Solid Hydrogen: The Ideal Analog of an Antiferromagnet

A physically meaningful analogy is drawn between the ordering and dynamical behavior of molecular rotations in solid hydrogen and the ordering and dynamical behavior of magnetic spin systems. In solid hydrogen the magnitude J of the rotational angular momentum is a good quantum number. Molecules having J = 1 resemble spin one magnetic moments, whereas those with J = 0 are "non‐magnetic." Solid solutions of these two species are analogs of magnetic alloys. The thermodynamic properties, in particular the order parameter, the specific heat, and the nuclear magnetic resonance properties are similar to those of magnetic systems. As for the dynamics, the small librations about the ground state are analogs of spin waves, and the interactions between the librational excitations are similar to, but larger than, the usual spin‐wave interactions. This circumstance enables the direct observation of the two‐libron spectrum and also leads to large anharmonic shifts in the single‐libron spectrum.