An iron thread stretched between quantum states. That's one way to describe a chain of iron atoms linked by organic dehydroindigo molecules. Each iron atom carries a magnetic moment, like a tiny compass that can point in different directions. In such a chain, the moments interact with one another, and quantum fluctuations give rise to an unusual state.

Calculations showed that the interaction between neighboring iron atoms is nearly isotropic: its strength is about 32 meV. It's as if neighboring compasses attract each other with equal force, regardless of which way they point. Weak anisotropy, meaning the dependence of energy on direction, only slightly distorts the picture.

To understand what happens in the chain, the scientists solved the quantum model exactly. For short chains, they used an exhaustive enumeration of all states, and for long chains, a method that orders states by importance. Both approaches gave the same result: the ground state is non-degenerate, and the first excited state is separated by a gap of 13–14 meV.

The dynamic response, which shows how the chain reacts to an external perturbation, is concentrated near the wave vector q=π. This is characteristic of antiferromagnetic chains, where neighboring moments tend to align oppositely. The iron thread turns out to be stretched between quantum states, and the Haldane gap keeps it from sagging.