足立 匡

Temperature dependence of the in-plane electrical resistivity, rho(ab), in various magnetic fields has been measured in the single-crystal La2-xBaxCuO4 with x=0.08, 0.10, 0.11 and La1.6-xNd0.4SrxCuO4 with x=0.12. It has been found that the superconducting transition curve shows a so-called fan-shape broadening in magnetic fields for x=0.08, while it shifts toward the low-temperature side in parallel with increasing field for x=0.11 and 0.12 where the charge-spin stripe order is formed at low temperatures. As for x=0.10, the broadening is observed in low fields and it changes to the parallel shift in high fields above 9 T. Moreover, the normal-state value of rho(ab) at low temperatures markedly increases with increasing field up to 15 T. It is possible that these pronounced features of x=0.10 are understood in terms of the magnetic-field-induced stabilization of the stripe order suggested from the neutron-scattering measurements in the La-214 system. The rho(ab) in the normal state at low temperatures has been found to be proportional to ln(1/T) for x=0.10, 0.11, and 0.12. The ln(1/T) dependence of rho(ab) is robust even in the stripe-ordered state.

Effects of the magnetic impurity Ni on the Cu-spin dynamics and superconductivity have been studied in La2-xSrxCu 1-yNiyO4 with x=0.13 changing y finely up to 0.10. Compared with the case of the non-magnetic impurity Zn, it has been found from the muon-spin-relaxation measurements that a large amount of Ni is required to stabilize a magnetic order of Cu spins. However, the evolution toward the stabilization of the magnetic order with increasing impurity concentration is qualitatively similar to each other. The area of the non-superconducting and slowly fluctuating or static region of Cu spins around Ni has been found to be smaller than that around Zn, suggesting that the pinning of rather long-ranged dynamical spin correlation such as the so-called dynamical stripe by Ni is weaker than that by Zn. This may be the reason why Zn destroys the superconductivity in the hole-doped high-Tc cuprates more markedly than Ni.

We have measured the thermal conductivity in the ab plane of La 2-xSrxCuO4 (x=0, 0.10,0.115,0.13) in magnetic fields up to 14 T parallel to the c axis and also parallel to the ab plane. By the application of magnetic fields parallel to the c axis, the thermal conductivity has been found to be suppressed at low temperatures below the temperature TK which is located above the superconducting transition temperature and is almost independent of the magnitude of the magnetic field. The suppression is marked in x=0.10 and 0.13, while it is small in x=0.115. Furthermore, no suppression is observed in the 1% Zn-substituted La 2-xSrxCu0.99Zn0.01O4 with x=0.115. Taking into account the experimental results that the temperature dependence of the relative reduction of the thermal conductivity is quite similar to the temperature dependence of the intensity of the incommensurate magnetic Bragg peak corresponding to the static stripe order and that the Zn substitution tends to stabilize the static order, it is concluded that the suppression of the thermal conductivity in magnetic fields is attributed to the development of the static stripe order. The present results suggest that the field-induced magnetic order in La2-xSrxCuO4 originates from the pinning of the dynamical stripes of spins and holes by vortex cores.

Precise zero-field muon-spin-relaxation measurements on La2-xSrxCuO4 have revealed that the dynamical muon-spin depolarization rate starts to increase monotonically with decreasing temperature at a high temperature around 100 K. Compared with the resistivity results in the normal state, it has been concluded that the localization of holes leads to the suppression of the Cu-spin fluctuations. (C) 2003 Elsevier B.V. All rights reserved.

Muon-spin-relaxation (muSR) and magnetic susceptibility measurements have been carried out in La2-xSrxCu1-yZnyO4 with, x = 0.13 changing y finely up to 0.10, with the aim at clarifying effects of the non-magnetic impurity Zn on the Cu-spin dynamics and superconductivity. It has been found that the y dependence of the volume fraction of the superconducting region estimated from susceptibility measurements highly correlates with that of the magnetically ordered region estimated from the muSR results. The rapid decrease in the superconducting region and the rapid increase in the magnetically ordered region by the doping of a small amount of Zn can be interpreted as follows; Zn pins the dynamical spin correlation or the dynamical stripe correlations and hence the superconductivity is destroyed around itself.

