足立 匡

The characteristic magnetic fields for the second peak (SP) on the dc magnetization curves of high-temperature superconductors with random quenched disorder increase with decreasing temperature T in the low-T range. It was argued that this aspect rules out the existence of an order-disorder transition in the vortex system as the primary cause for the occurrence of the SP, and a model based on a thermally induced square-to-rhombic vortex lattice transition was recently proposed. We investigated the T variation of the field H-on for the onset of the SP in the case of underdoped La2-xSrxCuO4 single crystals (x=0.08) with the external magnetic field oriented along the c axis. It was found that in the low-T domain H-on(T) proportional to 1/T-2, and an inflectionlike point in the H-on(T) dependence is still present, similar to that reported for overdoped specimens. We show that the observed behavior and the strong variation in the characteristic fields for the SP with the doping level are in agreement with a dynamic energy balance relation for the order-disorder transition in the vortex system at the SP.

Muon-spin-relaxation (μSR) measurements have been performed for the partially Zn-substituted electron-doped high- Tc superconductor Pr 0.86 LaCe0.14 Cu1-y ZnyO 4+α-δ with y=0-0.05 and the reduced oxygen content δ=0-0.09, in order to investigate nonmagnetic Zn-impurity effects on the Cu-spin dynamics. For all the measured samples with 0.01≤δ≤0.09, it has been found that a fast depolarization of muon spins is observed below 100 K due to the effect of Pr3+ moments and that the μSR time spectrum in the long-time region above 5 μsec increases with decreasing temperature at low temperatures below 30 K possibly due to slowing down of the Cu-spin fluctuations assisted by Pr3+ moments. No Zn-induced slowing down of the Cu-spin fluctuations has been observed for moderately oxygen-reduced samples with 0.04≤δ≤0.09, which is very different from the μSR results of La2-x Srx Cu1-y Zny O 4. The possible reason may be that there are no dynamical stripe correlations of spins and electrons in the electron-doped high- Tc cuprates or that the effect of Pr3+ moments on the μSR spectra is stronger than that of a small amount of Zn impurities. © 2010 The American Physical Society.

Spin correlations in the overdoped region of Bi1.75Pb0.35Sr1.90CuO6+z have been explored with Fe-doped single crystals characterized by neutron scattering, muon-spin-rotation spectroscopy, and magnetic-susceptibility measurements. Static incommensurate spin correlations induced by the Fe spins are revealed by elastic neutron scattering. The resultant incommensurability delta is unexpectedly large (similar to 0.2 r.l.u.), as compared with delta similar to 1/8 in overdoped superconductor La2-xSrxCuO4. Intriguingly, the large delta in this overdoped region is close to the hole concentration p. This result is reminiscent of the delta approximate to p trend observed in underdoped La2-xSrxCuO4; however, it is inconsistent with the saturation of delta in the latter compound in the overdoped regime. While our findings in Fe-doped Bi1.75Pb0.35Sr1.90CuO6+z support the commonality of incommensurate spin correlations in high-T-c cuprate superconductors, they also suggest that the magnetic response might be dominated by a distinct mechanism in the overdoped region.

The electronic ground state in high-T-c cuprates where the superconducting state is suppressed by Ni Substitution has been investigated in La2-xSrxCu1-yNiyO4 from the specific heat and muon spill relaxation measurements. It has been found that the ground state changes from a magnetically ordered state with the strong hole-trapping by Ni to a metallic state with the Kondo effect of Ni with increasing hole-concentration. Moreover, the analysis of the results has revealed that a phase separation into the magnetically ordered phase and the metallic phase occurs around the boundary of two phases.

The possible inhomogeneity of Superconductivity has been investigated by means of bulk-sensitive probes, using La2-xSrxCuO4 (LSCO) single crystals from the underdoped to optimally doped regime. Measurements of the magnetic susceptibility, chi, on field cooling and specific heat have revealed that both the absolute value of chi at 2 K, regarded as corresponding to the superconducting (SC) volume fraction in a sample, and the Sommerfeld constant, reflecting the density of states of quasiparticles at the Fermi level, exhibit significant dependence on x, namely. on the hole concentration. This is the first experimental work strongly suggesting that a phase separation into SC and normal-state regions takes place in the underodped regime of LSCO as well as in the overdoped regime.

