The relevant bosons at the liquid-solid transition

Abstract

Using the cubic alkali halogenides as model materials, it is shown that the cohesion of the solids up to the rather high melting temperatures, T_m, is not by the inter-atomic interactions but by a boson field. A reasonable measure of the absolute interatomic interaction strength is given by the Debye-temperature, Θ_D, which is much lower than T_m. It is explained that in the wide temperature range Θ_D < T < T_m, the dynamics is the dynamics of a boson field. This is evidenced by the observed universality in the temperature dependence of heat capacity and relative thermal length changes, ΔL/L₀ below T_m. The boson field orders at T_m and defines the perfect long-range atomic order of the crystalline state. Upon ordering all bosons condense in the lowest quantum state (Bose-Einstein condensation). This is the highest possible thermodynamic order, and provides a plausible entropy argument for the exclusion of the interatomic interactions at order-disorder phase transitions. Additionally, ordered boson fields contract themselves to a finite volume such as a domain. The mosaic blocks, occurring in, practically, all crystalline solids, have to be viewed as the domains of the bosons that order at T_m. Within each mosaic block, the bosons are in a stationary mode. The constricting force of the ordered boson field that compresses each mosaic block increasingly with decreasing temperature, guarantees the cohesion of the whole solid up to T_m. Plausible arguments are given that the bosons that order at T_m are elastic quadrupole radiation.