Yoshihiro Koike

High critical current density YBa2Cu3O7−δ films on surface-oxidized metallic substrates

Abstract YBa 2 Cu 3 O 7−δ (YBCO) films with high critical current density ( J c ) were fabricated on nickel tapes buffered with bi-axially textured NiO prepared by surface-oxidation epitaxy (SOE). The effects of oxide cap layers, such as YSZ, CeO 2 and MgO, on the SOE-grown NiO were investigated to improve the superconducting properties of the YBCO films on NiO. By inserting a thin MgO cap layer between NiO layer and YBCO film, a J c of 3×10 5 A/cm 2 (77 K, 0 T) was achieved. This result indicates the potentiality of the SOE method. In this paper, the application of the NiO/Ni substrate to non-vacuum processings, such as metal-organic deposition (MOD) and liquid phase epitaxy (LPE) will be also introduced.

Magnetic properties of a pressure-induced superconductor UGe2

We performed DC-magnetization and neutron scattering experiments under pressure P for a pressure-induced superconductor UGe 2 . We found that the magnetic moment is enhanced at a characteristic temperature T * in the ferromagnetic state, where T * is smaller than a Curie temperature T C . This enhancement becomes remarkable in the vicinity of P C * =1.20 GPa, where T * becomes 0 K and the superconducting transition temperature T SC shows a maximum. The characteristic temperature T * , which decreases with increasing pressure, also depends on the magnetic field.

Neutron-diffraction study of the unusual ordered phases of Ce0.75La0.25B6

Abstract Ce0.75La0.25B6 shows paramagnetic phase (phase I), 4f-electron quadrupolar ordered phase (II), magnetically ordered phase (III), and an unknown phase IV. In order to investigate order parameters in phases III and IV, neutron diffraction experiments were performed down to 74 mK . Magnetic diffraction pattern measured in phase III is reproduced by a model in which spin arrangement is modified from the previously suggested structure for CeB6. No magnetic reflections were observed in phase IV. Thus, the order parameter in phase IV is expected to be 4f-electron multipoles rather than magnetic dipoles.

Magnetization plateaux of the S = 1/2 two-dimensional frustrated antiferromagnet Cs2CuBr4

The field induced magnetic phase transitions of Cs2CuBr4 were investigated by means of a magnetization process and neutron scattering experiments. This system undergoes a magnetic phase transition at Neel temperature TN = 1.4 K at zero field, and exhibits a magnetization plateau at approximately one third of the saturation magnetization for the field directions and . In the present study, an additional symptom of the two-third magnetization plateau was found in the field derivative of the magnetization process. The magnetic structure was found to be incommensurate with the ordering vector Q = (0,0.575,0) at zero field. With increasing magnetic field parallel to the c-axis, the ordering vector increases continuously and is locked at Q = (0,0.662,0) in the plateau field range 13.1 T< H<14.4 T. This indicates that the collinear up–up–down spin structure is stabilized by quantum fluctuation at the magnetization plateau.

Magnetization dynamics in the normal and superconducting phases of UPd2Al3: I. Surveys in reciprocal space using neutron inelastic scattering

This topical review presents results from neutron inelastic scattering experiments on single crystals of UPd2Al3. The focus is on the experimental situation, while the sequel paper advances theoretical perspectives. We present a detailed and complete characterization of the wavevector- and energy-dependent magnetization dynamics in UPd2Al3 as measured by neutron inelastic scattering primarily in the form of extensive surveys in energy–momentum space under a wide range of experimental conditions, and put our observations in context with data that has been published previously by two independent groups. In this way we emphasize the robustness of the results, which indicate the intricate nature of the dynamic magnetic susceptibility of this material. This study yields unique insight into the low-temperature ground state, which exhibits a microscopic coexistence of antiferromagnetism and superconductivity, making UPd2Al3 one of the most accessible heavy-fermion superconductors that can be fully characterized by neutron spectroscopy.

Field-Induced Phase Transitions Driven by Quantum Fluctuation in S = 1/2 Anisotropic Triangular Antiferromagnet Cs2CuBr4

The field induced magnetic phase transitions of Cs 2 CuBr 4 were investigated by means of magnetization process and neutron scattering experiments. Cs 2 CuBr 4 should be characterized as S = 1/2 two-dimensional triangular antiferromagnet. Below the ordering temperature T N = 1.4 K, the Spin structure is the helical incommensurate structure almost within the triangular lattice plane. In the field direction within the triangular lattice plane, Cs 2 CuBr 4 exhibits the magnetization plateaux at one-third and two-thirds of the saturation magnetization. The spin structure in the one-third plateau phase is found to be almost collinear up-up-down structure which should be stabilized by quantum fluctuation as predicted by the theoretical studies.

MAGNETIC PROPERTIES OF R3PD20GE6 (R = PR AND ND)

Abstract We have succeeded in growing single crystals of new ternary compounds R 3 Pd 20 Ge 6 (R=Pr and Nd). The magnetic properties of R 3 Pd 20 Ge 6 (R=Pr and Nd) can be basically understood on the basis of the localized 4f-electron model. The magnetic phase diagram of Nd 3 Pd 20 Ge 6 consists of at least two antiferromagnetic phases II and III. The antiferromagnetic orderings of Nd 3 Pd 20 Ge 6 occur on the two R sites independently.

LPE growth of YBCO on silver substrates

The YBCO thick films were formed by LPE on the polycrystalline silver substrates, silver (110) single crystalline substrates and {100} cube textured silver tapes. The films on these substrates were grown at such a low growth temperature as 867 °C. For the polycrystalline silver substrates, c-axis oriented YBCO films were observed on particular grains which have specific orientation. On the silver (110) single crystalline substrates, epitaxially grown YBCO covered all over the surface. Critical temperature of 90 K was measured by SQUID but resitivity drop was also observed at 40 K with the measurement current density of ∼104A / cm2. Epitaxially grown YBCO were observed all over the surface of (100) |001]| cube textured silver tapes and thickness of YBCO film is about 2 μ m.

Neutron scattering study of the correlation of magnetism and superconductivity in heavy-fermion superconductor UPd2Al3

Abstract Neutron scattering experiments have been carried out in order to study the interplay between magnetism and superconductivity in the heavy-fermion superconductor UPd2Al3. We have observed 1% suppression of the (0 0 0.5) magnetic peak intensity below the critical temperature Tc. It is direct evidence for the coupling of the magnetic order with the superconducting order parameters. Spin excitation gap associated with superconductivity is observed for the first time in a heavy-fermion superconductor.

Carbon incorporated YBa2Cu3O7−d crystal; growth, phase separation and enhancement of irreversibility field

Abstract Carbon incorporated YBa2Cu3O7−d (YBCO) crystalline films have been grown from a high temperature solution of BaOCuO containing barium carbonate. Superconducting properties of YBCO crystal containing carbon have been measured. Transition temperature significantly decreases from about 90K both in transport and magnetic measurements when the carbon content exceeds 0.06wt%. Hysteresis loop has a peak at 1–2T for the sample with the carbon concentration of 0.045wt%. The irreversibility field increases as the carbon concentration decrease.