title: Collective spin 1 singlet phase in high-pressure oxygen.
abstract: Oxygen, one of the most common and important elements in nature, has an exceedingly well-explored phase diagram under pressure, up to and beyond 100 GPa. At low temperatures, the low-pressure antiferromagnetic phases below 8 GPa where O2 molecules have spin S = 1 are followed by the broad apparently nonmagnetic ε phase from about 8 to 96 GPa. In this phase, which is our focus, molecules group structurally together to form quartets while switching, as believed by most, to spin S = 0. Here we present theoretical results strongly connecting with existing vibrational and optical evidence, showing that this is true only above 20 GPa, whereas the S = 1 molecular state survives up to about 20 GPa. The ε phase thus breaks up into two: a spinless ε0 (20-96 GPa), and another ε1 (8-20 GPa) where the molecules have S = 1 but possess only short-range antiferromagnetic correlations. A local spin liquid-like singlet ground state akin to some earlier proposals, and whose optical signature we identify in existing data, is proposed for this phase. Our proposed phase diagram thus has a first-order phase transition just above 20 GPa, extending at finite temperature and most likely terminating into a crossover with a critical point near 30 GPa and 200 K.
authors.affiliation: The Abdus Salam International Center for Theoretical Physics, 34151 Trieste, Italy; and.
authors.forename: Yanier
authors.initials: Y
authors.lastname: Crespo
authors.affiliation: International School for Advanced Studies and Consiglio Nazionale delle Ricerche, Istituto Officina dei Materiali Democritos, 34136 Trieste, Italy.
authors.forename: Michele
authors.initials: M
authors.lastname: Fabrizio
authors.affiliation: The Abdus Salam International Center for Theoretical Physics, 34151 Trieste, Italy; and.
authors.forename: Sandro
authors.initials: S
authors.lastname: Scandolo
authors.affiliation: The Abdus Salam International Center for Theoretical Physics, 34151 Trieste, Italy; andInternational School for Advanced Studies and Consiglio Nazionale delle Ricerche, Istituto Officina dei Materiali Democritos, 34136 Trieste, Italy tosatti@sissa.it.
authors.forename: Erio
authors.initials: E
authors.lastname: Tosatti
keywords: IR modes; Raman modes; density functional; molecular magnetism; tetramer