Abstract

A theory of liquids and liquid-glass transition requires understanding most basic thermodynamics properties of the liquid state such as energy and heat capacity. This has turned out to be a long-standing problem in physics. Landau&Lifshitz textbook states that no general formulas can be derived for liquid thermodynamic functions because the interactions are both strong and system-specific. Phrased differently, liquids have no small parameter.

Recent experimental and theoretical results open a new way to understand liquid thermodynamics on the basis of collective modes (phonons) as is done in the solid state theory. There are important differences between phonons in solids and liquids, and we have recently started to understand and quantify this difference. I will review collective modes in liquids including high-frequency solid-like transverse modes and will discuss how a gap in the reciprocal space emerges and develops in their spectrum [2]. This reduces the number of phonons with temperature, consistent with the experimental decrease of constant-volume specific heat with temperature [1]. I will discuss the implication of the above theory for the liquid-glass transition and the change of heat capacity at Tg.

I will also mention how this picture can be extended above the critical point where the recently proposed concept of the Frenkel line on the phase diagram separates liquid-like and gas-like states of supercritical dynamics [1].

1. Trachenko, K., and Brazhkin, V. V., Collective modes and thermodynamics of the liquid state, Reports on Progress in Physics. 79, 016502 (2016).

2. C. Yang et al, PRL 118 , 215502 (2017).