Tadashi Adachi

The ability to efficiently control charge and spin in the cuprate high-temperature superconductors is crucial for fundamental research and underpins technological development. Here, we explore the tunability of magnetism, superconductivity, and crystal structure in the stripe phase of the cuprate La Ba CuO , with = 0.115 and 0.135, by employing temperature-dependent (down to 400 mK) muon-spin rotation and AC susceptibility, as well as X-ray scattering experiments under compressive uniaxial stress in the CuO plane. A sixfold increase of the three-dimensional (3D) superconducting critical temperature and a full recovery of the 3D phase coherence is observed in both samples with the application of extremely low uniaxial stress of 0.1 GPa. This finding demonstrates the removal of the well-known 1/8-anomaly of cuprates by uniaxial stress. On the other hand, the spin-stripe order temperature as well as the magnetic fraction at 400 mK show only a modest decrease under stress. Moreover, the onset temperatures of 3D superconductivity and spin-stripe order are very similar in the large stress regime. However, strain produces an inhomogeneous suppression of the spin-stripe order at elevated temperatures. Namely, a substantial decrease of the magnetic volume fraction and a full suppression of the low-temperature tetragonal structure is found under stress, which is a necessary condition for the development of the 3D superconducting phase with optimal . Our results evidence a remarkable cooperation between the long-range static spin-stripe order and the underlying crystalline order with the three-dimensional fully coherent superconductivity. Overall, these results suggest that the stripe- and the SC order may have a common physical mechanism.

To investigate proposed ferromagnetic fluctuations in the so-called single-layer Bi-2201 and La-214 high-Tc cuprates, we performed magnetization and electrical resistivity measurements using single-layer Tl-2201 cuprates Tl2Ba2CuO6+δ and La-214 La2−xSrxCuO4 in the heavily overdoped regime. Magnetization of Tl2Ba2CuO6+δ and La2−xSrxCuO4 exhibited the tendency to be saturated in high magnetic fields at low temperatures, suggesting the precursor behavior toward the formation of a ferromagnetic order. It was found that the power of temperature n obtained from the temperature dependence of the electrical resistivity is ~4/3 and ~5/3 for Bi-2201 and La2−xSrxCuO4, respectively, and is ~4/3 at high temperatures and ~5/3 at low temperatures in Tl2Ba2CuO6+δ. These results suggest that two- and three-dimensional ferromagnetic fluctuations exist in Bi-2201 and La2−xSrxCuO4, respectively. In Tl2Ba2CuO6+δ, it is suggested that the dimension of ferromagnetic fluctuations is two at high temperatures and three at low temperatures, respectively. The dimensionality of ferromagnetic fluctuations is understood in terms of the dimensionality of the crystal structure and the bonding of atoms in the blocking layer.

The magnetic hysteresis curve can provide information about magnetic properties, particle size, and its distribution. For superparamagnetic materials, the characteristics of the hysteresis curve are very typical, that is, Hc is equal to zero with a certain Ms value related to particle size in the order of nanometers. Here, we report the analysis of the magnetization hysteresis curve by applying a modified Langevin equation with and without a log-normal distribution for nearly superparamagnetic Fe3O4 and Fe3O4 encapsulated by SiO2 (Fe3O4–SiO2). To study the reliability of the modified Langevin equation, we compared the particle size values obtained from fittings analysis of the modified Langevin equation with those obtained from the TEM measurement. It was found that particle size from TEM measurement for Fe3O4 and Fe3O4–SiO2 is 11 and 14 nm, respectively. These values had a very high match with the particle size obtained from fitting analysis of the modified Langevin equation with or without a log-normal distribution, which is 12.6 and 12.9 nm for Fe3O4 and 13.8 and 14.5 nm for Fe3O4–SiO2, respectively. The fitting results of the modified Langevin equation with a log-normal distribution showed a value closer to the measurement results. By using the modified Langevin equation with a log-normal distribution, it was also obtained that the probability density function for Fe3O4 was 20.69% and that for Fe3O4–SiO2 was 0.55%. It was concluded that the modified Langevin equation with and without log-normal distribution was able to be used to obtain the particle size and its distribution for nearly superparamagnetic samples.

