Tadashi Adachi

We investigated the dc magnetization curves of overdoped La2-xSrxCuO4 single crystals in the low-temperature T range, with the external magnetic field H oriented along the c axis. It was found that the onset field for the second magnetization peak, associated with the order-disorder transition in the vortex system, increases significantly with decreasing T in the low-T region, where the superconductor parameters are independent of T. The upward curvature of the order-disorder transition line determined in standard magnetization measurements at low T is explained by considering the reduction of the actual pinning energy due to the macroscopic currents induced in the sample. A simple energy balance equation for the dynamic conditions generated in the widely performed magnetization studies is proposed.

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.

Our recent experimental works on the field-induced and impurity-induced magnetic order in La2-xSrxCuO4 are reviewed. The muon-spin-relaxation (mu SR) measurements of La2-xSrxCu1-yMyO4 (M = Zn, Ni) have revealed that both Zn and Ni impurities develop the magnetic order around x = 0.115, though the effect of Zn on the development of the magnetic order is more marked than that of Ni. The field-induced magnetic order around x = 0.115 pointed out from the neutron scattering experiments has also been investigated by the thermal conductivity measurements in magnetic fields. Marked suppression of the thermal conductivity by the application of magnetic fields parallel to the c-axis has been observed in x = 0.10 and x = 0.13, which appears to be in good correspondence to the development of the magnetic order. In conclusion, not only impurities but also vortex cores in the CUO2 plane seem to operate to pin the dynamical stripes of spins and holes, leading to the development of the magnetic order.

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.

We have found the 1/8 anomaly, namely, the anomalous suppression of superconductivity at p (the hole concentration per Cu) = 1/8 in the excess-oxygen-doped La1.8Nd0.2Cu1-yZnyO4+delta, where the excess oxygen is doped by the electrochemical technique and the phase separation of the excess oxygen is suppressed. The 1/8 anomaly has become marked by the 1% substitution of Zn for Cu. The muon-spin-relaxation measurements have revealed that the magnetic correlation develops at low temperatures below about 50 K in both Zn-free and 1% Zn-substituted samples with p = 1/8. Clear precession of muon spins suggesting the formation of a long-range magnetic order has been observed below 1 K in the 1% Zn-substituted sample with p = 1/8. These results are consistent with the stripe-pinning model.

We have investigated in detail dependence on p (the hole concentration per Cu) of the transport properties and T-c in the excess-oxygen-doped La(2-x)A(x)CuO(4+delta) (A = Nd, Bi, Pr), where the phase separation of the excess oxygen is suppressed. The so-called 1/8 anomaly has been found in La1.8Nd0.2CuO4+delta. The 1/8 anomaly has become marked through the 1 % substitution of Zn for Cu. The muSR measurements have revealed that the magnetic correlation develops at low temperatures below about 40 K in both Zn-free and 1 % Zn-substituted samples with p = 1/8 and that a long-range magnetic order is formed below 1 K in the 1 % Zn-substituted sample with p = 1/8. These results support the stripe-pinning model. Neither La2-xBixCuO4+delta nor La2-xPrxCuO4+delta has exhibited the clear 1/8 anomaly.

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 measurements have been carried out in order to investigate changes of the Cu-spin fluctuations in a wide hole-concentration range of La2-xSrxCuO4. An increase of the moon-spin depolarization rate with decreasing temperature has been observed between about 70 K and 140 K in the underdoped samples. The characteristic temperature, where the muon-spin depolarization rate starts to increase with decreasing temperature, has been found to decrease with increasing x and seems to vanish around x=0.15 after showing a local maximum around x=0.115. The present results are discussed in terms of the dynamical stripe and the pseudogap.

In order to investigate Zn-substitution effects on the dynamics of the Cu-spin fluctuations, zero-field muon-spin relaxation (muSR) measurements have been applied to Zn-substituted La2-xSrxCu1-yZnyO4 with x = 0.115 and 0.13, changing the Zn concentration in fine steps from y = 0 to 0.10. It has been found that the Cu-spin fluctuations are statically stabilized by a small amount of Zn (less than 1%), while a large amount of Zn (more than 1%) destroys the magnetic correlations between Cu spins. The present result can be well explained as due to the pinning effect of the dynamic stripes correlations of spins and holes by the substituted Zn. (C) 2002 Elsevier Science B.V. All rights reserved.

We report on the temperature dependence of the in-plane electrical resistivity rho(ab) and the in-plane Hall coefficient R-H in various magnetic fields of the single-crystal La2-xBaxCuO4 with x = 0.083 and 0.11. In x = 0.11, which is close to x = 1/8, where the superconductivity is strongly suppressed, a clear jump in rho(ab) and a drop in R-H have been observed at T-d2 similar to 53 K, where the structural phase transition between the orthorhombic mid-temperature and tetragonal low-temperature phases occurs. Moreover, a sign reversal of R-H has been observed below similar to25 K and the magnitude of the sign reversal increases with increasing magnetic field. In x = 0.083, on the other hand, there is neither jump in rho(ab) nor drop in R-H at T-d2, and also no sign reversal in R-H at low temperatures even in magnetic fields up to 9 T. In conclusion, there is no doubt that a static stripe order of holes and spins, observed in La1.6-xNd0.4SrxCuO4 with x similar to 1/8, is formed below T-d2 also in La2-xBaxCuO4 with x similar to 1/8. The R-H in the stripe-ordered state has a negative value, which is consistent with the recent theory by Prelovsek et al. (C) 2002 Elsevier Science Ltd. All rights reserved.

