In-plane Field Angle Dependence of the Critical Current of RBCO Wires at Low Temperatures

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The dependence of the critical current (Ic) of an RBCO superconductor on the angle of a magnetic field applied within its basal plane (coplanar with the transport current) is commonly held to follow a variable Lorentz force behavior. Ic varies trigonometrically from some maximum value determined by flux-line cutting or other effects in the force-free configuration of field parallel to current to its minimum value in the maximum Lorentz force configuration of field and current perpendicular. The reduction in Ic is considered to result from the increasing depinning force exerted by the current on the flux lines as their relative directions diverge.Here we show, for contemporary RBCO-based coated conductor wires, that this presumed behavior is only evident at high temperatures, notably including 77K. At lower temperatures as high as 60K in moderate magnetic fields (8 T), significant deviations from the expected behavior are observed, becoming stronger to lower temperatures and higher fields. An intermediate behavior of doubled periodicity is observed, incompatible with the simple trigonometric model, and around 30K, 3 T this resolves into a fully inverted behavior, with maximum Ic obtaining in the maximum Lorentz force configuration and minimum Ic in the force-free configuration.These experimental results have important implications for the design and operation of RBCO-based devices in the common operating regime of low temperatures and intermediate to high fields. In particular, it is commonly assumed that the maximum Lorentz force configuration is always the configuration of minimum Ic. In fact, we show here that this is not the case, with Ic values up to 30% lower being observed in the force-free configuration.