We study the electronic structure of electron-doped Pr1.3-xLa0.7CexCuO4 (PLCCO; Tc=27K, x=0.1) and hole-doped Bi2Sr2CaCu2O8+δ (Bi2212; Tc=90K) cuprate superconductors using x-ray absorption spectroscopy and resonant photoemission spectroscopy (Res-PES). From Res-PES across the O K-edge and Cu L-edge, we identify the O 2p and Cu 3d partial density of states (PDOS) and their correlation satellites, which originate in two-hole Auger final states. Using the Cini-Sawatzky method, analysis of the experimental O 2p PDOS shows an oxygen on-site Coulomb energy for PLCCO to be Up = 3.3±0.5eV, and for Bi2212, Up=5.6±0.5eV, while the copper on-site Coulomb correlation energy is Ud = 6.5±0.5eV for Bi2212. The expression for the Heisenberg exchange interaction J in terms of the electronic parameters Ud, Up, charge-transfer energy Δ, and Cu-O hopping tpd obtained from a simple Cu2O cluster model is used to carry out an optimization analysis consistent with J known from scattering experiments. The analysis also provides the effective one-band on-site Coulomb correlation energy U and the effective hopping t. PLCCO and Bi2212 are shown to exhibit very similar values of U/t∼9-10, confirming the strongly correlated nature of the singlet ground state in the effective one-band model for both materials.

We present an in situ uniaxial pressure device optimized for small angle x-ray and neutron scattering experiments at low-temperatures and high magnetic fields. A stepper motor generates force, which is transmitted to the sample via a rod with an integrated transducer that continuously monitors the force. The device has been designed to generate forces up to 200 N in both compressive and tensile configurations, and a feedback control allows operating the system in a continuous-pressure mode as the temperature is changed. The uniaxial pressure device can be used for various instruments and multiple cryostats through simple and exchangeable adapters. It is compatible with multiple sample holders, which can be easily changed depending on the sample properties and the desired experiment and allow rapid sample changes.

Muon spin rotation/relaxation measurements were performed to investigate magnetic properties of FeSe1-xSx thin films on LaAlO3. A drastic decrease of the initial asymmetry was observed together with the peak structure in the temperature dependence of the relaxation rate of muon spins almost at the same temperature where kink anomalies were observed in the temperature-dependent resistivity. With increasing S content, the anomaly temperature increased and the magnetic fluctuations at the lowest temperature were suppressed. These results show that the S substitution induces magnetism at low temperatures in FeSe1-xSx thin films. Although the behaviors of the magnetic and nematic phases in FeSe films toward chemical pressure by S substitution are similar to those for bulk FeSe toward hydrostatic pressure, the behavior of Tc is significantly different between these systems. Our results demonstrate that the detailed comparative investigation among physical and chemical pressure effects is essentially important to understand the interplay of the magnetism, the nematicity, and the superconductivity in iron chalcogenides.

The electron-doped cuprates are usually characterized by a more robust antiferromagnetic phase and a much narrower superconducting (SC) dome than those of the hole-doped counterparts. Recently, bulk single crystals of Pr1.3-xLa0.7CexCuO4-δ (PLCCO) prepared by the protect annealing method have been studied extensively and revealed many intriguing properties that were different from those obtained from samples annealed by the conventional methods. Here we report on a systematic angle-resolved photoemission spectroscopy study of PLCCO single crystals after protect annealing. The results indicate that the actual electron concentration (nFS) estimated from the Fermi-surface area is significantly larger than the Ce concentration x and the new nFS-based SC dome of PLCCO is more extended towards the overdoped side than the x-based SC dome derived for samples prepared using the conventional annealing method.

We present a combined soft x-ray and high-resolution vacuum-ultraviolet angle-resolved photoemission spectroscopy study of the electron-overdoped cuprate Pr1.3-xLa0.7CexCuO4 (PLCCO). Demonstration of its highly two-dimensional band structure enabled precise determination of the in-plane self-energy dominated by electron-electron scattering. Through analysis of this self-energy and the Fermi liquid cut-off energy scale, we find - in contrast to hole-doped cuprates - a momentum isotropic and comparatively weak electron correlation in PLCCO. Yet, the self-energies extracted from multiple oxide systems combine to demonstrate a logarithmic divergent relation between the quasiparticle scattering rate and mass. This constitutes a spectroscopic version of the Kadowaki-Woods relation with an important merit - the demonstration of Fermi liquid quasiparticle lifetime and mass being set by a single energy scale.

We report the results of the resistivity measurement on La2-xSrxCuO4 nanoparticles with x = 0, 0.05, and 0.20 evaluated by the four-point probe method. The high resistivity value shows the predominance of the inter-grain part. The temperature dependence of the conductivity can be analyzed by variable range hopping model showing the charge carriers are formed by thermal activation. There is no superconducting behavior that could be observed in La2-xSrxCuO4 nanoparticles with x = 0.05 and 0.20.

