The La3Ni2O7 crystal resembles a layered pie, where the filling of nickel and oxygen atoms alternates with lanthanum layers. At ambient pressure, it does not conduct without resistance, but squeeze it or grow it as a thin film, and high-temperature superconductivity emerges. To understand how this works, one must decipher the magnetic behavior of nickel atoms in the pristine, uncompressed state.
Physicists directed a beam of neutrons, particles that sense magnetic fields, at a single crystal. The neutrons bounced off the sample and revealed how the spins (tiny magnetic arrows) of electrons sway. It turned out that fluctuations only appear starting at an energy of about 5 meV, like a threshold below which the magnetic ripples freeze. Within a layer, waves run differently: along one direction they are stiff, while across they soften noticeably, as if the tension of a spring changes with angle.
The most surprising finding emerged between layers. The spin fluctuations showed clear periodicity with a two-layer step — direct evidence that magnetic moments of adjacent nickel planes are coupled antiferromagnetically, meaning they point in opposite directions. This is like two rows of dancers moving in opposite phases. Modeling confirmed that strong interlayer coupling plus competing interactions within the layer are needed to reproduce the observed pattern.
When the researchers recalculated the signal in absolute units, an intriguing detail surfaced. The amplitude of spin waves in the nickelate is only a quarter of what is seen in cuprates — another class of high-temperature superconductors based on copper. But the local magnetic susceptibility at the same energies turned out to be notably higher. The total fluctuating magnetic moment is comparable to that of cuprates. This means that strong electronic correlations hide in nickelates, giving rise to intense mid-energy magnetic fluctuations, and these are likely to play a key role in future superconductivity.
The energy of 5 meV is roughly a hundred billion times smaller than what is needed to boil a kettle, yet such tiny energy packets govern magnetic order in the crystal.
In Peter Watts's novel Blindsight, alien creatures use magnetic fields to communicate — much like spins in a nickelate exchange signals across layers.