Masafumi Furuyama

Crystal Structure of Magnetic Superconductor FeSr2YCu2O6+δ

The FeSr 2 YCu 2 O 6+δ compound with a Ba 2 YCu 3 O 6+δ -type structure exhibits superconductivity around 50 K, only when it is properly annealed in N 2 atmosphere and subsequently in O 2 atmosphere. The compound without N 2 -annealing, however, does not exhibit superconductivity. This compound consists of a sequence of alternating CuO 2 double sheets with nanometer-scale separation as those in high- T c superconductors, and FeO δ sheets similar to that in the SrFeO 3-δ magnetic compound. Neutron diffraction study shows that both N 2 -annealing and subsequent O 2 -annealing cause ordering of Cu and Fe, and that both O 2 -annealing and subsequent oxidization under high-O 2 pressure supply the charge. However, the charge was distributed not only to the CuO 2 sheet but also the FeO δ sheet. The bond valence sum calculation indicates that charges distributed to the FeO δ sheet more than to the CuO 2 sheet in the superconducting sample.

Ambient-pressure synthesis of single-crystal MgB2 and their superconducting anisotropy

We synthesized single crystalline and policrystalline MgB_2 under ambient pressures. The single crystals of MgB_2 were of good quality, where the crystal structure refinements were successfully converged with R = 0.020. The specific heat of policrystalline MgB_2 samples has been measured in a temperature range between 2 and 60 K in magnetic field up to 6 T. The measurement gave the coefficient of the linear term in the electronic specific heat, gamma = 3.51 mJ/K^2 mol, and the jump of the specific heat, 2.8 mJ/K^2 mol at 38.5 K. It is shown from the analysis of the specific heat that the electronic specific heat in the superconducting state differs largely from the conventional BCS weak coupling theory. From the results of measurements of the magnetic properties on single crystal samples, we found a sharp superconducting transition at 38 K with transition width Delta Tc = 0.8 K and the superconducting anisotropy ratio gamma increasing from about 1 near Tc to 4.0 at 25 K.

Ferromagnetism and superconductivity in RuSr2RCu2O8 (R=Sm, Eu, Gd)

Abstract Polycrystalline single phase RuSr 2 RCu 2 O 8 (R=Sm, Eu, Gd) samples have been synthesized by a new solid state reaction. We found superconductivity in two rare earths R=Sm and Eu with T s(R=0) =12 and 17 K, whereas ferromagnetism occurs at T c =146 and 139 K in these samples, respectively, in addition to the previously reported RuSr 2 GdCu 2 O 8 with T s =36 K and T c =136 K. Huge resistivity broadening phenomena common to the high T c superconductors were observed in magnetic fields, which suggest that strong enhancement of superconducting fluctuations due to magnetic field, suppresses the occurrence of superconducting phase coherence in these compounds.

Microwave response of YBaCuO/MgO/Pb planar Josephson tunnel junctions

Abstract Microwave response of YBaCuO (YBCO) /MgO/Pb planar Josephson tunnel junctions fabricated by an in situ deposition technique is presented. The observed Josephson critical current is 20–400μA.The current-voltage characterisitcs under microwave irradiation of 9GHz yield the Shapiro steps at integer multiples of hf/2e where f is the microwave frequency. Dependence of the Shapiro step height on the microwave voltage amplitude is consistent with that of the conventional theory. A series of subharmonic steps at half integer multiples of hf/2e are also observable, although their structures are weak.

Neutron Powder Diffraction Study on Mg11B2 Synthesized by Different Procedures

The crystal structure of a new superconducting material, MgB 2 , was studied by high-resolution neutron powder diffraction as a function of temperature from 8 K to 305 K. Two samples of Mg 11 B 2 were measured; one was synthesized for 2 d at 1050°C, and the other one was done for 1 d at 1100°C. All of the diffraction data were precisely refined by the Rietveld method, and we confirmed that there was no clear anomaly on the temperature dependence of the lattice constants around T c for both samples. The difference in sample synthesis procedures affects the profile shapes of the 00l reflections, whereas the h h 0 lines are almost the same.

Interaction between superconductivity and magnetic moments in RBa2(Cu1−xZnx)3O7−δ, (R=Gd, Dy and Y)

Low-temperature electrical resistances of Zn-doped oxide superconductors, RBa2(Cu1−xZnx)3O7−δ (R=Gd, Dy and Y) have been measured. The anomalous temperature dependence of resistance are observed in the samples with R=Gd and Dy but not Y. The temperature dependence of the susceptibility on the sample with R=Gd shows the antiferromagnetic ordering and this ordering temperature agrees with the temperature which the anomalous resistance appeared. These facts indicate the existence of the interaction between the conduction electrons and the local magnetic moments.

Tunneling measurements on the Y-Ba-Cu-O thin film junctions with artificial barrier

In-situ fabrication and measurements of tunnel junctions using a Y-Ba-Cu-O (YBCO) thin film, an artificial MgO tunnel barrier and a counterelectrode of Pb or Au are reported. These junctions show the gap opening structure at about 20mV, the zero bias anomaly and the asymmetry in the conductance, reproducibly. The abrupt change of the conductance appears at the superconducting transition temperature of YBCO film, which suggest the gap opening in the superconducting YBCO film.

Substitutional effects of 3d elements for Cu in YBa2Cu4O8

Abstract Structural and superconducting properties of YBa 2 (Cu 1−x M x ) 4 O 8 for M=Fe, Ni and Zn in 0≦ x≦ 0.06 have been investigated. The samples were prepared in oxygen at a pressure lower than an atmospheric pressure. The crystal structure was orthorombic and no structural phase transition was observed. The depression of the superconducting transition temperature, T c , was caused by the substitution for all samples. The substitution by the nonmagnetic Zn depressed T c larger than that by both the magnetic Fe and Ni. The depression of T c in 1-2-4 compounds was much larger than that of T c in 1-2-3 compounds.