Charge excitations in stripe-ordered 214 compounds La5/3Sr1/3NiO4 and 1/8-doped La2-x(Ba or Sr)(x)CuO4 are studied using resonant inelastic x-ray scattering in the hard x-ray regime. We observe similar or equal to 1 eV excitation with a momentum transfer corresponding to the charge stripe spatial period both for the diagonal (nickelate) and parallel (cuprates) stripes. They are interpreted as collective stripe excitations or anomalous softening of the charge excitonic modes of the in-gap states.

In order to investigate Ni-impurity effects on the Cu-spin dynamics in a wide range of hole concentration, we have performed zero-field muon-spin-relaxation measurements in La2-xSrxCu1-yNiyO4 (LSCNO) with x = 0.08-0.30 and y = 0-0.10. It has been found that the hole-trapping effect of Ni clearly appears in heavily Ni-substituted LSCNO with y >= 0.05, while the stripe-pinning effect of Ni is marked in lightly Ni-substituted LSCNO around x = 1/8. (C) 2008 Elsevier B.V. All rights reserved.

A central issue in high-T-c superconductivity is the nature of the normal-state gap (pseudogap) 1 in the underdoped regime and its relationship with superconductivity. Despite persistent efforts, theoretical ideas for the pseudogap evolve around fluctuating superconductivity(2), competing order(3-8) and spectral weight suppression due to many-body effects(9). Recently, although some experiments in the superconducting state indicate a distinction between the superconducting gap and pseudogap(10-14), others in the normal state, either by extrapolation from high-temperature data(15) or directly from La1:875Ba0:125CuO4 (LBCO-1/8) at low temperature(16), suggest the ground-state pseudogap is a single gap of d-wave(17) form. Here, we report angle-resolved photoemission data from LBCO-1=8, collected with improved experimental conditions, that reveal the ground-state pseudogap has a pronounced deviation from the simple d-wave form. It contains two distinct components: a d-wave component within an extended region around the node and the other abruptly enhanced close to the antinode, pointing to a dual nature of the pseudogap in this failed high-T-c superconductor that involves a possible precursor-pairing energy scale around the node and another of different but unknown origin near the antinode.

Measurements of the in-plane thermal conductivity,kappa(ab), in magnetic fields parallel to the c-axis have been carried out in the overdoped regime of La2-xSrxCuO4 with x = 0.18 - 0.30. At low temperatures,kappa(ab) is enhanced by the application of magnetic field for x < 0.22 due to the Volovik effect, while kappa(ab) is suppressed for x > 0.22. No Volovik effect for x > 0.22 is explained as being due to the phase separation into superconducting and normal-state regions. Moreover, the enhancement of kappa(ab) has been found to be singularly weak at x similar to 0.21 where T-c is a little suppressed in zero field. A so-called stripe order might be stabilized by the application of magnetic field at x similar to 0.21, as in the case of x similar to 0.115.

Our previous study of the magnetic susceptibility, chi, on zero-field and field cooling and magnetization-hysteresis-loop (MHL) for La(2-x)Sr(x)CuO(4) (LSCO) single crystals with x = 0.178 - 0.261 has suggested that a microscopic phase separation into superconducting (SC) and normal-state regions takes place in the overdoped regime of LSCO. In order to clarify the onset hole-concentration where the microscopic phase separation appears, we have carried out chi measurements on zero-field and field cooling and MHL measurements for LSCO single crystals with x = 0.125 and 0.147. From the chi measurements in a low magnetic field on field cooling, absolute values of chi at 2 K, vertical bar chi 2K vertical bar, in x = 0.125 and 0.147 have been found to be smaller than that of x = 0.178, which can be understood in terms of the vortex-pinning effect around twin boundaries in a crystal. Neither two-step SC transition in chi vs. T in a moderate field on zero-field cooling nor second magnetization peak in MHL has been observed in x = 0.125, while both characteristics have been observed in x = 0.147. These results imply no enhancement of the vortex pinning in a moderate field-range and therefore suggest the existence of a homogeneous superconducting state in x = 0.125. Accordingly, it is concluded that the onset hole-concentration of the microscopic phase separation is located between x = 0.125 and 0.147.

