Instantaneous charge and dielectric response to terahertz pulse excitation in TTF-CA

We present the results of exact numerical calculations of the dielectric properties of tetrathiafulvalene-p-chloranil (TTF-CA) using the extended Hubbard model. The electronic polarization (P) over bar (el) of the ionic ground state is obtained by directly calculating the adiabatic flow of current. The direction of (P) over bar (el) is opposite to polarization (P) over bar (ion) owing to ionic displacement, and | (P) over bar (el) | is much larger than | (p) over bar (ion) |, showing that, in the ionic phase, TTF-CA is an electric ferroelectric. Furthermore, we numerically calculate the dynamics induced by THz pulse excitation. In the ionic phase, there exists an almost exact linear relationship between Delta rho(t) and E(t), and between Delta P-el(t) and E(t) in the realistic range of the excitation magnitude, where Delta rho(t) [Delta P-el(t)] is the charge transfer (electric polarization) variation induced by the THz pulse and E(t) is the electric field of the pulse at time t. The absolute value of Delta rho(t) in the neutral phase is much smaller than that in the ionic phase. These results are consistent with those of experiments and originate from the adiabatic nature of the THz pulse excited state.