S. Okuma

First observation of an antiferromagnetic phase in the Y1Ba2Cu3Ox system

The magnetic properties of four different phases of the YBa2Cu3Ox system, including the famous 90 K superconductor, have been probed by the positive muon spin relaxation method (µ+SR). The oxygen-deficient tetragonal insulating phase (x~6.2) was found to be magnetically ordered, thus most likely antiferromagnetic, near room temperature.

Superconductor-insulator transition driven by magnetic-field and disorder in two-dimensional MoxSi1−x films

Abstract We have made systematic measurements for the field-swept superconductor-insulator (SI) transition over a wide range of disorder through the disorder-driven SI transition using very thin amorphous MoxSi1−x films with a fixed thickness of 4 nm. Critical resistances determined from both the field-driven and disorder-driven SI transitions are found to be nonuniversal and apparently smaller than R Q ∼ 6.5 k Ω .

μ+SR Studies of Magnetic Properties of the YBa2Cu3Ox System

Samples of YBa 2 Cu 3 O x in four different phases have been probed by positive muon spin relaxation (µ + SR) method. Each phase was found to show different magnetic behavior. In the famous orthorhombic 90K superconductor ( x ∼6.9), neither magnetic ordering nor magnetic fluctuations were observed. However, in another orthorhombic superconducting phase ( x ∼6.4, T c ≈60 K) a magnetic fluctuation was observed below 7 K via muon spin-nuclear spin double relaxation. Its magnetic ordering temperature was expected to be below 2.4 K. Similar behavior was observed in an oxygen-rich ( x ∼6.5) tetragonal insulating phase, whereas the oxygen-deficient ( x ∼6.2) tetragonal insulating phase was found to be magnetically ordered, most likely antiferromagnetic, near room temperature. This is the first observation of a magnetically ordered state in the YBa 2 Cu 3 O x system.

Observation of antiferromagnetic ordering in Bi2Sr2YCu2Oy above room temperature by the μSR method

Abstract We have observed a long-range magnetic ordering (probably an antiferromagnetic one) with a transition temperature above 300 K in Bi 2 Sr 2 YCu 2 O y by the μ + SR method. This is the first observation of the antiferromagnetic ordering with a high Neel temperature in the Bi-Sr-Ca-Cu-O system. The Bi 2 Sr 2 YCu 2 O y may be treated as an analogue of the high- T c superconductor-rela ted antiferromagnets such as YBa 2 Cu 3 O 6 or La 2 CuO 4−δ .

Evidence of the Vortex-Glass Transition in Homogeneously Disordered Thick Films of a-MoxSi1-x.

We have performed four-terminal ac complex resistivity measurements in the linear regime, as well as dc resistivity measurements, for thick a -Mo x Si 1- x films in constant magnetic fields ( B = 0.1 and 1 T). The critical behavior associated with the second-order transition has been observed for both dc and ac resistivities. This is the first convincing evidence for the vortex-glass transition (VGT) in the homogeneously disordered low- T C superconductors containing microscopic pinning centers. On the basis of this result, together with our previous finding of VGT in granular In films, we conclude that the VGT is universal in type-II superconductors with moderate disorder independent of whether pinning is microscopic or mesoscopic, or whether they are high- T C or low- T C superconductors.

Internal magnetic fields at μ+ sites and the magnetic phase diagram of Bi2Sr2Y1−xCaxCu2Oy system studied by the μSR method

Abstract The magnetic properties of the Bi 2 Sr 2 Y 1− x Ca x Cu 2 O y system have been studied by positive muon spin rotation or relaxation methods. The magnetic phase diagram has been obtained as a function of Ca content x and temperature T . A long-range magnetic ordering (probably antiferromagnetic) has been observed in Bi 2 Sr 2 Y 1− x Ca x Cu 2 O y with Ca content x ranging from 0.0 to about 0.3. The Neel temperature T N decreases with increasing Ca content; T N is above 300 K for x =0.0, 200–250 K for x =0.2 and drops abruptly to 10–20 K for x =0.3. Superconductivity sets in around the same Ca content ( x ≈0.5) where the magnetic ordering disappears completely. The magnetic phase diagram of the Bi 2 Sr 2 Y 1− x Ca x Cu 2 O y system is thus similar to that of YBa 2 Cu 3 O x or La 2− x A x CuO 4−δ (A=Sr or Ba) and suggests a common origin of the superconductivity in all these oxide superconductors. The temperature dependence of the internal magnetic field at μ + in Bi 2 Sr 2 YCu 2 O y is also discussed.

Superconducting Properties of Disordered Films of Zn

The superconducting transition temperature T c and the critical magnetic field H c of thin Zn films were measured. T c decreases almost linearly as the normal-state sheet resistivity increases. H c ( T ) is enhanced compared with the conventional value for dirty superconductors. These results are explained by the localization and electron-electron interaction theories for two-dimensional disordered superconductors.

Voltage noise of an amorphous MoxSi1-x film near the superconducting transition in magnetic fields

The voltage noise S V ( f ) in a thick amorphous Mo x Si 1- x film is studied over a broad frequency range spanning six decades ( f =0.1 Hz–100 kHz) in magnetic fields just above and below the melting field, as a function of a steady current. The origin of the low-frequency ( f ≈0.1–10 Hz) noise observed near the zero-resistance region is mainly due to temperature fluctuations and the contribution due to flux motion is very small. In contrast, S V ( f ) at higher frequencies ( f ≈10 Hz–100 kHz) appears to be dominated by the flux motion. These results are compared with recent experiments on YBa 2 Cu 3 O 7-δ films.

Scaling of current-resistivity isotherms of indium films in a magnetic field

We have studied the nonlinear resistivity (J-ϱ characteristics) of granular indium films with differennt thickness t and grain size d in various constant magnetic fields B down to 0.6 K. Films 1 and 2 with d=14 nm are viewed as a dirty superconductor while films 3 and 4 with d = 28 nm are modeled as a random Josephson network. The J-ϱ data are analyzed according to the vortex-glass (VG) theory. For film 1 with t = 100 nm, we have obtained the VG transition temperatures Tg(B) and scaling functions, which are similar to those obtained in high-Tc superconductors, suggesting that the VG transition also takes place in a conventional superconductor with sufficiently low Tc (< 4 K). A finite-size effect is visible for thinner film 2 with t=20 nm, where Tg appears to be zero. This is in accord with the two-dimensional (2D) VG theory. The VG behavior is not observed for films 3 (t = 60 nm) and 4 (t = 200 nm) with d = 28 nm. We ascribe it to the fact that the measuring current is too large to probe the film at lengths larger than d = 28 nm.

Dimensionality Crossover of the Nonlinear Resistance in Quench-Condensed Indium Films.

Thickness dependence of the nonlinear resistance ( I - V characteristics) has been measured in quench-condensed films of indium with thicknesses of 14 nm, 26 nm and 65 nm. The I - V curves for the 14 nm-thick film are qualitatively similar to the previous result for the much thinner film (5 nm) whose superconducting properties were well described by the two-dimensional (2D) Kosterlitz-Thouless (KT) theory. In contrast, those for the thicker films are markedly different. As I is raised from zero at low temperatures, there appears the nonlinear I - V curve (resistance) which exhibits negative curvature.