Weighing a piece of ice in a glass of water can tell you how much water has already frozen. If the scale is off by a factor of three, the ice will appear much larger than it really is. A similar story happened with calculations of the superconducting phase in a crystal of nickel, lanthanum, and samarium under high pressure.

Superconductivity is a state where a material conducts electricity without loss, like water flowing through a perfectly smooth pipe. To observe this phase, the sample is cooled in a magnetic field in two ways: first cool, then apply the field (ZFC), or the reverse (FC). By how the material expels the magnetic field, one calculates what fraction of the sample becomes superconducting. In the paper by Li and colleagues for La2SmNi2O7, they obtained 62.1% for ZFC and 14.4% for FC. But upon verification, it turned out that the calculations contained an error of exactly a factor of three.

Correcting this error matters because nickel-based superconductors are a new family of materials being studied in hopes of achieving superconductivity at not-too-low temperatures. If the superconducting fraction turns out to be smaller than thought, conclusions about how good this material is will need revision. But even with the correction, Li and colleagues' result remains important: it confirms that superconductivity in compressed nickelates is truly bulk, not surface.