For La2-xSrxCuO4 single-crystals in the underdoped and overdoped regimes, we have measured the electrical resistivity along the c-axis, pc, under constant magnetic fields, changing the field direction in the ab-plane. At x = 0.08, 0.12 and 0.16 in the underdoped and, almost optimally-doped regimes, clear anisotropy of pc with the fourfold symmetry in the ab-plane has been observed. This symmetry is regarded as the fourfold symmetry of the upper critical field, H-c2, and explained as being mainly due to the anisotropy of the superconducting energy gap owing to the d(x2)-(y2) pairing. The magnitude of the fourfold component of H-c2, DeltaH(c2)/H-c2, is largest in x = 0.08 among the three crystals. At x = 0.20 in the overdoped regime, no clear fourfold symmetry has been observed. It appears that the fourfold symmetry is easy to observe in the underdoped regime rather than in the overdoped regime.

Zero-field muon-spin-relaxation (μSR) measurements have been carried out in order to investigate the effect of nonmagnetic impurities on the dynamics of the Cu-spin fluctuations in the Zn-substituted La2-xSrxCu1-yZnyO4 with x=0.115 and 0.13, changing y from 0 to 0.10. A long-range-ordered state of Cu spins, which is observed in the Zn-free sample with x=0.115, disappears in the Zn-free sample with x = 0.13, but appears again in the Zn-substituted samples with x = 0.13 for y ≥ 0.0075. The long-range-ordered state disappears for y &gt 0.03 for both values of x, so that the Cu spins are in a paramagnetic state. The present μSR results support the suggestion that a small amount of nonmagnetic impurities produces a pinning of the dynamical stripe correlations of spins and holes and make them statically stabilized, while a large amount of nonmagnetic impurities destroy the stripe correlations themselves.

Anomalies. which we have recently discovered at x = 0.21 in L2-xSrxCu1-yZnyO4 single crystals, are reviewed. In the 1% Zn-substituted crystal with x = 0.21. the resistivity exhibits an upturn at low temperatures below about 80 K and the superconductivity (SC) is markedly suppressed. The muon-spin-relaxation (mu SR) measurements have revealed the existence of a static ordered state of Cu spins in this crystal at low temperatures below 2 K. From the elastic neutron-scattering experiment, incommensurate magnetic peaks around (pi, pi) have been observed in this crystal at low temperatures below 20 K. Even in the Zn-free crystal with x = 0.21. the SC is a little suppressed and the magnetic correlation has been found from the mu SR measurements to be enhanced at low temperatures below about 0.8 K. These results strongly suggest that the dynamical stripe correlations of spins and holes tend to become static singularly at x = 0.21 and are pinned by a small amount of Zn at x = 0.21, leading to the static stripe-order and the marked suppression of SC. as in the case of x = 0.115. (C) 2001 Elsevier Science B.V. All rights reserved.

We have attempted the crystal growth by the traveling-solvent floating-zone (TSFZ) method of La2-xBaxCuO4 (x∼1/8), where the superconductivity is strongly suppressed. Under flowing O2 gas at high pressure (4 bars), we have succeeded in growing single crystals for x=0.11 of 5 mm in diameter and 20 mm in length. Both in-plane and out-of-plane electrical resistivities of single-crystal La2-xBaxCuO4 (x=0.11) exhibit a clear jump at ∼53 K. The temperature corresponds to the structural phase transition temperature between the orthorhombic midtemperature and tetragonal low-temperature phases, Td2. Both in-plane thermoelectric power and Hall coefficient drop rapidly at Td2 and decrease below Td2 with decreasing temperature. These results are quite similar to those observed in single-crystal La1.6-xNd0.4SrxCuO4 (x∼1/8), suggesting that the so-called static stripe order of holes and spins in the CuO2 plane is formed below Td2 in La2-xBaxCuO4 (x∼1/8) as well as in La1.6-xNd0.4SrxCuO4 (x∼1/8).

