足立 匡

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.

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.

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.