[en] It is intriguing to investigate the behavior of vortices in multiple connected superconductors. A sample is a superconducting network made by the vacuum evaporating of Pb on a copper substrate. The substrate has a regular array of square lattice holes. The sample magnetization has been examined by a SQUID magnetometer under a small field regime. We found many periodic matching peaks at H=nH_phi. At T=7.23 K, the magnetization peak has the same polarity against the field reversal. These anomalous matching peaks cannot be understood by the extended Little-Parks effect. The vortex-vortex interaction becomes attractive in a low Ginzburg-Landau parameter κ (∼0.46) superconductor. We consider that the magnetization of a polarized vortex domain caused by the attractive interaction is responsible for the anomalous matching peak

[en] We have developed a SQUID microscope using a high-T_c SQUID sensor. The SQUID sensor has a sensitivity of 1 pT and the effective sensor area is 0.08 mm². An X-Y stage moves over 30x30 mm² area and its resolution is 0.05 μm at most. The SQUID sensor is cooled by liquid nitrogen, and can be held at 77 K for 1 h. The distance between the sensor and a sapphire window is less than 100 μm. The sample temperature is limited to room temperature. The device is controlled by a LabVIEW-6i software and a motion controller. The performance of the SQUID microscope is examined by a tiny Ni rod

[en] We prepared a sample of a square microhole lattice by evaporating a type-I superconductor lead on a copper micromesh sieve with 16.9-μm pitch. We explore the superconducting properties using a superconducting quantum interference device magnetization measurement. We found the anomalous matching effect in the magnetization curve where the matching peaks have the same polarity in both positive and negative field branches. The positive magnetization appears upon field cooling either in positive or negative field. The magnetization appears even upon cooling in zero fields. This might be a manifestation of spontaneous magnetization like a ferromagnetic substance. It was pointed out that the vortex current reversal occurs for a superconductor of a Ginzburg-Landau parameter κ∼1. This is the origin of the attractive vortex-vortex interaction. We consider that the attractive interaction plays a major role in forming spontaneous vortex domains

[en] Electron spin resonance (ESR) experiments with 15 GHz microwave have been done between 1.5 and 300 K for (AsMe₄)₂DMTCNQ₃ single crystals, where Me and DMTCNQ stand for methyl and dimethyl-tetracyano-quinodimethane, respectively. The crystal has one-dimensional structure in which the DMTCNQ molecules stack along the b-axis. Two kinds of resonance line are found. Although the main resonance line is observed in whole measured temperature range, the other resonance line appears only above 225 K. The magnetic field direction dependence of the line width for main one follows the dipole calculation which reflects one-dimensional character of localized spin. However that for the other one does not show the one-dimensional localized spin character. The results suggest the coexistence of localized spins and carrier electrons above 225 K

[en] Vortex lattice melting in the Hperpendicularc configuration of an YBa₂Cu₃O_6.94 single crystal has been investigated by means of the ac susceptibility χprime-iχdouble_prime and the magnetic torque τ. The melting transition of vortex lattice occurs in Hperpendicularc, too. Since the torque curve shows a sharp peak in the irreversible torque at θ_c ≅ 90degree due to intrinsic pinning at lower temperatures, the authors can determine the irreversibility line for the intrinsic pinning. The melting transition in the Hperpendicularc configuration appears at temperatures where the intrinsic-pinning peak is absent. They consider that the intrinsic pinning does not affect the nature of the vortex melting transition in the Hperpendicularc configuration

[en] A simple sample-inverting cryostat with no mechanical gears and motors has been introduced to a superconducting magnet up to 17 T combined with a variable temperature insert from 1.5 to 300 K for the Hall resistance measurements. An elliptical sample stage surrounded by a coil in 100 turns is turned upwards or downwards by switching the current to the coil under a magnetic field. The direction of the stage is controlled by the minimum current from 100 mA at 0.2 T to 1 mA at 15 T

[en] Electromagnetic response of CuIr₂S₄ at 34 GHz has been measured by a cavity perturbation method. Clear anomalies appear in the temperature dependences of the resonant frequency and the full-width at half-maximum of the Q-curves at the metal-insulator phase transition. These anomalies of the microwave responses are very suggestive of a simultaneously charge-ordering phase transition with the metal-insulator phase transition of CuIr₂S₄

[en] We examine the superconducting anisotropy γ=√m_c/m_ab of a metallic high-T_c superconductor MgB₂ by measuring the magnetic torque of a single crystal. The anisotropy γ does not depend sensitively on the applied magnetic field at 10 K. We obtain the anisotropy parameter γ=4.31±0.14. The torque curve shows the sharp hysteresis peak when the field is applied parallel to the boron layers. We consider that this comes from the intrinsic pinning and is direct evidence for the occurrence of superconductivity in the boron layers. This is consistent with what the band calculations predict in the electronic states

[en] We have measured the magnetic torque of an MgB₂ single crystal in the various different fields below 10 kG by using a torque magnetometer and a 4 K closed cycle refrigerator. The MgB₂ single crystal was synthesized by the vapor transport method. The torque can be measured as an off-balance signal of the Wheatstone bridge of the four piezoresistors on a Si cantilever. The torque curves are analyzed by the Kogan model. The superconducting anisotropy γ is rather independent of temperature in 5 and 10 kG, but is dependent on field up to 60 kG. We consider that the field dependence of γ comes from the nature of the multiple superconducting gaps. The experimental results show that the π-band superconducting gaps have been deteriorated gradually up to a crossover field H^*(π) ∼ 20 kG at 10 K when the magnetic field increases

[en] Half vortices spontaneously emerge in corners of d-wave superconductor dot embedded in s-wave superconducting matrix due to the phase interference between superconducting order parameters with different symmetries. Since these half vortices are robust to external perturbations, high potentialities toward novel device applications are expected. In order to clarify quantum features of these half vortices we perform numerical calculations for the electronic structure of d-wave superconductor dot embedded in s-wave superconducting matrix. Consequently, we find that discrete levels appear inside the superconducting gap of the s-wave superconducting matrix due to the coupling between the core states of half vortices and the localized quantized states inside d-dot. These isolated levels may be favorable for the realization of qubits