In a Cornell University laboratory, physicists unexpectedly discovered that superconductivity in iron selenide FeSe depends not on the number of electrons, but on how perfect the material's crystal lattice is. This conclusion changes the familiar picture of high-temperature superconductors and opens a new path to controlling them.
Usually in such materials scientists change the number of electrons — so-called doping — in order to reach the superconductivity "dome," where the material loses its resistance at relatively high temperatures. In FeSe, however, things turned out differently. Researchers led by Kyle Shen showed that the curve of this dome is closely tied to the scattering of electrons on lattice defects, rather than to the charge-carrier concentration.
Picture city streets: if the roads are smooth and free of potholes, cars travel fast and unimpeded. Let potholes and obstacles appear, and traffic slows down, even if no more cars have been added. So it is in the FeSe crystal: electrons "stumble" over imperfections in the lattice, and this is precisely what determines when the material becomes a superconductor.
Postdoc Paul Malinowski, the first author of the work published in PNAS, explains: "We found that the dome in this material is governed by something entirely different from what people are used to. Not by how many electrons you add, but by what obstacles they encounter along the way — by how perfect or imperfect the crystal lattice is."
The experimenters used a new technique for controlling FeSe and measured how the resistance changes at different levels of disorder. It turned out that minimal disorder allows the maximum superconducting temperature to be reached, whereas simply changing the number of electrons produces a far smaller effect. This sets FeSe apart from most other iron-based superconductors.
The discovery matters not only for fundamental physics. Understanding the role of structural disorder may help create more stable and controllable materials for future technologies — from powerful magnets to quantum devices. The cleaner the lattice, the closer we come to the practical use of high-temperature superconductivity.
