足立 匡

In the hole-doped cuprates, a small number of carriers suppresses antiferromagnetism and induces superconductivity. In the electron-doped cuprates, on the other hand, superconductivity appears only in a narrow window of high-doped Ce concentration after reduction annealing, and strong antiferromagnetic correlation persists in the superconducting phase. Recently, Pr1.3-xLa0.7CexCuO4 (PLCCO) bulk single crystals annealed by a protect annealing method showed a high critical temperature of around 27 K for small Ce content down to 0.05. Here, by angle-resolved photoemission spectroscopy measurements of PLCCO crystals, we observed a sharp quasi-particle peak on the entire Fermi surface without signature of an antiferromagnetic pseudogap unlike all the previous work, indicating a dramatic reduction of antiferromagnetic correlation length and/or of magnetic moments. The superconducting state was found to extend over a wide electron concentration range. The present results fundamentally challenge the long-standing picture on the electronic structure in the electron-doped regime.

For the understanding of Ce-doped high-<italic>T</italic>_ccuprates with the so-called T′-structure, we have performed ^63,65Cu and ¹³⁹La NMR measurements in T′-Pr_1.3−<italic>_x</italic>La_0.7Ce<italic>_x</italic>CuO_4+δsingle crystals with <italic>x</italic>= 0.10 and 0.15. The Néel temperature (<italic>T</italic>_N) was evaluated from the temperature (<italic>T</italic>) dependence of the ¹³⁹La NMR line width. For <italic>x</italic>= 0.10, <italic>T</italic>_Ndecreased from 190 K in the as-grown crystal to 60 K in the oxygen-reduced crystal annealed at 800 °C. For <italic>x</italic>= 0.15, <italic>T</italic>_Nof 170 K in the as-grown crystal decreased to zero through the same heat treatment. The oxygen reduction strongly suppressed the antiferromagnetic (AF) order. ^63,65Cu NMR spectra had a narrow center line and broad satellites with a small electric field gradient in the paramagnetic state. In the AF state, in contrast, ^63,65Cu NMR spectra had a large electric field gradient. The <italic>T</italic>dependence of the ⁶³Cu nuclear spin–lattice relaxation rate (1/<italic>T</italic>₁) showed the existence of large spin fluctuations in the paramagnetic state. The spin fluctuations seem to play a key role in the appearance of superconductivity in the T′-cuprates.

Angle-resolved photoemission spectroscopy (ARPES) is typically used to study only the occupied electronic band structure of a material. Here we use laser-based ARPES to observe a feature in bismuth-based superconductors that, in contrast, is related to the unoccupied states. Specifically, we observe a dispersive suppression of intensity cutting across the valence band, which, when compared with relativistic one-step calculations, can be traced to two final-state gaps in the bands 6 eV above the Fermi level. This finding opens up possibilities to bring the ultrahigh momentum resolution of existing laser-ARPES instruments to the unoccupied electron states. For cases where the final-state gap is not the object of study, we find that its effects can be made to vanish under certain experimental conditions.

To investigate the origin of the enhanced T-c (approximate to 110 K) of the trilayer cuprate superconductor Bi2Sr2Ca2Cu3O10+delta (Bi-2223), its underdoped single crystals are a critical requirement. Here, we demonstrate the first successful in-plane resistivity measurements of heavily underdoped Bi-2223 (zero-resistivity temperatures approximate to 20 similar to 35 K). Detailed crystal growth methods, the annealing process, as well as X-ray diffraction (XRD) and magnetic susceptibility measurement results are also reported. (C) 2015 The Authors. Published by Elsevier B.V.

Ultrafast spectroscopy is an emerging technique with great promise in the study of quantum materials, as it makes it possible to track similarities and correlations that are not evident near equilibrium. Thus far, however, the way in which these processes modify the electron self-energy-a fundamental quantity describing many-body interactions in a material-has been little discussed. Here we use time-and angle-resolved photoemission to directly measure the ultrafast response of self-energy to near-infrared photoexcitation in high-temperature cuprate superconductor. Below the critical temperature of the superconductor, ultrafast excitations trigger a synchronous decrease of electron self-energy and superconducting gap, culminating in a saturation in the weakening of electron-boson coupling when the superconducting gap is fully quenched. In contrast, electron-boson coupling is unresponsive to ultrafast excitations above the critical temperature of the superconductor and in the metallic state of a related material. These findings open a new pathway for studying transient self-energy and correlation effects in solids.

