Collective modes in simple melts: Transition from soft spheres to the hard sphere limit

Area of Science:

• Condensed matter physics
• Statistical mechanics
• Computational physics

Background:

• Collective modes are crucial for understanding material properties.
• Inverse-power-law (IPL) interactions model systems from soft to hard spheres.
• Fluid-solid coexistence is a key phase transition in many materials.

Purpose of the Study:

• To investigate collective modes in classical particle systems with repulsive inverse-power-law (IPL) interactions.
• To evaluate the accuracy of the quasi-crystalline approximation (QCA) for these systems.
• To identify reliable indicators for freezing in dense particle systems.

Main Methods:

• Molecular dynamic (MD) simulations were used to study systems with IPL exponents (n) from 10 to 100.
• Longitudinal dispersion relations were calculated and compared between MD simulations and QCA.
• High-frequency elastic moduli and elastic velocities were analyzed.

Main Results:

• The quasi-crystalline approximation (QCA) shows significant inaccuracy for systems with high IPL exponents (n > 60).
• Conventional expressions for elastic moduli are not meaningful in the hard-sphere-like regime.
• The study discusses the relationship between QCA elastic velocities and adiabatic sound velocity from MD simulations.

Conclusions:

• The QCA model is limited in its applicability to systems with steep repulsive interactions.
• New freezing indicators are proposed for classical particle systems near the fluid-solid transition.
• Understanding collective dynamics is essential for predicting phase behavior in dense matter.