Collective spin 1 singlet phase in high-pressure oxygen

Area of Science:

• Condensed matter physics
• Materials science
• Quantum chemistry

Background:

• The high-pressure phase diagram of oxygen is extensively studied, with the epsilon phase (8-96 GPa) being a key area of interest.
• This phase was previously thought to be nonmagnetic, with oxygen molecules transitioning to a spin S=0 state.

Purpose of the Study:

• To investigate the spin state of oxygen molecules within the epsilon phase under high pressure.
• To refine the understanding of oxygen's phase diagram between 8 and 96 GPa.

Main Methods:

• Theoretical calculations were employed to analyze the electronic and magnetic properties of oxygen.
• Existing vibrational and optical spectroscopy data were re-examined in light of the new theoretical findings.

Main Results:

• The epsilon phase is reclassified into two distinct phases: epsilon0 (20-96 GPa) and epsilon1 (8-20 GPa).
• Oxygen molecules retain a spin S=1 state up to approximately 20 GPa within the epsilon1 phase, exhibiting short-range antiferromagnetic correlations.
• A local spin liquid-like singlet ground state is proposed for the epsilon1 phase, supported by optical signatures in existing data.

Conclusions:

• The established understanding of the epsilon phase of oxygen is revised, revealing a previously unrecognized spin state.
• A first-order phase transition is proposed just above 20 GPa, potentially terminating in a critical point near 30 GPa and 200 K.
• This research provides a more accurate and detailed phase diagram for high-pressure oxygen.