We have measured the temperature dependence of the magnetic susceptibility, chi vs. T, and the magnetization curve, M vs. H, for NbSe2 single crystals, in order to compare the superconducting (SC) state in the overdoped regime of La2-xSrxCuO4 (LSCO) with the SC state of the layered conventional superconductor NbSe2. While a plateau in chi vs. T in a moderate magnetic field and a so-called second peak in M vs. H, which is due to the marked enhancement of vortex pinning, have been observed in the overcloped regime of LSCO, these behaviors have not been observed in NbSe2. The present results indicate that the anomalously marked enhancement of vortex pinning is a characteristic feature in the overcloped LSCO where a microscopic phase separation into SC and normal-state regions takes place. (C) 2008 Elsevier Ltd. All rights reserved.

Zero-field and longitudinal-field muon-spin-relaxation measurements have been carried out in order to investigate the electronic and magnetic states in La2-xSrxCuO4 over a wide range of hole concentration from x = 0.024 to 0.15. It has been found that the dynamic depolarization rate of muon spins in zero field starts to increase monotonically with decreasing temperature at a high temperature around 100 K. This effect is enhanced around the hole concentration of 1/8 per Cu but easily suppressed by the application of a small external field of a couple of ten gauss. The present study suggests that the dynamics of fluctuating internal fields at the muon site in the normal state of La2-xSrxCuO4 changes at a high around 100 K and is correlated with the mobility of holes. Possible relationships with spin-hole stripes and the spin-gap state in the normal state are discussed.

We report an anomalous temperature T variation of the field H(on) for the onset of the second magnetization peak in La(1.81)Sr(0.19)CuO(4) single crystals with the external magnetic field H oriented parallel to the c axis. While the peak field H(p) has a continuous decrease with increasing T, H(on) exhibits a sudden decrease for T similar to 11-15 K. This behavior appears to be related to the particular T dependence of the superfluid density in the case of two-band superconductivity affecting the T variation of the elastic energy of the vortex system at low H.

We have measured the thermal conductivity of the one-dimensional (1D) $S=1/2$ Heisenberg antiferromagnetic spin system of Sr2Cu1-xPdxO3 single crystals including nonmagnetic impurities of Pd2+. It has been found that the mean free path of spinons along the 1D spin chain at low temperatures is very close to the average length of finite spin chains between spin defects estimated from the magnetic susceptibility measurements. This proves that the thermal conduction due to spinons at low temperatures in Sr2CuO3 is ballistic as theoretically expected [Zotos et al.: Phys. Rev. Lett. 55 (1997) 11029].

Our recent experimental works on the inhomogeneity of superconductivity in both lnderdoped and overdoped regimes of La2-xSrxCuO4 are reviewed. In order to estimate the superconducting (SC) volume fraction in a sample, bulk-sensitive magnetic-susceptibility and specific-heat measurements have been carried out, using La2-xSrxCuO4 single crystals of good quality with x = 0.05-0.3. As a result, it has been found that the SC volume fraction is not 100% in both underdoped and overdoped regimes, suggesting the occurrence of a phase separation into SC and non-SC regions. Moreover, strong vortex-pinning effects have been observed in moderate magnetic fields in the overdoped regime, suggesting that the phase separation in the overdoped regime is not macroscopic but microscopic.

In order to investigate the inhomogeneity of the superconducting (SC) state in the overdoped high-T-c cuprates, we have measured the magnetic susceptibility, chi, of La2-xSrxCuO4 (LSCO) single crystals in the overdoped regime in magnetic fields parallel to the c-axis up to 7 T on warming after zero-field cooling. It has been found for x = 0. 198 and 0.219 that the temperature dependence of chi in 1 T shows a plateau, that is, chi is almost independent of temperature in a moderate temperature-range in the SC state. Moreover, a so-called second peak in the magnetization curve has markedly appeared in these crystals. These results indicate an anomalous enhancement of the vortex pinning and strongly suggest the occurrence of a microscopic phase separation into SC and normal-state regions in the overdoped high-T-c cuprates.

