Kenji Kurahashi

Heat Treatment Effects on the Superconductivity and Crystal Structure of Nd1.85Ce0.15CuO4 Studied Using a Single Crystal

We performed systematic post-growth heat treatments on sizable single crystals of Nd 1.85 Ce 0.15 CuO 4 . The superconducting transition temperature, T c , and its distribution in the sample are both sensitive to details of the heat treatment and sample size. With refinement of the heat treatment conditions we have succeeded in preparing two distinct superconducting samples in which T c equals 18 K and 25 K under identical Ce-concentrations. Neutron scattering experiments initially observed nonmagnetic superstructure peaks at (π,π) only in the reduced superconducting phase. Atomic displacement in this structure is different from that induced by in-plane rotation of CuO 4 squares as previously reported for Gd 2- x Ce x CuO 4 . A possible relationship between atomic displacements and superconductivity and the role of reduced heat treatments are discussed from the viewpoint of carrier localization.

Growth of large single crystals using the improved lamp-image floating-zone furnace: application to

We grew large single crystals of reproducibly over a wide doping range using an improved travelling solvent floating zone (TSFZ) method. We performed a simulation on the lamp-image focusing to find an optimum condition for a steep temperature gradient along the rod. Based on the results of simulation, we adapted smaller lamps and shielded the partial area of the mirror surfaces. The improvement of the focusing had a great effect on keeping the molten zone stable for more than 100 h. Stabilization of the molten zone improves both the reproducibility of the crystal growth and the quality of the grown crystals.

Anisotropy of the Superconducting Order Parameter of Nd 2- xCe xCuO 4 Studied by STM/STS

STM/STS studies have been carried out on single crystals of the electron-doped high-T c oxide Nd 2-x Ce x CuO 4 (x ∼ 0.15, T c ∼ 20.5 K) to clarify the symmetry of its superconducting order parameter Δ. The observed gap structure of the quasi-particle excitation is found to depend on the distance between the tip and the sample surface and its characteristics indicate that the in-plane anisotropy of Δ is basically similar to that reported previously for hole-doped high-T c oxides: It is, at first sight, xy-like but can be explained by the (x 2 - y 2 ) symmetry by applying a model proposed previously [J. Phys. Soc. Jpn. 64 (1995) 1476], which takes account of the electron wave vector dependence of the tunneling probability. The zero bias conductance peak has also been observed.STM/STS studies have been carried out on single crystals of the electron-doped high- T c oxide Nd 2- x Ce x CuO 4 ( x ∼0.15, T c ∼20.5 K) to clarify the symmetry of its superconducting order parameter Δ. The observed gap structure of the quasi-particle excitation is found to depend on the distance between the tip and the sample surface and its characteristics indicate that the in-plane anisotropy of Δ is basically similar to that reported previously for hole-doped high- T c oxides: It is, at first sight, x y -like but can be explained by the ( x 2 - y 2 ) symmetry by applying a model proposed previously [J. Phys. Soc. Jpn. 64 (1995) 1476], which takes account of the electron wave vector dependence of the tunneling probability. The zero bias conductance peak has also been observed.

Coexistence of antiferromagnetic ordering and high-Tc superconductivity in electron-doped superconductor Nd2−xCexCuO4

Abstract Muon spin relaxation and magnetic susceptibility measurements were performed for the electron-doped superconductor Nd2−xCexCuO4 (NCCO) with a particular attention to the vicinity of the boundary in phase diagram between the antiferromagnetic (AF) and the superconducting phases. The obtained phase diagram clearly exhibits a competitive change in the volume fraction between the two phases around the phase boundary; a dramatic decrease of the AF phase and a sudden appearance of bulk superconductivity upon doping beyond x∼0.13. However, the spatial coexistence of both phases is also suggested around the boundary. In contrast to the hole-doped system, the onset of Tc is approximately independent on the doping in the underdoped region of NCCO.

