Quantumness and state boundaries hidden in supercritical helium-4: A path integral centroid molecular dynamics study

Area of Science:

• Thermodynamics
• Quantum Mechanics
• Fluid Dynamics

Background:

• Supercritical fluids exhibit unique properties near their critical points.
• Understanding the distinct states within the supercritical region is crucial.
• Helium-4 (4He) is a key quantum fluid for studying these phenomena.

Purpose of the Study:

• To reveal hidden state boundaries in supercritical 4He near the critical point.
• To analyze the thermodynamic, dynamic, and quantum properties of 4He.
• To investigate the role of quantum effects in defining supercritical states.

Main Methods:

• Isothermal-isobaric path integral centroid molecular dynamics simulations.
• Analysis of over 600 state points near the critical point.
• Examination of thermodynamic properties (e.g., isobaric heat capacity), dynamic properties, and quantum wavelength (λ_quantum).

Main Results:

• Identified hidden liquid-like and gas-like state boundaries in supercritical 4He.
• The Widom line (maxima of isobaric heat capacity) acts as a quantum boundary, correlating with changes in quantum wavelength.
• Observed the Frenkel line originating similarly to classical fluids, and new state boundaries emanating from the critical point.
• Quantified quantumness through depressed heat capacity, slopes of phase boundaries, and pseudo-boiling heat across the Widom line.

Conclusions:

• Supercritical 4He displays distinct liquid-like and gas-like regions separated by identifiable boundaries.
• Quantum effects significantly influence the thermodynamic and dynamic behavior, particularly around the Widom line.
• The study provides a deeper understanding of quantum fluid behavior in the supercritical state.