M. Matsuda

Sr-induced oxygen defects in La2−xSrxCuO4-δ a neutron powder diffraction study

Abstract The validity so Sr-induced oxygen-defect structure in La 2− x Sr x CuO 4- δ claimed by Tan et el. was judged by the Rietveld analysis of TOF neutron powder diffraction data of La 2− x Sr x CuO 4- δ ( x =0, 0.11, 0.15, 0.3 and 0.6); both oxidized and reduced samples were used for the composition x =0.11. The samples with x =0.15, 0.3 and 0.6 were tetragonal (14/mmmm) at room temperature, while La 2 CuO 4-δ at room temperature anf La 1.89 Sr 0.11 CuO 4-δ at 115 K were orthorhombic (Cmca). No interstitial oxygen trapped near Sr could be detected in any of the samples. In La 1.4 Sr 0.6 CuO 4-δ , an equational oxygen site is 8% deficient, and the distance between Cu and apical oxygen is as short as 2.337 A, which is ascribed to the formation of oxygen defects related structurally to Sr 2 CuO 3 .

Crystal growth and characterization of Pr2−xCexCuO4

Abstract Single crystals of Pr 2− x Ce x CuO 4 were grown using the CuO flux method. Large crystals in which the Ce distribution was homogeneous 50O×3 mm 3 and 10×10×0.1 mm 3 , respectively. A small but high quality reduced Pr 1.84 Ce 0.16 CuO 4 crystals show superconductivity with a T c of 18 K and a transition width of 1.5 K. We have examined the effects of Pt contamination from the Pt crucible. Only a 1% substitution of Pt for Cu eliminates the superconductivity entirely. Pt suppresses T c more quickly in the Pr 2− x Ce x CuO 4 system than in the La 2− x Sr x CuO 4 system, where a Pt substitution of 2% eliminates the superconductivity.

MAGNETIC EXCITATIONS AND STRUCTURAL CHANGE IN THE S = 1/2 QUASI-ONE-DIMENSIONAL MAGNET SR14-XYXCU24O41 (0X1)

Neutron scattering measurements have been performed on the S=1/2 quasi-one-dimensional system Sr_{14-x}Y_{x}Cu_{24}O_{41}, which has both simple chains and two-leg ladders of copper ions. We observed that when a small amount of yttrium is substituted for strontium, which is expected to reduce the number of holes, the dimerized state and the structure in the chain are changed drastically. The inelastic peaks originating from the dimerized state of the chain becomes broader in energy but not in momentum space. This implies that the dimerized state becomes unstable but the spin correlations are unchanged with yttrium substitution. Furthermore, it was observed that nuclear Bragg peak intensities originating from the chain show strong temperature and x dependence, which suggests that the chains slide along the c axis as temperature and x are varied.

Flux pinning and flux creep in La2−xSrxCuO4 with splayed columnar defects

The effects of columnar defects with splayed configurations on flux pinning and flux creep were investigated experimentally. Parallel columnar defects and splayed columnar defects were introduced into specimens of La2−xSrxCuO4 by high-energy heavy-ion irradiation. A large enhancement of the flux pinning and the critical current density Jc due to the splayed columnar defects was observed. At temperature T<15 K and a magnetic field H=0.1 T, the effective pinning potential U0 of a specimen with splayed columns was larger than that of a specimen with parallel columns. On the other hand, at 15 K≤T≤30 K and H=0.1 T, U0 of a specimen with splayed columns was smaller than that of a specimen with parallel columns.

Spin correlations in high-temperature superconductors

Abstract Recent neutron magnetic scattering experiments from single crystals of both undoped and doped cuprous “214” oxides are reviewed. The magnetic properties in the undoped crystals are essentially interpreted by the quantum two-dimensional Heisenberg antiferromagnetic model. The three-dimensional long-range order occurs because of small but finite interplanar magnetic coupling plus various anisotropies. The doping of charge modifies the spin correlations significantly as well as the spin fluctuations, which suggests close correlations between the charge and spin degrees of freedom.

MAGNETIC EXCITATIONS IN THE S=1/2 QUASI-ONE-DIMENSIONAL MAGNET SR14-XCAXCU24O41

Neutron scattering experiments have been performed on Sr14−xCaxCu24O41 (x=3 and 6), which consists of CuO2 chains and Cu2O3 ladders, in order to study the effect of Ca substitution on the dimerized state in the chains. The Ca substitution broadens the excitation peak although the excitation energy is almost unchanged. The effects of Ca and Y substitution on the dimerized state are compared.

Suppression of vortex motion in La1.85Sr0.15CuO4 with the splayed configuration of columnar defects

Abstract Remarkable enhancements of the critical current density J c were obtained in polycrystalline La 1.85 Sr 0.15 CuO 4 with the splayed configuration of the columnar defects which were introduced by 3.5 GeV Xe ion irradiation with splay angles θ = 0°, ±5° and ±10°. In addition J c increased with the irradiation dose from 1.0×10 11 to 3.0×10 11 ions/cm 2 . Larger upward shift of the irreversibility line for the splayed configuration, in comparison with conventional parallel configuration, was observed. The results suggest that the vortex motion in La 1.85 Sr 0.15 CuO 4 is suppressed effectively by the splayed irradiation.

Spin fluctuations of La2−xSrxCu1−yZnyO4−δ (x=0.14, y=0.012) (abstract)

Magnetism of high‐Tc superconductors is still the subject of intensive investigation. Zn substitution for Cu together with removal of oxygen suppresses the superconductivity quickly. We prepared two crystals to perform a neutron scattering study of the spin fluctuations of La2−xSrxCu1−yZnyO4−δ. One is a Zn‐doped and deoxygenated crystal La1.86Sr0.14Cu0.988Zn0.012O4−δ (♯1) which does not show superconductivity above 1.5 K. The other is a Zn‐doped crystal La1.86Sr0.14Cu0.988Zn0.012O4 (♯2) which shows superconductivity below 16 K. The incommensurate spin excitations observed previously in superconducting samples1,2 containing no Zn remain in both samples. However, there is significant difference in the spectral weight χ‘(q,ω) at low temperatures between the non‐superconducting and superconducting Zn‐doped crystals. In crystal ♯1 the spectral weight at low temperatures is qualitatively similar to that of Zn‐undoped non‐superconducting samples.3 In crystal ♯2 the spectral weight at low temperatures is qualitat...

Magnetic structures and their phase transitions in lanthanide cuprates and related compounds

Magnetic structures and phase transitions in lanthanide compounds with La 2 Cu0 4 (T)- and Nd 2 CuO 4 (T′)-type structures are reviewed. The spin structures can be summarized by two types of structure: La 2 CuO 4 ( S ⊥τ) and La 2 NiO 4 -type ( S ∥τ). Successive magnetic phase transitions and order of the rare-earth moments are observed in both T- and T′-type compounds.

Magnetic excitations in the S=1/2 quasi-one-dimensional magnet Sr{sub 14{minus}x}Ca{sub x}Cu{sub 24}O{sub 41}

Neutron scattering experiments have been performed on Sr{sub 14{minus}x}Ca{sub x}Cu{sub 24}O{sub 41} (x = 3 and 6), which consists of CuO{sub 2} chains and CuO{sub 2}O{sub 3} ladders, in order to study the effect of Ca substitution on the dimerized state in the chains. The Ca substitution broadens the excitation peak although the excitation energy is almost unchanged. The effects of Ca and Y substitution on the dimerized state are compared.