Taichiro Nishio

Type-1.5 Superconductivity

We demonstrate the existence of a novel superconducting state in high quality two-component MgB2 single crystalline superconductors where a unique combination of both type-1 (lambda{1}/xi{1}<1/sqrt[2]) and type-2 (lambda{2}/xi{2}>1/sqrt[2]) superconductor conditions is realized for the two components of the order parameter. This condition leads to a vortex-vortex interaction attractive at long distances and repulsive at short distances, which stabilizes unconventional stripe- and gossamerlike vortex patterns that we have visualized in this type-1.5 superconductor using Bitter decoration and also reproduced in numerical simulations.

Scanning SQUID microscopy of vortex clusters in multiband superconductors

In type-1.5 superconductors, vortices emerge in clusters, which grow in size with increasing magnetic field. These novel vortex clusters and their field dependence are directly visualized by scanning SQUID microscopy at very low vortex densities in MgB2 single crystals. Our observations are elucidated by simulations based on a two-gap Ginzburg-Landau theory in the type-1.5 regime.

Giant vortices, rings of vortices, and reentrant behavior in type-1.5 superconductors

We predict that in a bulk type-1.5 superconductor, the competing magnetic responses of the two components of the order parameter can result in group-stabilized giant vortices and individual rings of vortices in the absence of any extrinsic pinning or confinement mechanism. We also determine within the Ginzburg-Landau theory a condition for the robustness of type 1.5 in the vicinity of the critical temperature, and we find a rich phase diagram with successions of behaviors such as type 1{yields} type 1.5{yields} type 2{yields} type 1.5 when temperature decreases.

Vortex imaging in microscopic superconductors with a scanning SQUID microscope

Direct observation of vortex arrangement in microscopic superconductors is achieved with a scanning SQUID microscope. Newly 2 /spl mu/m diameter sensing pickup coil is developed for the observation of giant vortex state. Vortex arrangements in 10 /spl mu/m triangles is investigated with the new pickup device. No giant vortex state, however, can be seen under this experimental condition. Further investigation is needed.