click to enlarge False-color plots of the superconducting gap distribution of BaFe2(As0.7P0.3)2. The image on the left shows the superconducting energy gap approaching zero (indicated by color), but at a circular horizontal line-node. This immediately rules out the d-wave pairing symmetry, which would give four vertical line nodes in the diagonal directions. Image: D.L. Feng, et al.

Condensed-matter physicists the world over
are in hot pursuit of a comprehensive understanding of high-temperature superconductivity,
not just for its technological benefits but for the clues it holds to strongly
correlated electron systems.

One important avenue of investigation is
pairing symmetry. It’s a property of Cooper pairs, the bound electron pairs
that are a hallmark of all superconductors, whether high-temperature or
conventional. The paired electrons act as if they were a single particle, and
the energy required to break Cooper pairs is measured by the superconducting
gap. The symmetry of the superconducting gap, known as the pairing symmetry, is
an important characteristic of Cooper pairs that is intimately related to the
mechanism of superconductivity.

In conventional superconductors, the Cooper
pairs have s-wave pairing
symmetry, which takes the shape of a sphere. In contrast, Cooper pairs in the
cuprate family of high-temperature superconductors exhibit d-wave
pairing symmetry, which looks a bit like a four-leaf clover. The leaves, or
lobes, are areas where the superconducting gap is finite. At the points where
two leaves join, known as nodes, the superconducting gap goes to zero.

However, iron-based superconductors do not
fall nicely into either of these two categories. Some members of this group
exhibit characteristics of superconducting gaps with s-wave pairing
symmetry, while others show signatures of nodes where the gap becomes zero, as
with d-wave pairing symmetry.

The key to resolving this discrepancy
remained unknown until recently, when a team of scientists from Fudan
University used an instrument at the Stanford Synchrotron Radiation
Lightsource’s Beam Line 5-4 to measure the detailed superconducting gap
structure of the ferropnictide superconductor BaFe₂(As_0.7P_0.3)₂.
They discovered a signature that could not have originated from a d-wave
pairing—a striking difference from the cuprate family.

This
finding, the first measurement of its kind, provides solid experimental
evidence that iron-based superconductors fall into the regime of s-wave pairing symmetry seen in conventional
superconductors, and suggests that both nodal and nodeless gaps could arise
from the same mechanism. This could lead to a unified theoretical framework for
both phenomena, making the research an important step toward unveiling the
mechanism of iron-based superconductivity.

SLAC National Accelerator Laboratory