Roles of intrinsic anisotropy and pi-band pairbreaking effects on critical currents in tilted c-axis MgB2 films probed by magneto-optical and transport measurements

Investigations of MgB2 and Fe-based superconductors in recent years have revealed many unusual effects of multiband superconductivity but manifestations of anisotropic multiband effects in the critical current density Jc have not been addressed experimentally, mostly because of the difficulties to measure Jc along the c-axis. To investigate the effect of very different intrinsic anisotropies of sigma and pi electron bands in MgB2 on current transport, we grew epitaxial films with tilted c-axis (THETA ~ 19.5{\deg}), which enabled us to measure the components of Jc both along the ab-plane and the c-axis using magneto-optical and transport techniques. These measurements were combined with scanning and transmission electron microscopy, which revealed terraced steps on the surface of the c-axis tilted films. The measured field and temperature dependencies of the anisotropic Jc(H) show that Jc,L parallel to the terraced steps is higher than Jc,T perpendicular to the terraced steps, and Jc of thinner films (50 nm) obtained from transport experiments at 0.1 T reaches ~10% of the depairing current density Jd in the ab plane, while magneto-optical imaging revealed much higher Jc at lower fields. To analyze the experimental data we developed a model of anisotropic vortex pinning which accounts for the observed behavior of Jc in the c-axis tilted films and suggests that the apparent anisotropy of Jc is affected by current pairbreaking effects in the weaker {\pi} band. Our results indicate that the out-of-plane current transport mediated by the {\pi} band could set the ultimate limit of Jc in MgB2 polycrystals.Comment: 21 pges, 13 figure

Evidence for electromagnetic granularity in polycrystalline Sm1111 iron-pnictides with enhanced phase purity

We prepared polycrystalline SmFeAsO1-xFx (Sm1111) bulk samples by sintering and hot isostatic pressing (HIP) in order to study the effects of phase purity and relative density on the intergranular current density. Sintered and HIPped Sm1111 samples are denser with fewer impurity phases, such as SmOF and the grain boundary wetting phase, FeAs. We found quite complex magnetization behavior due to variations of both the inter and intragranular current densities. Removing porosity and reducing second phase content enhanced the intergranular current density, but HIPping reduced Tc and the intragranular current density, due to loss of fluorine and reduction of Tc. We believe that the HIPped samples are amongst the purest polycrystalline 1111 samples yet made. However, their intergranular current densities are still small, providing further evidence that polycrystalline pnictides, like polycrystalline cuprates, are intrinsically granular.Comment: 14 pages, 6 figure

Composition Operators and Endomorphisms

If $b$ is an inner function, then composition with $b$ induces an endomorphism, $\beta$, of $L^\infty(\mathbb{T})$ that leaves $H^\infty(\mathbb{T})$ invariant. We investigate the structure of the endomorphisms of $B(L^2(\mathbb{T}))$ and $B(H^2(\mathbb{T}))$ that implement $\beta$ through the representations of $L^\infty(\mathbb{T})$ and $H^\infty(\mathbb{T})$ in terms of multiplication operators on $L^2(\mathbb{T})$ and $H^2(\mathbb{T})$. Our analysis, which is based on work of R. Rochberg and J. McDonald, will wind its way through the theory of composition operators on spaces of analytic functions to recent work on Cuntz families of isometries and Hilbert $C^*$-modules

Neutron Irradiation of Sm-1111

SmFeAsO$_{1-x}$F$_x$ was irradiated in a fission reactor to a fast (E > 0.1 MeV) neutron fluence of 4x10^${21}$ m$^{-2}$. The introduced defects increase the normal state resistivity due to a reduction in the mean free path of the charge carriers. This leads to an enhancement of the upper critical field at low temperatures. The critical current density within the grains, Jc, increases upon irradiation. The second maximum in the field dependence of Jc disappears and the critical current density becomes a monotonically decreasing function of the applied magnetic field