Psuedo-isotropic upper critical field in cobalt-doped SrFe2As2 epitaxial films

TL;DR

This study investigates the temperature and angular dependence of the upper critical field in cobalt-doped SrFe2As2 epitaxial films, revealing near-isotropic behavior at low temperatures and providing insights into the two-band superconductivity model.

Contribution

It presents the first detailed phase diagram and modeling of Hc2 in cobalt-doped SrFe2As2 films, highlighting the unusual diffusivity relationship between two anisotropic bands.

Findings

Hc2 is nearly linear in temperature for H parallel c

Hc2 ratio decreases linearly with temperature, approaching 1 at low T

Observation of a local maximum of Hc2 parallel at low temperature

Abstract

The temperature and angular dependence of the upper critical field (Hc2) is reported for cobalt-doped SrFe2As2 epitaxial films between Tc and 0.5 K in pulsed magnetic fields up to 50 T. For H parallel c, Hc2 is close to a linear function of temperature, while in the perpendicular direction there is significant downward curvature that results in an Hc2 ratio (gamma = Hc2(perpendicular)/Hc2(parallel) that decreases nearly linearly with temperature, approaching gamma = 1 at low temperature with Hc2(0) = 47 T. We measure the complete upper-critical field phase diagram including angular dependence and model the data using a two band theory allowing us to determine the anisotropy of both bands, their relative diffusivities, and the relationship between BCS coupling constant matrix elements. We find an unusual relationship between the diffusivities of the two bands, with two anisotropic and opposite bands. This relationship is supported by the observation of a local maximum for Hc2(parallel) at low temperature.