Condensed Matter Seminars

Abstract: While physicists have a good theoretical understanding of the heat capacity of both solids and gases, a general theory of the heat capacity of liquids has always remained elusive. Apart from being an awkward hole in our knowledge of condensed-matter physics, heat capacity – the amount of heat needed to change a substance's temperature by a certain amount – is a technologically relevant quantity that it would be nice to be able to predict¹. I will introduce a phonon-based approach to liquids² and supercritical fluids³, and describe its thermodynamics in terms of phonon excitations. I will show that the effective Hamiltonian⁴ has a transverse phononic gaps and explain their evolution with temperature variations. I will explain how the introduced formalism covers the Debye theory of solids, the phonon theory of liquids, and thermodynamic limits such as the Delong-Petit and the ideal gas thermodynamic limits. The experimental evidence for the new thermodynamic boundary (the Frenkel line) on the pressure-temperature phase diagram will be demonstrated⁵. Finally, I will discuss the phonon propagation and localization effects in liquids above and below the Frenkel line, and outline new directions towards phonon band gaps engineering and sound manipulation.