Pseudoisotropic Upper Critical Field in Cobalt-Doped SrFe2As2 Epitaxial Films

We present resistivity measurements of the complete superconducting upper critical field (H-c2) phase diagram as a function of angle (theta) and temperature (T) for cobalt-doped SrFe2As2 epitaxial films to 0.5 K and 50 T. Although H-c2(theta) at 10 K is indistinguishable from that derived from a single-band anisotropy model, the apparent anisotropy H-c2(perpendicular to c)/H-c2(parallel to c) linearly decreases to 1 at low T, with H-c2(0)=47 T. The data are well described by a two-band model with small, opposing anisotropies for the bands. This unusual relationship is confirmed by the observation of a local maximum for H-c2(parallel to c) at low T.