arXiv subjects

J. Albrecht

Publications and source records attributed to J. Albrecht.

870 records · Page 49Linked to original sources

arXiv2008

Superconducting phase formation in random neck syntheses: a study of the Y-Ba-Cu-O system by magneto-optics and magnetometry

Magneto-optical imaging and magnetization measurements were applied to investigate local formation of superconducting phase effected by a random neck synthesis in Y-Ba-Cu-O system. Polished pellets of strongly inhomogeneous ceramic samples show clearly the appearance of superconducting material in the intergrain zones of binary primary particles reacted under different conditions. Susceptibility measurements allows evaluation of superconducting critical temperature, which turned out to be close to that of optimally doped YBCO.

cond-mat.supr-con↗

arXiv2006

Dramatic role of critical current anisotropy on flux avalanches in MgB2 films

Anisotropic penetration of magnetic flux in MgB2 films grown on vicinal sapphire substrates is investigated using magneto-optical imaging. Regular penetration above 10 K proceeds more easily along the substrate surface steps, anisotropy of the critical current being 6%. At lower temperatures the penetration occurs via abrupt dendritic avalanches that preferentially propagate {\em perpendicular} to the surface steps. This inverse anisotropy in the penetration pattern becomes dramatic very close to 10 K where all flux avalanches propagate in the strongest-pinning direction. The observed behavior is fully explained using a thermomagnetic model of the dendritic instability.

cond-mat.supr-con↗

arXiv2005

Spin-polarized quasiparticle injection effects in YBCO thin films

We report detailed transport studies on ferromagnet-superconductor heterostructures. Epitaxial heterostructures of half-metal colossal magnetoresistive La2/3Ca1/3MnO3 (HM-CMR) and high Tc superconducting YBa2Cu3O7 (YBCO) are grown on SrTiO3 (100) single crystal substrates by pulsed laser deposition. Using the HM--CMR layer as source for spin-polarized quasiparticles, we show the effect of injection of spin-polarized quasiparticles into the ab-plane and along the c-axis of YBCO. The results show a drop in the ab-plane resistance Rab (T) in the case of injection along the c-axis that is discussed to be related to the opening of a pseudogap.

cond-mat.supr-con↗

arXiv2004

Ferromagnetic/superconducting bilayer structure: A model system for spin diffusion length estimation

We report detailed studies on ferromagnet--superconductor bilayer structures. Epitaxial bilayer structures of half metal--colossal magnetoresistive La$_{\mathrm{2/3}}$Ca$_{\mathrm{1/3}}$MnO$_{\mathrm{3}}$ (HM--CMR) and high--$T_{\mathrm{c}}$ superconducting YBa$_{\mathrm{2}}$Cu$_{\mathrm{3}}$O$_{\mathrm{7-δ}}$(HTSC) are grown on SrTiO$_3$ (100) single--crystalline substrates using pulsed laser deposition. Magnetization $M$(T) measurements show the coexistence of ferromagnetism and superconductivity in these structures at low temperatures. Using the HM--CMR layer as an electrode for spin polarized electrons, we discuss the role of spin polarized self injection into the HTSC layer. The experimental results are in good agreement with a presented theoretical estimation, where the spin diffusion length $ξ_{\mathrm {FM}}$ is found to be in the range of $ξ_{\mathrm{FM}} \approx$ 10 nm.

physics.class-ph↗

arXiv2004

Enhancement of flux-line pinning in all-oxide superconductor/ferromagnet heterostructures

We have studied the local critical current density, jc, in the superconductor thin film of bilayer structures consisting of YBa2Cu3O7 and the ferromagnets La2/3Ca1/3MnO3 and SrRuO3, respectively, by means of quantitative magneto-optics. A pronounced hysteresis of jc was observed which is ascribed to the magnetization state of the ferromagnetic layer. The results are discussed within the frame of magnetic vortex - wall interactions.

cond-mat.supr-con↗

arXiv2000

Influence of vortex-vortex interaction on critical currents across low-angle grain boundaries in YBa2Cu3O7-delta thin films

Low-angle grain boundaries with misorientation angles theta < 5 degrees in optimally doped thin films of YBCO are investigated by magnetooptical imaging. By using a numerical inversion scheme of Biot-Savart's law the critical current density across the grain boundary can be determined with a spatial resolution of about 5 micrometers. Detailed investigation of the spatially resolved flux density and current density data shows that the current density across the boundary varies with varying local flux density. Combining the corresponding flux and current pattern it is found that there exists a universal dependency of the grain boundary current on the local flux density. A change in the local flux density means a variation in the flux line-flux line distance. With this knowledge a model is developped that explains the flux-current relation by means of magnetic vortex-vortex interaction.

cond-mat.supr-con↗