In order to clarify the effect of non-magnetic impurities on the dynamics of the Cu spin, muon-spin relaxation measurements have been carried out on the Zn-substituted La2-xSrCu1-yZnyO4 with x = 0.13, changing the Zn concentration (0 less than or equal to y less than or equal to 0.10). Although the Cu spins at y = 0 have been continued to be in the paramagnetic state down to at least 2 K, clear muon-spin precession has been observed at the same temperature at y = 0.01. This muon-spin precession has disappeared at y greater than or equal to 0.05. and the muon-spin depolarization at low temperatures has become small with increasing y at y > 0.01, indicating that the magnetic correlation between Cu spins becomes weaker as the value of y increases at y > 0.01. The present result proves that a small amount of non-magnetic impurities operate to statically stabilize the dynamical stripe correlations of spins and holes but a large quantity of non-magnetic impurities destroy the stripe correlations themselves. (C) 2002 Published by Elsevier Science Ltd.

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.

Static and dynamical properties of Cu spins in the overdoped single-crystals of La2-xSrxCu1-yZnyO4 have been investigated by the muon-spin-relaxation (mu SR) method. In the Zn-substituted single-crystal with x = 0.21 and y = 0.01, where the high-T-c superconductivity is anomalously suppressed, a coexisting state of statically ordered Cu-spins and localized holes has been found at low temperatures below about 2 K. The dynamical depolarization rate of the muon spin at 3.0 K has been found to be nearly proportional to minus-root-square of the external magnetic-field applied along the initial muon-spin polarization. This field-dependence can be explained by assuming the existence of an excited state of the Cu spins diffusing in a one-dimensional direction. The present study suggests a high possibility of the existence of a new one-dimensional stripe ordered state of the Cu spins and doped holes at x = 0.21 in the overdoped region of La2-xSrxCu1-yZnyO4.

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.

Zero-field (ZF) and longitudinal-field (LF) muon spin relaxation (mu SR) measurements have been carried out in the overdoped single crystals of La2-xSrxCu1-yZnyO4 with x=0.21, y=0 and 0.01. In the case of the Zn-substituted crystal with x=0.21 and y=0.01 in which the high-T-c superconductivity is almost suppressed, the ZF- and LF-mu SR measurements have revealed the existence of a static ordered state of Cu spins at low temperatures below 2 K. No clear coherent precession of the muon spin has been observed down to 20 mK. Slowing down of the Cu-spin fluctuations has also been observed in the non-Zn-substituted crystal with x=0.21 and y=0 at low temperatures below about 0.8 K in ZF, indicating that the observed magnetic anomaly is an intrinsic property at x=0.21. These mu SR results suggest a strong possibility of the existence of a spin/charge ordered state at x=0.21 in the overdoped region.

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.

Anomalies in the thermoelectric power and the Hall coefficient, which are analogous to those observed in La2-xBaxCuO4 with x similar to 1/8, have been observed in the Zn-substituted La2-xSrxCu1-yZnyO4 with x = 0.115 and y = 0 - 0.025 and with x = 0.15 and y similar to 0.02. Based on the so-called stripe model, these are explained as being due to the formation of the static order of the stripe-patterned CDW and SDW on account of the pinning by Zn. Accordingly, the long-standing problem on the conspicuous suppression. of superconductivity in the Zn-substituted La2-xSrxCu1-yZnyO4 with x similar to 0.115 is likely to be attributed to this static order.

We have measured the electrical resistivity rho and the thermal conductivity kappa of Sr(14-x)A(x) Cu24O41 (A = Ca and La) single crystals. The Arrhenius plot of ln rho vs T-1 gives two kinds of activation energy with a boundary temperature T-rho. The activation energy at T < T-rho is in approximate agreement with the spin gap in the ladder estimated from the NMR measurements, suggesting that holes in the ladder are paired and localized at T < T-rho. The observed kappa has been analyzed to be composed of kappa(ph), kappa(spin) and kappa(hole) due to phonons, spins and holes, respectively. The kappa(ph) exhibits a small peak at similar to 30 K in every direction of every single-crystal. The contribution of kappa(spin) is observed along the c-axis except for x(Ca) greater than or equal to 6, and the spin gap, which corresponds to the spin excitation from spin-singlet to spin-triplet, has been estimated to be similar to 420 K. For x(Ca) greater than or equal to 6, the spin gap, which corresponds to the destruction of spin-singlet pairs i.e. the dissociation of hole pairs, has been estimated from kappa along the c-axis at T > T-rho to decrease with increasing x(Ca).

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.

