Zinc doping preserves two energy gaps in iron selenide

The takeaway: zinc-related disorder and the material’s multiband structure pull the transition temperature in opposite directions.

What did the heat measurements reveal?

Specific heat measures how much heat a material absorbs for each 1-degree rise in temperature. The researchers measured it at low temperatures to test models of FeSe’s superconducting state. Their measurements fit a two-gap model better than either a single-gap model or a d-wave model, in which the gap’s sign and magnitude vary by direction.

An energy gap is a range of normally available electron energies that becomes unavailable in the superconducting state. The researchers identified two distinct forms:

• An anisotropic, extended s-wave gap: its value changes with direction, and the gap extends over a broad range of electronic states.
• An isotropic s-wave gap: it has the same value in every direction.

The two gaps also contributed nearly the same shares to the low-temperature fit as zinc concentration changed. According to the researchers, this stability suggests that zinc-induced scattering between the different electronic bands remained relatively weak.

Han-Shu Xu and colleagues used the phrase “robust multigap superconductivity” in the paper title. The researchers’ conclusion was that FeSe “retains its two-gap superconducting state despite zinc doping.” Together, those statements capture the central measurement: local disorder and a nonmonotonic transition did not remove the two-gap signature.

The takeaway: the heat-capacity evidence points to two distinct gaps that survive zinc doping, rather than one gap or a d-wave state.

Who ran the study, and what was published?

The team included researchers at the Hefei Institutes of Physical Science of the Chinese Academy of Sciences, with collaborators from Anhui Medical University, the University of Science and Technology of China, the Hong Kong University of Science and Technology, and other institutions. The experiments used the Steady High Magnetic Field Facility.

The paper appeared in Physical Review B in 2026 as “Nonmonotonic evolution of the superconducting transition temperature and robust multigap superconductivity with extended s-wave and s-wave gaps in Zn-incorporated FeSe.” Its DOI is 10.1103/rw7j-zxw6.

The takeaway: a multi-institution, high-field experiment tested how zinc changes FeSe’s transition and superconducting energy gaps.

What does the result mean for materials researchers?

For practitioners, the practical lesson is to track more than transition temperature when assessing a dopant. A sample can show a lower transition at one zinc concentration and a higher one at another while its two-gap signature stays largely intact. Comparing heat-capacity evidence with transition data helps distinguish a change in when superconductivity begins from a change in its internal gap structure.

The result does not close every question about FeSe. The available findings give no exact zinc concentrations, numerical transition temperatures, sample counts or uncertainty ranges. Readers therefore cannot locate the dip and rebound or assess reproducibility from these findings alone. Nor does the study provide a complete microscopic explanation for why the two gaps remain stable.

Within those limits, the evidence shows that zinc can alter local crystal structure, electron scattering and the transition temperature without destroying the multigap state.

The closing message is simple: a material’s transition temperature can move while its two-gap superconducting state stays largely intact.