453 tion and produces superconductivity. But so does the doping that leaves the electron density unchanged, suggesting that “you can get the same effect by making the structural change only,” Rosner says. The structural changes may be all that’s relevant in the iron-and-arsenic compounds, Johrendt says. Rosner hesitates to go that far. In any case, such experiments underscore a difference between the iron-and-arsenic superconductors and the cuprates: Replace copper atoms in a cuprate’s planes with anything else, and the material’s superconductivity vanishes.