Evidence for a first-order liquid-liquid transition in high-pressure hydrogen from ab initio simulations

Area of Science:

• Condensed matter physics
• Quantum chemistry
• Materials science

Background:

• Liquid hydrogen exhibits complex phase behavior under extreme conditions.
• Understanding the transition to metallic hydrogen is crucial for planetary science and fusion energy.

Purpose of the Study:

• To investigate the phase transitions in liquid hydrogen using advanced quantum simulation methods.
• To determine the critical point of the molecular-to-atomic transition and the melting curve.
• To identify the conditions under which hydrogen metallizes.

Main Methods:

• Quantum simulation techniques, including density functional theory (DFT) and quantum Monte Carlo (QMC).
• Analysis of the temperature dependence of electronic conductivity to identify phase transitions.
• Calculation of the melting curve of molecular hydrogen up to 200 GPa.

Main Results:

• Clear evidence of a first-order phase transition in liquid hydrogen from a low-conductivity molecular state to a high-conductivity atomic state.
• Estimation of the critical point for this transition at approximately 2,000 K and 120 GPa.
• Determination of a reentrant melting line for molecular hydrogen, crossing the metallization line at specific temperature and pressure points (e.g., 700 K and 220 GPa with DFT).

Conclusions:

• Liquid hydrogen undergoes a significant electronic transition under high pressure and temperature.
• The metallization of hydrogen is predicted to occur at accessible, albeit extreme, conditions.
• Quantum simulation methods provide valuable insights into the fundamental properties of matter under extreme conditions.