Collective spin 1 singlet phase in high-pressure oxygen
Among the elemental diatomic molecules, O2 is the only one carrying a spin 1 magnetic moment. In the high pressure phases of oxygen the magnetic moment conspires with intermolecular forces to generate a rich phase diagram. Whereas, up to 80,000 atmospheres the moment persists, at pressures between 80,000 and 200,000 atmospheres, molecular magnetism apparently disappears, however with a number of unexplained vibrational and optical anomalies. Through a fully quantum treatment of the electronic states of the dense crystalline state we find that in this pressure range oxygen still retains a spin moment in an unconventional and rare state of matter dominated by the quantum fluctuations. This state, a special case of so-called spin liquids, explains most of the observed anomalies.