Universal lower bound on the dissipation of superconductors

Despite their name, not all superconductors have zero resistance below their transition temperature T_c, at least when placed in a sufficiently strong magnetic field. For so-called type 2 superconductors—a class that includes high-temperature cuprate, iron-based, and magnesium diboride superconductors—the field penetrates and forms a lattice of vortices. Each vortex is an eddy of supercurrent that encircles a quantized amount of magnetic flux. Crystal defects, often intentionally introduced, will tend to pin the vortex lattice in place, but a sufficiently high current will force the vortices to move. That motion dissipates energy and manifests itself as a finite resistance. For currents slightly below the threshold, thermal fluctuations can provide the extra kick needed to knock the lattice free. Known as creep, thermally activated vortex motion can limit the operating range in applications such as high-field magnets and power transmission. The discovery of iron-based superconductors a decade ago challenged the understanding of vortex creep: The materials’ observed creep rate was significantly higher than expected. Serena Eley (Los Alamos National Laboratory) and colleagues now report on their study of BaFe₂(As_0.67P_0.33)₂. The research did not explain the high creep rates in iron superconductors—indeed, the team observed the lowest rate yet seen for those materials. But the researchers did find a universal lower bound for the low-temperature creep rate, one that depends only on the ratio of temperature to T_c and on the square root of the Ginzburg number, which parameterizes the scale of thermal fluctuations with respect to the superconductor’s magnetic properties. The figure shows how the derived limits (dashed lines) compare with measured creep rates for different superconductors. The researchers conclude that any new high-T_c superconductor will have high creep; the work may also help guide materials design for superconductor applications. (S. Eley et al., Nat. Mater., in press, doi:10.1038/nmat4840 .)