Precise measurements of the magnetic susceptibility and the in-plane electrical resistivity in the overdoped regime of La2-xSrxCuO4 have been carried out using single crystals of good quality, in order to investigate the inhomogeneity of the superconductivity in the overdoped regime of high-T-c cuprates. From the magnetic-susceptibility measurements, it has been found that the Meissner volume fraction decreases with increasing x, indicating that phase separation into superconducting (SC) and normal-state regions takes place in the overdoped regime. From the in-plane resistivity measurements, it has been found that the SC transition width becomes broad with increasing x, which is possibly due to the increase of the volume fraction of the proximity region between SC and normal-state regions in the phase-separated CuO2 plane. It appears that microscopic phase separation takes place in the overdoped regime and that the proximity region becomes dominant with increase x. (C) 2007 Elsevier B.V. All rights reserved.

In order to investigate the inhomogeneity of the superconductivity, we have grown a large-sized single-crystal of La2-xSrxCuO4 (LSCO), in which the Sr concentration, x, continuously changes in the underdoped regime, and measured the magnetic susceptibility on field cooling for several pieces of single crystal with different x values by slicing the single crystal. It has been found that the Meissner volume fraction, regarded as corresponding to the superconducting (SC) volume fraction in a sample, increases with increasing x. This suggests that a phase separation into SC and normal-state regions takes place in the underdoped regime as well as in the overdoped regime of LSCO. (c) 2007 Elsevier B.V. All rights reserved.

The low-temperature specific heat (SH) of overdoped La2-xSrxCuO4 single crystals (0.178 <= x <= 0.290) has been measured. For the superconducting samples (0.178 <= x <= 0.238), the derived gap values (without any adjusting parameters) approach closely onto the theoretical prediction Delta(0)=2.14k(B)T(c) for the weak-coupling d-wave BCS superconductivity. In addition, the residual term gamma(0) of SH at H=0 increases with x dramatically when beyond x similar to 0.22, and finally evolves into the value of a complete normal metallic state at higher doping levels, indicating a growing amount of unpaired electrons. We argue that this large gamma(0) cannot be simply attributed to the pair breaking induced by the impurity scattering, instead the phase separation is possible.

In order to investigate the possible inhomogeneous superconductivity in the overdoped high-T-c cuprates, suggested from the muon-spin-relaxation measurements, we have grown a single crystal of La2-xSrxCuO4 in which the Sr concentration, x, continuously changes in the overdoped regime and measured the magnetic susceptibility of several pieces of single crystals with different x values by slicing the single crystal. It has been found that the Meissner volume fraction, regarded as corresponding to the superconducting (SC) volume fraction in a sample, as well as T-c rapidly decreases with increasing x. It has been concluded that a phase separation into SC and normal-state regions takes place in a sample of La2-xSrxCuO4 in the overdoped regime. The separated two phases in the overdoped regime may be a hole-rich normal Fermi-liquid state region and a hole-poor superconducting one. Otherwise, holes doped into the Cu 3d orbital may destroy the superconductivity around themselves, producing normal-state regions in the superconducting sea. (c) 2006 Elsevier B.V. All rights reserved.

In order to prove the possible phase separation in the overdoped high-T-c superconductors, suggested from the muon-spin-relaxation measurements, the superconducting (SC) volume fraction has been investigated in detail in the overdoped regime of La2-xSrCuO4. From measurements of the magnetic susceptibility, chi, using a single crystal in which the Sr concentration, x, continuously changes in the overdoped regime, both the absolute value of chi at 2 K, vertical bar chi(2K)vertical bar, on field cooling reflecting the SC volume fraction and T-c have been found to decrease with increasing x. Moreover, it appears that vertical bar chi(2K)vertical bar has a roughly linear relation to T-c. It has been concluded that phase separation into SC and normal-state regions takes place in the overdoped regime of La2-xSrxCuO4.