We have systematically measured the thermal conductivity kappa of Sr(14-x)A(x)Cu(24)O(41) (A=Ca, La: 0 less than or equal to x(Ca) less than or equal to 9: 0 less than or equal to x(La) less than or equal to 5) single crystals, in order to investigate the spin gap and hole pairing. The observed kappa has been analyzed to he composed of kappa (ph),kappa (spin) and kappa (hole) due to phonons, spins and holes, respectively. The contribution of kappa (spin) is observed only along the c-axis except for x(Ca) greater than or equal to 6. The spin gap in the ladder estimated from kappa (spin) is similar to 400 K and independent of x. This is regarded as corresponding to the spin excitation from spin-singlet to spin-triplet, as those estimated from the neutron scattering and Raman scattering measurements. The contribution of kappa (hole) is observed at high temperatures for larger x(Ca) values. The activation energy of holes in the ladder estimated from kappa (hole) decreases with increasing x(Ca). This is regarded as corresponding to the dissociation of hole pairs, as that estimated from the electrical resistivity measurements. These results are consistent with the recent suggestion that most of holes are paired and localized in the ladder at low temperatures.

Our recent experimental studies on the 1/8 anomaly in the Bi-2212 and Y-123 phases and on the new anomaly at x = 0.21 in La2-xSrxCu1-yZnyO4 are reviewed. In the Zn-substituted Bi-2212 and Y-123 phases, we have found anomalous suppression of superconductivity at p (the hole concentration per Cu in the CuO2 plane) similar to 1/8. and it has been found from the muon spin relaxation (mu SR) measurements that the magnetic correlation between Cu spins is enhanced singularly at p similar to 1/8. Furthermore. we have found marked suppression of superconductivity singularly at x = 0.21 in the 1% Zn-substituted single-crystals of La2-xSrxCu1-yZnyO4, and it has been found from the it SR measurements that the magnetic correlation is also enhanced at x = 0.21. These results suggest that the dynamical stripe correlations of holes and spins exist in the Bi-2212 and Y-123 phases as well as in the La-214 phase and that they tend to be pinned by Zn, leading to the suppression of superconductivity. If this tendency is the ease in a wide range of p, it is possible to understand the universal results in the high-T-c cuprates that Zn is a strong scatterer and that the Zn substitution markedly suppresses superconductivity.

Effects of the Cu-site substitution on the 1/8 anomaly in the Bi-2212 and Y-123 phases and also on the anomaly at x = 0.21 in La2-xSrxCuO4 (LSCO) have been investigated from the transport, muon-spin relaxation and neutron-scattering measurements. It is suggested that the dynamical stripe correlations of spins and holes exist in the Bi-2212 and Y-123 phases as well as in the La-214 phase and that they tend to be pinned by Zn at the hole concentration of ∼1/8 per Cu in the high-Tc cuprates and also at x = 0.21 in LSCO. Based upon the stripe-pinning model, universal properties in the high-Tc cuprates that partially substituted Zn is a strong scatterer and that the Zn substitution markedly suppresses the superconductivity are also discussed. © 2001 Elsevier Science B.V. All rights reserved.

Our recent experimental studies on the so-called 1/8 anomaly in the Bi-2212 and Y-123 phases and also on the new anomaly in the overdoped region of the La-214 phase are reviewed. In the partially Zn-substituted Bi-2212 and Y-123 phases, we have found anomalous suppression of superconductivity (SC) at p (the hole concentration per Cu in the CuO2 plane) similar to 1/8, and it has been found from the muon-spin-relaxation (mu SR) measurements that the magnetic correlation between Cu spins is enhanced singularly at p similar to 1/8 Furthermore, we have found giant suppression of SC singularly at x = 0.21 in the 1% Zn-substituted single-crystals of La2-xSrxCu1-yZnyO4 (LSCZO), and it has been found from the mu SR measurements that the magnetic correlation is also enhanced at x = 0.21. These results suggest that the dynamical stripe correlations of holes and spins exist in the Bi-2212 and Y-123 phases as well as in the La-214 phase and that they tend to be pinned by a small amount of Zn, leading to quasi-static stripe order and the suppression of SC at p similar to 1/8 in the Bi-2212, Y-123, La-214 phases and also at x = 0.21 in LSCZO. If this tendency is the case in a wide range of p, it is possible to understand the universal experimental results in the high-rr, cuprates that Zn is a strong scatterer in the unitarity limit and that the Zn substitution markedly suppresses SC.