We use resonant inelastic x-ray scattering to investigate charge-stripe correlations in La1.675Eu0.2Sr0.125CuO4. By differentiating elastic from inelastic scattering, it is demonstrated that charge-stripe correlations precede both the structural low-temperature tetragonal phase and the transport-defined pseudogap onset. The scattering peak amplitude from charge stripes decays approximately as T-2 towards our detection limit. The in-plane integrated intensity, however, remains roughly temperature independent. Therefore, although the incommensurability shows a remarkably large increase at high temperature, our results are interpreted via a single scattering constituent. In fact, direct comparison to other stripe-ordered compounds (La1.875Ba0.125CuO4, La1.475Nd0.4Sr0.125CuO4, and La1.875Sr0.125CuO4) suggests a roughly constant integrated scattering intensity across all these compounds. Our results therefore provide a unifying picture for the charge-stripe ordering in La-based cuprates. As charge correlations in La1.675Eu0.2Sr0.125CuO4 extend beyond the low-temperature tetragonal and pseudogap phase, their emergence heralds a spontaneous symmetry breaking in this compound.

Abstract We present the results of the investigations of high conducting area and superconductivity at the interfaces between ferroelectric oxide and insulating oxide in heterostructures, isostructural to BaTiO₃/La₂CuO₄. The performed numerical simulations of BaTiO₃/La₂CuO₄ heterostructure show the possibility of high conductivity state at the interface. The temperature dependence of the measured electrical resistance of the Ba_0.8Sr_0.2TiO₃/La₂CuO₄ heterostructure interface have been studied and the superconducting behaviour with transition temperature T_c about 30K has been found. Therefore, the transition to the state with 2DEG at the interface is demonstrated. The results offer the possibility to design novel electronic devices.

Abstract The results of the investigations of high conducting and superconducting areas at the interfaces between ferroelectric oxide and insulating oxide are presented. The numerical simulations of BaTiO₃/LaMnO₃ and BaTiO₃/La₂CuO₄ heterostructures have been performed. It is found that in the samples of the Ba_0.8Sr_0.2TiO₃/LaMnO₃ heterostructure the electrical resistance decreases significantly and exhibits metallic behaviour at low temperatures for the case when the c axis of ferroelectric film is directed along the normal to the surface of the single crystal. The superconducting behaviour with transition temperature T_c about 30K has been found at the interface of the Ba_0.8Sr_0.2TiO₃/La₂CuO₄ heterostructure. The proposed concept promises the ferroelectrically controlled interface conductivity and superconductivity.

The upper critical field of a cuprate high-temperature superconductor, La1.84Sr0.16CuO4, was investigated by high-frequency self-resonant contactless electrical conductivity measurements in magnetic fields up to 102 T. An irreversible transition was observed at 85T (T = 4.2 K), defined as the upper critical field. The temperature-dependent upper critical field was argued on the basis of the Werthamer-Helfand-Hohenberg theory. The Pauli-limiting pair-breaking process with a small contribution of the spin-orbit coupling explained the first-order phase transition exhibiting a hysteresis observed at low temperatures.

Recently, hole-doped superconducting cuprates with the TA-structure La1.8−xEu0.2SrxCuO4 (LESCO) have attracted a lot of attention. We have performed x-ray photoemission and absorption spectroscopy measurements on as-grown and reduced TA-LESCO. Results show that electrons and holes were doped by reduction annealing and Sr substitution, respectively. However, it is shown that the system remains on the electron-doped side of the Mott insulator or that the charge-transfer gap is collapsed in the parent compound.

Nano-size effect on structure of crystal of free-standing La2-xSrxCuO4 (LSCO) nanoparticles was observed to reveal size effects of La-based high-Tc superconducting oxides. Lattice parameters were estimated by changing temperature and time of sintering. It is concluded that heat treatment plays a key role in controlling particle size of sample. Lattice distortions will occur when the particle size of sample becomes smaller.

We report the first observation of superconductivity in a heterostructure consisting of an insulating ferroelectric film (Ba0.8Sr0.2TiO3) grown on an insulating parent compound of La2CuO4 with [001] orientation. The heterostructure was prepared by magnetron sputtering on a nonatomically flat surface with inhomogeneities of the order of 1-2 nm. The measured superconducting transition temperature T-c is about 30 K. We have shown that superconductivity is confined near the interface region. Application of a weak magnetic field perpendicular to the interface leads to the appearance of the finite resistance. That confirms the quasi-two-dimensional nature of the superconductive state. The proposed concept promises ferroelectrically controlled interface superconductivity which offers the possibility of novel design of electronic devices.