The buckling of CuO2 plane in single crystals of La-based high-T-C cuprates LSCO (x=0.15) and LBCO (x=0.08) was directly observed by Cu-NMR. In both the cases, buckling patterns obtained by NMR disagree with those expected in the averaged structure at the vicinity of the structural phase transitions.

Magnetic-field effects on the so-called charge-spin stripe order, namely, the charge stripe order and spin stripe order in the La-214 system have been investigated from the in-plane electrical-resistivity, rho(ab), measurements. In La2-xBaxCuO4 (LBCO) with x = 0.10 and La2-xSrxCuO4 with x = 0.115 where the incommensurate charge peaks are weak and unobservable in zero field from the elastic neutron-scattering measurements, respectively, the normal-state value of rho(ab) at low temperatures markedly increases with increasing field up to 27 T. As for x = 0.11 in LBCO where the charge stripe order is fairly stabilized in zero field, the increase in rho(ab) with increasing field is negligibly small. In conclusion, the magnitude of the increase in rho(ab) by the application of magnetic field is well correlated with the stability of the charge stripe order in zero field for the La-214 system around x = 1/8. Our understanding is as follows. When the charge-spin stripe order is not fully stable in zero field, magnetic field operates to stabilize the charge-spin stripe order. The value of rho(ab) increases with increasing field depending on the stability of the charge stripe order.

We have succeeded in the crystal growth of the superconducting La2-x-yBa3SryCaCu2O6 of the La2126 phase by only the traveling-solvent floating-zone (TSFZ) method without the hot-isostatic-pressure (HIP) treatment. In the TSFZ growth, we added a small amount of B2O3 to the solvent for the stabilization of the growth and used high-pressure oxygen of similar to 10 atm as a growth atmosphere for the suppression of oxygen defects. We have obtained single crystals with the size of the order of 2 x 2 x 3 mm(3) and determined the composition to be La1.80Ba0.07Sr0.1Ca0.97Cu1.97O6 by the inductively-coupled-plasma atomic-emission-spectrometry. The powder x-ray diffraction has revealed that the crystals are of the single phase of La2126 and include no impurity phases such as La2-xMxCuO4 (M: alkaline earth). The as-grown single crystals have shown superconductivity of the bulk with the transition temperature T-C = 20 K.

We have measured the thermal conductivity along the b axis, k(b), parallel to the one-dimensional (1D) spin chain of Sr2Cu1-xPdxO3 single crystals including nonmagnetic impurities of Pd2+. in order to investigate possible ballistic thermal conduction. It has been found that the mean free path of spinons estimated from the thermal conductivity due to spinons, which is obtained by subtracting the thermal conductivity due to phonons from K-b using the Debye model, is comparable with the average length of finite spin chains between spin defects estimated from the magnetic susceptibility measurements. This proves that the thermal conduction due to spinons at low temperatures in the 1D spin system Sr2CuO3 is ballistic.

Magnetic-field effects on the charge-spin stripe order in La-214 high-T-c cuprates have been investigated from measurements of the in-plane electrical-resistivity, rho(ab). In La2-xBaxCuO4 with x = 0.10 and La2-xSrxCuO4 with x = 0.115 where the incommensurate charge peaks are weak and unobservable in zero field in elastic neutron-scattering measurements, respectively, the normal-state value of rho(ab) at low temperatures markedly increases with increasing field up to 27 T. For La2-xBaxCuO4 with x = 0.11 and Zn-substituted La2-xSrxCu1-yZnyO4 with x = 0.115 and y = 0.02 where the charge stripe order is fairly stabilized in zero field, on the other hand, the increase in rho(ab) with increasing field is negligibly small. In conclusion, when the charge-spin stripe order is not fully stable in zero field, magnetic field operates to stabilize the charge-spin stripe order. The value of rho(ab) increases with increasing field depending on the stability of the charge stripe order.