We have found a little suppression of superconductivity at x=0.21 in La2-xSrxCuO4 single crystals and, moreover, a giant suppression of superconductivity at x=0.21 through the 1% substitution of Zn for Cu. In the Zn-substituted La2-xSrxCu1-yZnyO4 single crystal with x=0.21 and y=0.01, the resistivity exhibits an upturn at low temperatures below similar to 80 K. These results suggest that the dynamical stripe correlations of spins and holes are pinned by a small amount of Zn at x=0.21, leading to a static stripe order and the giant suppression of superconductivity, as in the case of x=0.115.

Raman spectra of Sr14-xCaxCu24O41 have been investigated in a number of well-defined single-crystal and polycrystalline samples. From the polarization and doping dependence, and from a comparison with previous reports on isostructural compounds, we identify the (4A(g) + 4B(1g) + 3B(2g) + B-3g) modes that are Raman allowed within the simple Fmmm structure, consisting of two Cu-O sub-lattices. With increasing Ca content, we observe the intensity decrease in the modes involving the atoms of the ladders. This effect, analogous to the CuO2-plane phonons in the high-T-c cuprates, can be attributed to the hole transfer from the chains to the ladders. (C) 2000 Elsevier Science B.V. All rights reserved.

We have investigated the electrical resistivity rho and the magnetic susceptibility of Sr(14-x)A(x)Cu(24)O(41) (A = Ca, La) single crystals. The Arrhenius plot of In rho versus T-1 exhibits a bend at a temperature T-rho and gives two kinds of activation energy above and below T-rho. The activation energy at low temperatures below T-rho is in approximate agreement with the spin gap in the ladder. Accordingly, it appears that hole pairs are localized at T < T-rho and that thermally activated single-holes contribute to the electrical conduction at T < T-rho. (C) 2000 Elsevier Science B.V. All rights reserved.

We have investigated the electric-field dependence of the electrical conductivity in La2-xBaxCuO4 with x = 0.03 and 0.125 (= 1/8) where the superconductivity is suppressed. For x = 0.03, nonlinear conduction has been observed at low temperatures where the low-current resistivity increases with decreasing temperature. The observed nonlinearity can be well represented by the electric-field dependence of the conductivity in the Anderson-localized state. For x = 0.125, on the other hand, the conductivity is independent of the applied field within the measured field-range. We speculate that this result at x = 1/8 is due to the formation of static stripe-order of holes and spins, as discovered in La1.6-xNd0.4SrxCuO4 with x = 1/8. (C) 2000 Elsevier Science B.V. All rights reserved.

We report on Raman scattering study of changes in the phonon spectrum and low-energy electronic excitations for Sr14Cu24O41. From the polarization and Ca-doping dependence, and from a comparison with previous reports on isostructural compounds, we identify the Raman-allowed modes within the simple structure, consisting of two orthorhombic CuO subcells. At low temperatures, the spectra exhibit a number of normally forbidden phonons as well as additional low-energy excitations. For T < similar to 200 K, we observe a large loss of low-energy spectral weight that linked to changes in the linewidth of the 246-cm(-1) phonon. These effects, analogous to the pseudogap effect in the high-T-c cuprates, can be correlated with recently reported resistivity experiments that show the pair formations in the ladders for T < similar to 200 K.

Recent experimental work on the 1/8 problem by our group is reviewed not only in the La-based cuprate but also in the Bi- and Y-based ones. In the partially Zn-substituted Bi2Sr2Ca1-xYx (Cu1-xZny)(2)O8+delta, we have found anomalous suppression of superconductivity at x = 0.30 - 0.35, where the hole concentration per Cu p similar to 1/8. Moreover, it has been revealed from the mu SR measurements that the Cu-spin fluctuations exhibit slowing-down behavior at low temperatures in these samples. These results are suggestive of the stripe correlations of holes and spins tending to be pinned by Zn at p similar to 1/8 in the Bi-based cuprate as well as in the La-based cuprate. In Y1-xCaxBa2Cu3O7-delta, the 60 K plateau of T-c has been found not to be correlated with the oxygen content but to be interpreted as being due to the suppression of superconductivity at p similar to 1/8. Accordingly, we conclude that the 1/8 problem is common to all high-T-c cuprates with the CuO2 plane. Moreover, we have found transport and T-c anomalies around x = 0.22 in La2-xSrxCu1-yMyO4 (M = Zn, Ga), suggesting a possibility that an order of holes and/or spins is formed or fluctuates in these samples in the overdoped region.