Fluctuations of various order parameters are considered as the significant components of understanding the mechanism of high-T-c superconductivity. To study these fluctuations in the cuprate and Fe-based superconductors we use the joint measurements of direct current resistivity and non-resonant microwave absorption. Comparing data obtained with both methods allowed us to extract the contribution of dynamic charge density waves in La2-xSrxCuO4, superconducting fluctuations in Bi2Sr2Ca1-xYxCu2O8 and nematic fluctuations in FeTe1-xSex.

We present a study of the charge order of 214 stripe ordered superconducting cuprates (La1.6-xNd0.4)SrxCuO4 and La2-xBaxCuO4 for doping levels 0.11≤p≤0.14 by means of resonant x-ray scattering. Up to 6 T, we find no field dependence on either the integrated intensity or the correlation length of the charge modulations, providing evidence for strong stability of charge order under applied fields. The magnetic field data support a strong pinning scenario induced by the low-temperature tetragonal distortion and static disorder, and they highlight the role of the symmetry of the lattice on the stabilization of electronic periodicities.

A recent progress of reduction process for electron-doped cuprates enabled us to get superconducting samples at very low doping levels. In order to clarify the electronic state of strongly reduced Pr1.3−xLa0.7CexCuO4+δ (x = 0.05, 0.10) which exhibit high Tc (∼27 K) superconductivity, we have measured their optical spectra. The reflectivity of these samples was found much higher than the published data for the moderately reduced and non-superconducting samples with the same Ce concentrations. Moreover, the estimated effective electron numbers Neff for x = 0.05 and 0.10 were close to that of the optimally doped and superconducting sample with x = 0.15. Given that the parent compound is a Mott insulator, these results indicate that in the electron-doped cuprates only a small amount of carrier doping changes the system to a high Tc superconductor with a large Fermi surface. At low temperatures, a broad mid-infrared peak appeared even in the superconducting samples.

We performed Cu-63,Cu-65 and La-139 NMR measurements of T'-La1.8Eu0.2CuO4+delta (T'-LECO) with the Nd2CuO4-type structure (so-called T'-structure). As a result, we detected the Cu-63,Cu-65 NMR signal under finite magnetic fields and found superconductivity without antiferromagnetic (AF) order only in the reduced T'-LECO, where excess apical oxygen atoms are properly removed. This indicates that the intrinsic ground state of the ideal T'-LECO is a paramagnetic and superconducting (SC) state. Below T-c, the Knight shift was found to rapidly decrease, which indicates the emergence of bulk superconductivity due to spin-singlet Cooper pairs in the reduced T'-LECO. In the SC state of the reduced T'-LECO, moreover, a characteristic temperature dependence of the spin-lattice relaxation rate 1/T-1 was observed, which implies the existence of nodal lines in the SC gap. These findings suggest that the superconductivity in the reduced T'-LECO probably has d-wave symmetry. In the normal state of the reduced T'-LECO, on the other hand, AF fluctuations were found to exist from the temperature dependence of 1/T1T, though no clear pseudogap behavior was observed. This suggests that the AF correlation plays a key role in the superconductivity of undoped high-T-c cuprate superconductors with the T'-structure. (C) 2017 Elsevier B.V. All rights reserved.

High-pressure neutron powder diffraction, muon-spin rotation, and magnetization studies of the structural, magnetic, and the superconducting properties of the Ce-underdoped superconducting (SC) electron-doped cuprate system with the Nd2CuO4 (the so-called T') structure T'-Pr1.3-xLa0.7CexCuO4 with x = 0.1 are reported. Astrong reduction of the in-plane and out-of-plane lattice constants is observed under pressure. However, no indication of any pressure-induced phase transition from T' to the K2NiF4 (the so-called T) structure is observed up to the maximum applied pressure of p = 11 GPa. Large and nonlinear increase of the short-range magnetic order temperature T-so in T'-Pr1.3-xLa0.7CexCuO4 (x = 0.1) was observed under pressure. Simultaneous pressure causes a nonlinear decrease of the SC transition temperature T-c. All these experiments establish the short-range magnetic order as an intrinsic and competing phase in SC T'-Pr1.3-xLa0.7CexCuO4 (x = 0.1). The observed pressure effects may be interpreted in terms of the improved nesting conditions through the reduction of the in-plane and out-of-plane lattice constants upon hydrostatic pressure.