The buckling of CuO2 plane in single crystals of La2-xSrxCuO4 (LSCO) [x = 0.08] and La2-x-yBaySrxCuO4 (LBSCO) [x = 0.06, y = 0.065] was directly observed by high-field Cu-NMR spectra. The split pattern in the NMR spectra of LSCO [x = 0.08] shows that the local structure is Low Temperature Orthorhombic (LTO) like at low temperature, consistent with the results reported by diffraction experiments. In LBCO [x = 0.08], the local structure is Low Temperature Tetragonal (LTT) like, while the averaged structure is reported to be LTO. In LBSCO, the local structure is found to be either LTT-like or Low Temperature Less-Orthorhombic (LTLO) between 35 and 70 K, while the averaged structure is reported to be LTC. With decreasing temperature, the resonance peak disappeared below 35 K, indicating the appearance of Charge Density Wave (CDW) or Spin Density Wave (SDW).

We have grown a single crystal of Lag(2-x)Sr(x)CuO(4) in which the Sr concentration, x, continuously changes from 0.24 to 0.29 in the overdoped regime and obtained many pieces of single crystals with different x values by slicing the single crystal. From detailed measurements of the magnetic susceptibility, chi, of each piece, it has been found that the absolute value of chi at the measured lowest temperature 2K, vertical bar chi(2K)vertical bar, on field cooling rapidly decreases with increasing x as well as the superconducting (SC) transition temperature. As the value of vertical bar chi(2K)vertical bar is regarded as corresponding to the SC volume fraction in a sample, it has been concluded that a phase separation into SC and normal-state regions occurs in a sample of La2-xSrxCuO4 in the overdoped regime.

We have performed neutron scattering experiment on the single crystals of La2-xBaxCuO4 (x = 0.10 and 0.125) in order to study the doping dependence of stripe correlations. At low temperatures, clear incommensurate (IC) peaks from spin-density-wave orders (SDW) were observed in both samples. The incommensurability (5) of the SDW in the x = 0.10 sample was found to be 0.110, which slightly deviates from the simple relation of 6 = x seen in La2-xSrx- CuO4, possibly due to the large stripe pinning effect in the low-temperature tetragonal (LTT) structure. The inelastic magnetic signal at the low-energy region below 6 meV drastically changes around the structural transition temperature between LTT and low-temperature orthorhombic (LTO) phases T-d2. Temperature dependences of the peak-width and the incommensurability also show an anomalous behavior at T-d2 rather than the spin ordering temperature, T-sp. These results suggest that spin correlations are well stabilized in the LTT phase as expected from the stripe model. With further increasing temperature in LTO phase, the incommensurability decreases and the peak-width grows due to the degradation of stripe correlations. However, the stiffness of stripe correlation seen in the low-energy IC spin fluctuations depends on the doping. In the x = 0.10 sample no well-defined magnetic excitation peak was observed below 6 meV at 200 K, while a flat top shaped single peak remains at 6 meV in the x = 0.125 sample. Deviation from the 1/8 doping more easily lose the spatial coherence of spin correlations above T-d2. (c) 2005 Elsevier B.V. All rights reserved.

Zero-field muon-spin-relaxation measurements have been carried out in order to investigate the Zn- and Ni-substitution effects on the Cu-spin dynamics in the electron-doped Pr0.86LaCe0.14Cu1-y(Zn, Ni)(y)O4+alpha-delta with y = 0, 0.01, 0.02, 0.05 and different values of the reduced oxygen content delta (delta <= 0.09). For the samples with y = 0 and very small delta values of delta < 0.01, a muon-spin precession due to the formation of a long-range antiferromagnetic order has been observed at low temperatures below similar to 5 K. For the moderately oxygen-reduced samples of 0.01 <= delta <= 0.09, on the contrary, no muon-spin precession has been observed and the temperature dependence of the spectra is similar to one another regardless of the y value. That is, no impurity-induced slowing down of the Cu-spin fluctuations has been detected, which is very different from the results of the hole-doped high-T-c cuprates. The reason is discussed. (c) 2005 Elsevier B.V. All rights reserved.