The pristine, cleaved surface of a crystal superconducts at a higher temperature than its bulk

When studying superconducting materials, scientists sometimes assume a single critical temperature for the entire volume of a material, which may not be correct.

Parzyck et al. suspected that the surface of Ba(Fe_0.95Co_0.05)₂As₂ — a widely-studied material that belongs to a class of iron-based superconductors — inherently superconducts differently than its bulk, because the configuration of the atoms at the surface differs from the configuration of atoms in the bulk.

Typically, ambient gas molecules contaminate the surface of a material, preventing measurement of a surface’s inherent properties. The authors devised an apparatus to measure the surface superconductivity, as well as the bulk superconductivity, and found that the surface of Ba(Fe_0.95Co_0.05)₂As₂ becomes superconducting at a temperature at least 4 K higher than the bulk.

To measure this difference, the authors placed a crystal of Ba(Fe_0.95Co_0.05)₂As₂ in an ultrahigh vacuum chamber, cleaved it open to reveal a clean surface uncontaminated by gas molecules, and then applied electrical current and measured the resistivity of the surface layer and bulk simultaneously, which showed the enhanced superconductivity of the surface.

Next, the authors intend to use their method to examine other superconductors that could exhibit this phenomenon, with the hopes that some of those surfaces demonstrate either exotic superconductivity or much higher superconducting temperatures than their bulk counterparts.

Source: “Enhanced surface superconductivity in Ba(Fe_0.95Co_0.05)₂As₂,” by Christopher T. Parzyck, Brendan D. Faeth, Gordon N. Tam, Gregory R. Stewart, and Kyle M. Shen, Applied Physics Letters (2020). The article can be accessed at https://doi.org/10.1063/1.5133647 .