Thermal conductivity measurements in magnetic fields have been performed for FeSe0.3Te0.7 single crystals as-grown and annealed at 400 degrees C for 100 h in vacuum (similar to 10(-4) Pa). It has been found that the thermal conductivity in the ab-plane, kappa(ab), of the annealed crystal shows an enhancement at low temperatures just below the superconducting transition temperature, T-c, owing to the thermal conductivity due to quasiparticles, while the kappa(ab) of the as-grown crystal does not. This suggests that FeSe1-xTex is a strongly correlated electron system. It has also been found that both the degree of the enhancement of kappa(ab) just below T-c and the behavior of the suppression of kappa(ab) by the application of magnetic field for the annealed crystal depend on the thickness of the crystal. These results indicate that bulk superconductivity is absent in the as-grown crystal, appears only in the surface area in the annealed-thick crystal and appears not only in the surface area but also inside of the annealed-thin crystal. It has been concluded that excess Fe included in the as-grown crystal is removed from the surface by annealing.

Low-temperature specific heat measurements in various magnetic fields parallel and perpendicular to the c-axis for annealed single crystals of the iron-chalcogenide superconductor FeSe1-xTex have revealed that the anisotropy of the enhancement of the residual electronic-specific- heat-coefficient in the ground state by the application of magnetic field, namely, the anisotropy of the upper critical field, H-c2, is 2.2 for x = 0.6, which is different from the anisotropy of H-c2 estimated from the resistivity measurements in non-annealed single crystals of x = 0.6 [Khim et al. 2010 Phys. Rev. B 81 184511]. It appears that the anisotropy of H-c2 strongly depends on the content of excess iron included in a crystal.

In order to investigate the possible ferromagnetic phase in the non-superconducting (non-SC) heavily overdoped (HOD) regime of high-T-c cuprates suggested by Sonier et al. [Proc. Natl. Acad. Sci. USA 107, 17131 (2010)1 in La2-xSrxCuO4, both the magnetization and in-plane electrical resistivity, rho(ab), have been measured using single crystals of Bi1.76Pb0.35Sr1.89CuO6+delta covering a wide range of hole concentration. Magnetization curves in the non-SC HOD regime have shown a tendency to be saturated in high magnetic fields at low temperatures, which is probably a precursor of the possible ferromagnetic transition at a lower temperature. In addition, it has been found that rho(ab) exhibits the temperature dependence being proportional to T-4/3, which is characteristic of a metal with two-dimensional ferromagnetic fluctuations, in a wide range of hole concentration from the superconducting overdoped to the non-SC HOD regime. It has been concluded that two-dimensional ferromagnetic correlation develops at low temperatures in HOD Bi-2201 and that therefore the development of the ferromagnetic correlation is probably an intrinsic feature in the HOD cuprates.

In order to investigate the electronic homogeneity in electron-doped high-T-c cuprates, magnetization and specific heat measurements have been performed for Pr1-xLaCexCuO4 single crystals with x = 0.13 - 0.17. It has been found from the magnetization curve that the hysteresis is relatively small and that the so-called second magnetization peak is absent for every x. Moreover, the Sommerfeld constant in the ground state has shown HlnH dependence on magnetic field H for every x, suggesting that the superconductivity is not pure but dirty. These results are in contrast to those observed in the overdoped regime of hole-doped La2-xSrxCuO4 where an inhomogeneous superconducting state is formed in spite of the superconductivity being pure. Therefore, it has been concluded that a rather homogeneous electronic state is realized in the electron-doped cuprates, suggesting different electronic states between hole-and electron-doped cuprates.

