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 O-2 molecules have spin S = 1 are followed by the broad apparently nonmagnetic epsilon 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 epsilon phase thus breaks up into two: a spinless epsilon(0) (20-96 GPa), and another epsilon(1) (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.

Collective spin 1 singlet phase in high-pressure oxygen

Crespo, Yanier;Fabrizio, Michele;Scandolo, Sandro;Tosatti, Erio
2014-01-01

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 O-2 molecules have spin S = 1 are followed by the broad apparently nonmagnetic epsilon 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 epsilon phase thus breaks up into two: a spinless epsilon(0) (20-96 GPa), and another epsilon(1) (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.
2014
111
29
10427
10432
https://arxiv.org/abs/1409.4956
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4115506/
https://arxiv.org/abs/1409.4956
Crespo, Yanier; Fabrizio, Michele; Scandolo, Sandro; Tosatti, Erio
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/20.500.11767/30184
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