Transport and T-c anomalies around x = 0.22 in La2-xSrxCuO4 have been studied through the partial substitution of Zn, Ga and Ni for Cu. It has been found that the value of x where the anomalies occur shifts to lower x values through the Zn substitution, while it shifts to larger x values through the Ga substitution, and it disappears through the Ni substitution. There is a possibility that an order of holes and/or spins, such as the so-called stripe order, is formed or fluctuates around x = 0.22 in La2-xSrxCuO4. PACS numbers: 74.25.Fy, 74.62.-c, 74.62.Dh, 74.72.Dn.

Raman spectra of Sr14-xCaxCu24O41 in single-crystal and polycrystalline samples were investigated. At first, from the polarization and a comparison with previous reports on isostructural compounds, we identify the (4Ag+4B1g+3B2g+B3g) modes that are Raman active within the simple Fmmm structure. Second, it is clear that hole carriers transfer from chain to ladder with substituting Sr for Ca in Sr14Cu24O41, because on the one hand the intensity for modes associated with oxygen vibrations for chain increases and on the other hand the one associated with ladder decreases with Ca content x increasing. Third, we observe new additional modes at low temperature in this system. Especially, for both Sr14Cu24O41 and Sr7.2Ca6.8Cu24O41, the temperature, at which the new mode near 500 cm-1 shows up, is nearly identical to the temperature, at which the electrical and magnetic anomalies have been observed. This result is consistent with the picture that holes in the ladder are paired and localized below the temperature.

We have re-investigated the electrical resistivity rho and the magnetic susceptibility chi of the spin-ladder cuprate Sr(14-x)A(x)Cu(24)O(41) (A=Ca, Y, La), consisting of two-dimensional Cu2O3 ladder planes and one-dimensional CuO2 chains. The dln rho/d(1/T) vs. T plot exhibits a broad peak around 200 K for x = 0. The peak temperature T-rho goes down with increasing hole-concentration p in the ladder for x(Ca)> 0, while it goes up with decreasing p in the ladder for x(Y, La)> 0. The observed T-rho is regarded as a temperature below which holes in the ladder are localized. On the other hand, the chi(-1) vs. T plot exhibits inflection indicating deviation from the Curie-Weiss law around 200 K for x = 0. The inflection temperature T-chi also goes down with increasing p in the ladder, and goes up with decreasing p in the ladder. The observed T-chi is regarded as a temperature above which Cu2+ spins in the ladder notably contribute to the susceptibility. Values of T-chi and its x-dependence have been found to be almost the same as those of T-rho. This coincidence strongly suggests that holes in the ladder are paired and localized at low temperatures below T-chi (and T-rho) where almost all Cu2+ spins in the ladder complete to form spin-singlet pairs. (C) 1998 Elsevier Science Ltd.

An anomaly in the thermoelectric power, which is analogous to that observed below about 60 K in La2-xBxCuO4 with x similar to 1/8, has been observed below about 60 K also in the Zn-substituted La2-xSrxCu1-yZnyO4 with x similar to 0.115 and y = 0.01 - 0.02. According to conclusions by Tranquada et al. [Nature, 375 (1995) 561], this is explained as being due to the formation of the static order of the stripe pattern of holes and spins on account of the pinning by Zn. Moreover, the long-standing problem on the conspicuous suppression of superconductivity in the Zn-substituted La2-xSrxCu1-yZnyO4 with x similar to 0.115 is likely to be attributed to the static order of the stripe pattern owing to the pinning by Zn.

We have measured the magnetic susceptibility chi of Sr14-yYyCu24O41-delta consisting of two-dimensional Cu2O3 planes, namely, so-called spin-1/2 ladders with two legs and also of one-dimensional CuO2 chains. The susceptibility chi(chain)(T) due to the CuO2 chains, obtained by subtracting the susceptibility chi(ladder)(T) due to the Cu2O3 planes from the experimental data of chi(T), is described by the Curie-Weiss law at T > T-max similar to 80 K, where chi exhibits the maximum for y = 0. With increasing y, chi(chain) becomes large owing to the increase of Cu2+ spins introduced into the CuO2 chains. Moreover, the antiferromagnetic interaction between Cu2+ spins in the CuO2 chains becomes weak with the increase in y, namely, with the increase of Cu2+ spins introduced into the CuO2 chains, leading to suppression of the spin gap of the CuO2 chains. As the result, the broad peak of chi, observed around 80 K for y = 0, is found to disappear for y greater than or equal to 1.0.

We have investigated the transport and magnetic properties of Sr(14-x)A(x)Cu(24)O(41) (A = Ca, Ba, Y, La) consisting of two-dimensional Cu2O3 planes, namely, so-called spin-ladders with two legs and also of one-dimensional CuO2 chains. Sr14Cu24O41 exhibits a semiconductive behavior. For x(Ca) greater than or equal to 8.4, metallic behavior is observed in the thermoelectric power measurements, but superconductivity does not appear. The susceptibility exhibits a broad peak around 80 K for x = 0. With incresing s, the Curie component at low temperatures increases and the broad peak disappears. It has been concluded that the broad peak is due to spin gap of the CuO2 chains and that the Cu2+ spins in the Cu2O3 planes have already formed spin-singlet pairs below room temperature. A model on the spin configuration in the CuO2 chains at low temperatures is proposed, depending on the value of x.