Muon positions in La2CuO4 were examined by using the density functional theory. Potential minimum positions near apical and plane oxygen have been determined as possible initial muon stopping positions. We found that final muon stopping positions were different from those initial positions due to effects of the local deformation of crystal structure which was induced by injected muons. This means that injected muons relax their positions deforming local crystal structures and minimizing the total energy of the system. We also found that the estimation of those final muon positions had to be done in the large scale area as a supercell which contained 27 unit cells in order to achieve realistic situations of the system with the muon as a dilute impurity.

Stage-2 MnCl2 GIC magnetically behaves like a quasi 2D XY antiferromagnet on the triangular lattice with the spin freezing temperature TSG (= 1.1 K). We have undertaken an extensive study on the T dependence of specific heat in the absence of an external field H and in the presence of H along the c-plane. The magnetic specific heat Cmag at H = 0 shows no anomaly at TSG, but exhibits double broad peaks around 5 and 41 K. The anomaly at 41K is the onset of short range spin correlation. The magnetic specific heat C mag at H = 0 is described by Cmag / α(1/T 2) exp(- ΔT ) with Δ = 1.41 ± 0.03 K, while C mag at H = 10 kOe is proportional to T. The entropy due to the broad peak around 5K is 1/3 of the total entropy. The residual entropy is 63% of the total entropy because of highly frustrated nature of the system. These results are consistent with the previous DC susceptibility, neutron scattering, and electron spin resonance (ESR) experiments. © 2013 The Physical Society of Japan.

We have investigated the suppression of the superconducting transition temperature, T-c, with an increase of the residual resistivity, rho(0), through the substitution of M (M = Co, Ni, Zn) for Fe in Fe1-yMySe0.3Te0.7 single crystals, in order to clarify the symmetry of the superconducting gap in FeSe1-xTex. Small, large and very small suppression of T-c have been observed through the Co, Ni, and Zn substitution, respectively. The magnitude of the suppression rate is hardly explained in terms of the pair-breaking effect due to potential scattering calculated based on the Abrikosov-Gor'kov theory, even if errors in the estimation of the carrier concentration, effective mass and rho(0) were taken into account. Accordingly, these results suggest that the superconducting symmetry is the s(++)-wave in FeSe1-xTex.

We have performed resonant inelastic x-ray scattering (RIXS) near the Cu-K edge on cuprate superconductors La2-xSrxCuO4, La2-xBaxCuO4, La2-xSr xCu1-yFeyO4, and Bi 1.76Pb0.35Sr1.89CuO6+δ, covering underdoped to heavily overdoped regimes and focusing on charge excitations inside the charge-transfer gap. RIXS measurements of the 214 systems with Ei=8.993 keV have revealed that the RIXS intensity at 1-eV energy transfer has a minimum at (0,0) and maxima at (±0.4π,0) and (0,±0.4π) for all doping points regardless of the stripe-ordered state, suggesting that the corresponding structure is not directly related to stripe order. Measurements with Ei=9.003 keV on metallic La 1.7Sr0.3CuO4 and Bi1.76Pb 0.35Sr1.89CuO6+δ exhibit a dispersive intraband excitation below 4 eV, similar to that observed in the electron-doped Nd1.85Ce0.15CuO4. This is the first observation of a dispersive intraband excitation in a hole-doped system, evidencing that both electron- and hole-doped systems have a similar dynamical charge correlation function. © 2013 American Physical Society.

