction and the Rietveld analysis. It has been found that both the rhombic distortion of the CuO4 basal plane and the change of the thermoelectric power in the TLT phase are greatest at x ∼ 0.115. Therefore, the local minimum of the superconducting transition temperature at x = 0.115 is concluded to be due to the drastic change in the electronic structure in cooperation with the large rhombic distortion of the CuO4 basal plane in the TLT phase.