Spraying cesium on iron selenide films ties its superconducting dome to lattice disorder

Every conventional doping step means growing a new crystal ^[10], and adding electrons brings disorder with it, Shen said ^[16]. A chemical doping series therefore moves carrier count and lattice imperfection at the same time ^[22]. A dome plotted against electron count that way also contains an uncontrolled change in how perfect the lattice is.

Cesium is what let the Cornell group split those two. The films were grown by molecular beam epitaxy and then dosed with alkali metal at the surface, with electrical transport and angle-resolved photoemission measured in situ in ultrahigh vacuum ^[12]. No point on the curve required a fresh compound, because the sample was never removed or regrown ^[13]. "The novel thing here is we can do it all at once in a very comprehensive way," Malinowski said ^[14]. Shen said researchers had tried and failed for years to understand iron selenide's dome, because the material's chemistry resisted the standard methods ^[9].

What came out was a dome that tracked resistance from lattice imperfections more closely than the number of electrons moving through the crystal ^[2]. "It's not driven by how many electrons you're adding in, but rather, it's driven by the obstacles the electrons are hitting," Malinowski said ^[6]. Disorder here means any deviation from strict atomic order that disrupts electron flow, an atom out of place or a break in the pattern, and in a real crystal it is always present ^[15].

The scope is narrow on purpose. Malinowski said the prominence of disorder makes iron selenide stand out from all others ^[17], and in the cuprates, Shen said, the electron count is still believed to be the primary driver of the phase diagram and the dome ^[18]. The paper's own framing is that iron selenide could be different from other unconventional superconductors ^[23], not that carrier density has stopped mattering elsewhere. The Cornell release does not report transition temperatures or a numerical measure of the disorder ^[24].

The part I would expect to travel is the method rather than the verdict. In any material that can be grown as a film and dosed in vacuum, the same setup separates the two axes that chemical doping fuses, and in iron selenide, once they were separated, disorder was the one that moved with the dome ^[1]. The authors say the result can inform theoretical descriptions of how superconductivity develops in this compound ^[19]. Shen, the study's corresponding author ^[5], said: "The key is the ability to precisely control the doping in the fashion Paul has been able to accomplish" ^[21].