Impurity and magnetic field effects on the stripes are reviewed not only in the La-based cuprates but also in the other high-T-c cuprates. It has turned out that the so-called 1/8 anomaly takes place not only in the La-based cuprates but also in the hole-doped high-T-c cuprates universally, when adequate pinning centers are introduced. Impurity-induced magnetism tends to emerge at low temperatures in a wide range of hole concentration in LSCO, Bi2201 and YBCO where superconductivity appears. Accordingly, it is possible that the dynamically fluctuating stripes exist universally in the hole-doped high-T-c cuprates and play an important role in the appearance of the high-T-c superconductivity. For the hole-doped high-T-c cuprates, it has turned out that magnetic field effects on the stripes are small in samples where the stripes are well stabilized statically in zero field, while field-induced magnetism is observed under magnetic fields parallel to the c-axis in underdoped samples where magnetism is not well developed in zero field. The field-induced magnetism has been observed in some optimally doped samples also, but it has never been observed in overdoped samples. Field-induced CDW has been observed in slightly overdoped BSCCO. These results are understood fundamentally in terms of the theoretical phase diagram as a function of magnetic field, based on the Ginzburg-Landau theory with competing antiferromagnetic and superconducting order parameters. It is also possible to understand that the field-induced magnetism and CDW are due to pinning of the dynamically fluctuating stripes by vortex cores as in the case of the pinning by impurities. In the electron-doped high-T-c cuprates, on the other hand, impurity-induced magnetism has never been observed. The nature of the field-induced magnetism observed in the electron-doped cuprates is different from that in the hole-doped ones. (C) 2012 Elsevier B. V. All rights reserved.

In an effort to elucidate the origin of the Curie-like paramagnetism that is generic for heavily overdoped cuprates, we have performed high-transverse-field muon spin rotation (TF-μSR) measurements of La 2-xSr xCuO 4 single crystals over the Sr content range 0.145≤x≤0.33. We show that the x dependence of the previously observed field-induced broadening of the internal magnetic field distribution above the superconducting transition temperature T c reflects the presence of two distinct contributions. One of these becomes less pronounced with increasing x and is attributed to diminishing antiferromagnetic correlations. The other grows with increasing x, but decreases above x∼0.30, and is associated with the Curie-like term in the bulk magnetic susceptibility χ. In contrast to the Curie-like term, however, this second contribution to the TF-μSR linewidth extends back into the underdoped regime. Our findings imply a coexistence of antiferromagnetically correlated and paramagnetic moments, with the latter becoming dominant beyond x∼0.185. This suggests that the doped holes do not neutralize all Cu spins via the formation of Zhang-Rice singlets. Moreover, the paramagnetic component of the TF-μSR linewidth is explained by holes progressively entering the Cu 3d x2-y2 orbital with doping. © 2012 American Physical Society.

We report in-plane Cu-O atomic displacements as a function of doping in the La2-xSrxCuO4 system, determined by polarized Cu K-edge extended x-ray absorption fine structure measurements. We find a wide distribution of the Cu-O bonds showing a single peak asymmetric function for the system at low doping, which gets two peak function for the underdoped and optimally doped superconducting phases before turning to a single peak function for the overdoped phase. The results provide a direct evidence of critical local lattice fluctuations for an optimum transition temperature in the superconducting phases hosting quantum stripes. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4726245]

We have investigated the low-frequency Cu-spin dynamics of partially Fe-substituted La2-xSrxCu1-yFeyO4 with x = 0.06 - 0.235 and y = 0.01 from zero-field muon-spin-relaxation measurements. We find that the magnetic transition temperature, T-N(mid), is enhanced through the 1% Fe substitution in a wide range of hole concentration per Cu, p, where superconductivity appears in La2-xSrxCuO4. The T-N(mid) exhibits a local maximum at p similar to 0.115 and exists even in the overdoped regime of p &gt; 0.15. To the surprise, T-N(mid) in the overdoped regime of 0.15 &lt; p &lt;= 0.225 is larger than in the underdoped regime of 0.05 &lt;= p &lt;= 0.09. These results suggest that the substitution of Fe as a magnetic impurity is effective for the stabilization of the so-called stripe order in high-T-c cuprates in the underdoped regime and that the nature of the magnetic order induced through the Fe substitution in the overdoped regime might be different from that in the underdoped regime.

