On the dynamics of band Jahn-Teller systems with A15-structure

We construct a thermodynamic model for a system of electronic d-bands coupled to the elastic lattice. For the electronic fluctuations a Debye-type of relaxation, for the lattice displacement a modified elastic equation of motion is assumed. As a result a coupling of certain acoustic and relaxational modes is found leading to a soft mode instability. For non-vanishing external fields displacement and dielectric response functions are derived. The special case for wavevector q[110] is worked out explicitly. A comparison between the present phenomenological model and a microscopic multiple-band electron-phonon transport theory, recently given by the author, reveals remarkable agreement between both approaches.