Intraparticle structure in ultra-fine ZnFe2O4 particles

Abstract We have performed small-angle polarized-neutron scattering from ultra-fine ZnFe 2 O 4 particles to confirm the intraparticle magnetic structure, which has been considered the origin of its peculiar magnetic features. On the basis of the Q -dependent differential neutron-scattering intensity, we were unable to detect any magnetic structure consisting of magnetically inactive surface layer with magnetic core. Thus, the magnetic behavior of ultra-fine ZnFe 2 O 4 particles is explained in terms of a deviation in cation distribution from the normal spinel structure.

Electron-doping effect on magnetic order and superconductivity in Nd2−xCexCuO4 single crystals

Abstract Doping dependences of magnetic properties such as the Neel temperature, staggered moment of Cu2+ spin and antiferromagnetic coherence length in the electron-doped Nd2−xCexCuO4 have been investigated using elastic neutron scattering measurement. Near the antiferromagnetic–superconducting phase boundary for the heat-treated samples antiferromagnetism remarkably degrades upon electron-doping. The effect of heat treatment on the Neel temperature is clearly seen when x exceeds ∼0.10 where fractional superconductivity appears by the heat treatment.

Spatial modulation of low-frequency spin fluctuations in hole-doped La2CuO4

Systematic neutron scattering measurements have been performed on the Sr-doped La{2-x}SrxCuO4 to study the doping dependence of spatially modulated dynamical spin correlations or so-called incommensurate spin fluctuations. The modulated spin correlations appear beyond x ≈ 0.05 which is close to the lower boundary of the superconducting phase. First evidence was observed for the linear relation between the degree of spatial modulation or the incommensurability Σ(x) and the maximumTc atx. We present a universal curve for Σ(x) by adding data from other La2CuO2 systems such as oxygen-doped superconductors, oxygen-reduced or Zn-substituted nonsuperconductors, and La1.6-xNd0.4SrxCuO4.

X-ray and Electron Diffraction Study of a Precipitation Phase in Nd1.85Ce0.15CuO4+y Single Crystal

X-ray diffraction measurements have been performed to investigate a crystal structure of a precipitation phase in oxygen-reduced Nd 1.85 Ce 0.15 CuO 4+ y (NCCO). A detailed structure analysis of the precipitation phase as well as the parent NCCO phase revealed that the precipitation phase is a cubic (Nd,Ce) 2 O 3 , which is distorted tetragonally by the surrounding NCCO phase. The averaged grain size of the precipitation phase exceeds 200 A parallel to the CuO 2 plane of the parent NCCO, while the size along layered direction is short-ranged (∼30 A). The grain thickness along layered direction is strongly correlated with the homogeneity of superconducting state. The volume fraction of the (Nd,Ce) 2 O 3 phase is about 2%. We also performed electron diffraction as well as microscopy experiment, which confirms that the (Nd,Ce) 2 O 3 phase segregate from the parent NCCO phase and exists as an epitaxial layer along the c -axis.

Nondestructive Analysis and Control of Doping Rate in La2-x(Bi,Sr)xCuO4.+-.y.

Systematic heat treatments of single crystals of La 2- x Bi x CuO 4+ y ( x =0.05) and La 2- x Sr x CuO 4- y ( x =0.15) combined with accurate measurements of the sample weight demonstrated that the doping rate can be controlled and determined without decomposing the sample. For the former, the absolute doping rate 2 y was determined in the region of 0≤2 y ≤0.020 and the detailed doping dependence of the Neel temperature was obtained. For the latter, the effective doping x eff ≡ x -2 y can be reduced continuously from 0.15 to around 0.07 by deoxygenation. It is confirmed that the superconductivity was destroyed at x eff ≈0.10 irrespective of the details of the heat treatments.

Superconducting anisotropy in Nd1.85Ce0.15CuO4 single crystals

Abstract The magnetic torque of the Nd 1.85 Ce 0.15 CuO 4 crystal consists of three contributions in the superconducting state, i.e., (1) a torque of the anisotropic magnetic moment of the Nd 3+ ions, (2) a torque coming from the mass difference between m ab and m c and (3) a torque of the vortex lock-in states appeared when the field is applied parallel to the CuO 2 planes. We analyze a torque component arising from the mass anisotropy by using the anisotropic London model and obtain an anisotropy parameter γ c =28±3. The irreversibility line of Nd 1.85 Ce 0.15 CuO 4 is in excellent accordance with that of the La-based cuprate.