Superconductors are characterized by zero resistance and Meissner effect. At below critical temperature (Tc), these materials exhibit diamagnetic properties. On the other hand, materials in nano-crystal size have specific properties that differ from bulk state. Nanomaterials are characterized by surface effect which influences physical and chemical properties of the materials. Combining these two mayor fields, it can be obtained superconductors in nano-crystal size (below 200 nm) using simple method (called as nano-superconductors). Generally, ceramic-oxides in nano-crystal size, even in cuprate-superconductors, may have ferromagnetic behavior at room temperature. In this research, Bi and Bi,Pb-based nano-superconductors synthesized by wet mixing technique have Tc similar to 80 K for 2212 (Bi2Sr2CaCu2O8-delta and Bi1,6Pb0,4Sr2CaCu2O8-delta) phases. They also exhibit ferromagnetism effect and hysteresis curve although at well above Tc. This is unusual phenomenon because superconductor materials are generally diamagnetic at below Tc and paramagnetic at normal state. This phenomenon is possibly due to magnetic moments which may possibly come from oxygen vacancies of the nanoparticles surface.

Muon-spin-relaxation measurements have been performed in the overdoped regime of hole-doped high-T-c superconductor La(2-x)Srx(C)uO(4) and electron-doped Pr1-xLaCexCuO4 with Cu partially replaced by Zn, in order to understand the role of stripe pinning to the superconductivity. It has been found that dynamically fluctuating stripes of spins and charges probably exist in a wide range of hole concentration in the hole-doped cuprates and that it is possible that they play an important role in the appearance of superconductivity. On the other hand, no clear symptom of stripe pinning has been observed in the electron-doped cuprates.

With the aim at investigating the relationship between the anomalous decrease in T-c, the development of the Cu-spin correlation and the inhomogeneity of superconductivity at x similar to 0.21 in the overdoped regime of La2-xSrxCuO4, we have performed the in-plane electrical-resistivity, rho(ab), and magnetization measurements. Measurements of the temperature dependence of rho(ab) in magnetic fields have revealed that the superconducting (SC) transition curve shifts to the low-temperature side almost in parallel with increasing field above similar to 9 T at x similar to 0.21, which is similar to that observed for stripe-ordered La-214 systems with the hole concentration of similar to 1/8 per Cu. On the other hand, it has been found that the magnetization curve shows the so-called second peak in the SC state at x similar to 0.21, which is due to the strong vortex pinning in normal-state regions in the nano-scale inhomogeneous SC state. Accordingly, it is likely that the stripe correlations are developed in the nano-scale inhomogeneous SC state at x similar to 0.21.

Ni-substitution effects on the superconductivity and Cu-spin correlation have been investigated in La2-xSrxCu1-yNi yO4 from the electrical resistivity and muon spin relaxation measurements, taking into account the hole trapping by Ni recently suggested. It has been found that the Ni substitution suppresses the superconductivity, induces the localization of holes, and develops the Cu-spin correlation to the same degree as the Zn substitution. These suggest that Ni with a trapped hole tends to give rise to potential scattering of holes in the CuO2 plane to the same degree as Zn, being discussed in relation to the so-called dynamical stripe correlations of spins and holes. © 2011 American Physical Society.

In order to investigate the local structure in the CuO2 plane of the La-based lhigh-T-c cuprate La2-xBaxCuO4, we have carried out measurements of Cu-63-NMR on single crystals with x=0.08 and 0.10 at low temperatures down to 4 K. In both samples, the local structure deduced from observed NMR spectra agreed with the averaged structure. In high-temperature region around 300K, detailed investigation of NMR line width has shown the existence of the local structure different from the averaged structure; that is, CuO6 octahedra tilt from the c-axis toward the random direction by approximately 1.4 deg.

We performed zero-field muon spin rotation/relaxation (ZF-mu SR) measurements on high-quality single-crystalline samples of Bi1.76Pb0.35Sr1.89CuO6+delta [(Bi,Pb)2201] over a wide doping region controlled by oxygen depletion (delta). We find that there is no static magnetic order above similar to 2 K in any of these samples including underdoped samples. The absence of slow antiferromagnetic fluctuation in the underdoped region allows reliable muon Knight shift measurements, which provides a direct evidence of reduced density of states at lower temperatures or "pseudogap". (C) 2009 Elsevier B.V. All rights reserved.