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We revisit the critical penetration of Pearl vortices in narrow superconducting flat rings cooled in magnetic fields. Scanning superconducting quantum interference device microscopy measurements showed how magnetic field penetrates and vortices are trapped in flat rings made of amorphous MoGe thin films. Counting the number of trapped vortices for each image, we found that the vortices are completely excluded from the ring annulus when the applied field $H$ is below a threshold field $H_{\rm{p}}$: Above this field, the vortices increase linearly with field. The obtained values of $H_{\rm{p}}$ depend on the annulus width $w_{\rm{ring}}$ and follow the relation $\mu_0H_{\rm{p}} = (1.9 \pm 0.1) \Phi_0/w_{\rm{ring}}^2$ with the superconducting flux quantum $\Phi_0$. This relationship provides an insight into the effect of the net-current circulating in the annular region, and also leads to a precise control to trap or eliminate vortices in flat rings., Comment: 13 pages, 5 figures, accepted for publication

Direct observation of vortex states confined in mesoscopic regular triangle dots of amorphous MoGe thin films was made with a scanning superconducting quantum interference device microscope. The observed magnetic images illustrate clearly how vortices are distributed over the triangle dots by forming not only commensurate triangular clusters, but also unique patterns imposed by incommensurability. We discuss the results in terms of vortex shells and construct the packing sequence of vortices in the multiple shell structure., Comment: 2 figures, 1 table, 28th International Symposium on Superconductivity(2015)

We report the first realization of columnar defects in Co-doped BaFe$_{2}$As$_{2}$ single crystals by heavy-iron irradiation. The columnar defects are confirmed by transmission electron microscopy and their density is about 40 % of the irradiation dose. Magneto-optical imaging and bulk magnetization measurements reveal that the critical current density is strongly enhanced in the irradiated region. We also find that vortex creep rates are strongly suppressed by the columnar defects. We compare the effect of heavy-ion irradiation into Co-doped BaFe$_{2}$As$_{2}$ and cuprate superconductors., Comment: 5 pages, 4 figures

We have experimentally found related anomalies in electrical resistivity, dc and ac magnetic susceptibility, appearing deeply within ferromagnetically ordered state in SrRuO3. Lack of Jahn-Teller distortion in this regime rules out conventional orbital order, forcing one to describe these in terms of an orbital liquid ground state coexisting with ferromagnetic spin order. We suggest that weak spin-orbit coupling in such an unusual state underpins the observed anomalies., Comment: 4 pages, 3 figures submitted to PRL

We study the Josephson plasma resonance (JPR) in Bi$_2$Sr$_2$CaCu$_2$O$_{8+y}$ (BSCCO) with spatially dependent interlayer-phase coherence (IPC). The half-irradiated BSCCO (HI-BSCCO), in which columnar defects are introduced only in a half of the sample, shows several resonance peaks, which are not simple superposition of the peaks in irradiated- and pristine-parts. JPR in HI-BSCCO changes its character from irradiated- to pristine-type at a crossover frequency ($\omega_{cr}$). We demonstrate that the one-dimensional \LSGE, which takes into account the spatial dependence of IPC, can reproduce most of the experimental findings including the presence of $\omega_{cr}$., Comment: 4 figures

Magneto-optical imaging in YBa2Cu3O7-d with tilted columnar defects (CD's) shows an asymmetric critical-state field profile. The observed hysteretic shift of the profile ridge (trough) from the center of the sample is explained by in-plane magnetization originated from vortex alignment along CD's. The extracted ratio of the in-plane to out-of-plane magnetization component has a maximum at 1/5 of matching field ($B_\Phi$) and disappears above $B_\Phi/3$, suggesting a reduction of interlayer coherence well bellow $B_\Phi$ in the Bose glass phase. Implications are discussed in comparison with the vortex liquid recoupling observed in irradiated Bi2Sr2CaCu2O8+y., Comment: Revtex, 4 pages, 5 figures, also see a movie at (http://www.ap6.t.u-tokyo.ac.jp/kitaka/Research/d-line/index_e.htm). This manuscript will appear in Phys. Rev. Lett