Phase Separation in Cold Para-H_{2} D_{2} Clusters

Area of Science:

• Quantum Fluids and Condensed Matter Physics
• Low-Temperature Physics
• Spectroscopy and Molecular Dynamics

Background:

• Low-temperature phase separation is a known phenomenon in quantum fluids like helium-3 and helium-4 mixtures.
• Hydrogen isotopes, specifically para-hydrogen (H₂) and deuterium (D₂), have been theoretically predicted to exhibit quantum liquid behavior and superfluidity at low temperatures (below 1 K).
• Phase separation in H₂-D₂ mixtures was predicted at temperatures below 3 K, but experimental verification has been challenging due to freezing.

Purpose of the Study:

• To investigate the potential for phase separation in mixtures of para-hydrogen (H₂) and deuterium (D₂) at low temperatures.
• To experimentally confirm the quantum liquid nature of H₂-D₂ mixtures and observe isotope phase separation, deferring the freezing point.
• To validate theoretical predictions of quantum behavior in hydrogen isotope mixtures.

Main Methods:

• Production of para-H₂ and D₂ clusters at an estimated temperature of approximately 2 K to prevent freezing.
• Vibrational Raman spectroscopy was employed to study the state and properties of the produced clusters.
• Quantum molecular dynamics simulations were conducted to corroborate experimental findings.

Main Results:

• Experimental evidence indicates that the para-H₂ and D₂ clusters exist in a liquid state at the studied low temperatures.
• The vibrational Raman spectroscopy results clearly demonstrate the phase separation of para-H₂ and D₂ isotopes within the clusters.
• Quantum molecular dynamics simulations independently support and confirm the observed phase separation phenomenon.

Conclusions:

• The study provides the first experimental observation of phase separation in liquid para-hydrogen and deuterium mixtures.
• These findings strongly suggest that hydrogen isotopes can indeed behave as quantum liquids, exhibiting properties analogous to helium isotopes.
• The research validates theoretical predictions and opens new avenues for exploring quantum phenomena in molecular hydrogen systems.