We report the effect of protected vacuum annealing on the superconducting (SC) and magnetic properties of Pr1-xLaCexCuO4 (PLCCO) single crystals in the overdoped regime (x = 0.13 -0.20). The powders and single crystals of PLCCO with all x were synthesized by the solid state reaction and grown by the travelling solvent floating zone (TSFZ) method, respectively. The powder x-ray diffraction (XRD) results analyzed by the Rietveld method have shown that there is a systematic decrease in the c-axis length with increasing x, suggesting the proper substitution of Ce for Pr in overdoped PLCCO up to x = 0.20. The socalled two-step annealing performed in the overdoped regime was essential for increasing the SC volume fraction estimated from the magnetic susceptibility measurements. Furthermore, the temperature dependence of the normal-state magnetic susceptibility for reduced crystals of PLCCO with all x has exhibited a Curie-Weiss-like behavior regardless of various steps of the protected vacuum annealing process.

To investigate the relationship between the so-called stripe correlations and the high-<italic>T</italic>_csuperconductivity, we have carried out magnetic susceptibility and zero-field muon spin relaxation measurements in partially Fe-substituted La₂₋<italic>_x</italic>Sr<italic>_x</italic>Cu₁₋<italic>_y</italic>Fe<italic>_y</italic>O₄(LSCFO). It has been found that the Fe substitution induces successive magnetic transitions in the overdoped regime: a spin-glass transition of Fe³⁺spins due to the Ruderman–Kittel–Kasuya–Yosida interaction based on itinerant spins at high temperatures and a stripe-order transition of localized Cu²⁺spins at low temperatures. The stripe-order transition disappears at the hole concentration per Cu of ∼0.28 which coincides with the endpoint of the superconductivity in pristine La₂₋<italic>_x</italic>Sr<italic>_x</italic>CuO₄, suggesting that the stripe correlations are closely related to the high-<italic>T</italic>_csuperconductivity in the overdoped regime as well as in the underdoped regime. For the spin-glass state, it has been found that the contribution of polarized itinerant spins to the muon-spin depolarization is not negligible. Taking into account neutron-scattering results obtained by He et al. [<ext-link ext-link-type="uri" xlink:href="http://doi.org/10.1103/PhysRevLett.107.127002" xlink:type="simple">Phys. Rev. Lett. <bold>107</bold>, 127002 (2011)</ext-link>] that a spin density wave (SDW) order has been observed in overdoped LSCFO, the present results demonstrate the presence of a novel coexistent state of an SDW order of itinerant spins and a spin-glass state of Fe³⁺spins.

To investigate the electronic state of Ce-free and Ce-underdoped high-<italic>T</italic>_ccuprates with the so-called T′ structure, we have performed muon spin relaxation (μSR) and specific heat measurements of Ce-free T′-La_1.8Eu_0.2CuO_4+δ(T′-LECO) polycrystals and Ce-underdoped T′-Pr_1.3−<italic>_x</italic>La_0.7Ce<italic>_x</italic>CuO_4+δ(T′-PLCCO) single crystals with <italic>x</italic>= 0.10. The μSR spectra of the reduced superconducting samples of both T′-LECO with <italic>T</italic>_c= 15 K and T′-PLCCO with <italic>x</italic>= 0.10 and <italic>T</italic>_c= 27 K have revealed that a short-range magnetic order coexists with the superconductivity in the ground state. The formation of a short-range magnetic order due to a very small amount of excess oxygen in the reduced superconducting samples strongly suggests that the Ce-free and Ce-underdoped T′-cuprates are strongly correlated electron systems.

It is inferred from bulk-sensitive muon Knight shift measurement for a Bi1.76Pb0.35Sr1.89CuO6+delta single-layer cuprate that the metal-insulator (MI) transition (in the low-temperature limit, T -&gt; 0) occurs at the critical hole concentration P = PMI = 0.09(1), where the electronic density of states (DOS) at the Fermi level is reduced to zero by the pseudogap irrespective of the Neel order or spin glass magnetism. Superconductivity also appears for P &gt; PMI, suggesting that this feature is controlled by the MI transition. More interestingly, the magnitude of the DOS reduction induced by the pseudogap remains unchanged over a wide doping range (0.1 &lt;= p &lt;= 0.2), indicating that the pseudogap remains as a hallmark of the MI transition for P &gt; PMI.