Modified Barrett formula near the neutral-ionic quantum phase transition

A quantum version of the Blume-Emery-Griffiths (BEG) model is investigated by mean-field theory in order to characterize the newly-found neutral-ionic (NI) quantum phase transition in pressured-DMTTF-QBr4 (4,4'-dimethyltetrathiafulvalene-p-bromanil). In the quantum BEG model, a finite tunneling between neutral and ionic states changes the quantum NI transition of first order in the BEG model into that of second order. In the vicinity of this continuous NI quantum critical point, the dielectric permittivity in the neutral phase is shown to follow the Barrett formula characteristic of quantum paraelectricity. These features in the quantum BEG model are in good agreement with the experimental results, which suggests the importance of quantum NI fluctuations in DMTTF-QBr4.