Strong Coupling of the Proton–Phonon System in KDP. I. Isotope Effect on the Transition Temperature TC

A mixed mode model is proposed for KDP (KH 2 PO 4 ), which includes explicitely the coupling of the proton to the O…︁O vibrations of its hydrogen bond. The Hamiltonian is formulated in the framework of the tunnelling model using delocalized states of single H‐bonds as a basis set. The linear transverse coupling term is handled by a canonical transformation in infinite order perturbation theory. Uncoupling the protonic and vibrational motion a Tokunaga‐Matsubara type Hamiltonian is obtained with a renormalized tunnelling frequency and a renormalized interaction between different hydrogen bonds, both effects being mass dependent. This mass dependence explains fully the isotope effect of the transition temperature.