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

Hiroki Gomi, Naoto Yamagishi, Tomohito Mase, Takeshi J. Inagaki, and Akira Takahashi

Phys. Rev. B 95, 094116 – Published 20 March 2017

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.95.094116

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 ${\overline{P}}_{\mathrm{el}}$ of the ionic ground state is obtained by directly calculating the adiabatic flow of current. The direction of ${\overline{P}}_{\mathrm{el}}$ is opposite to polarization ${\overline{P}}_{\mathrm{ion}}$ owing to ionic displacement, and $|{\overline{P}}_{\mathrm{el}}|$ is much larger than $|{\overline{P}}_{\mathrm{ion}}|$, 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 $\mathrm{\Delta }\rho \left(t\right)$ and $E\left(t\right)$, and between $\mathrm{\Delta }{P}_{\mathrm{el}}\left(t\right)$ and $E\left(t\right)$ in the realistic range of the excitation magnitude, where $\mathrm{\Delta }\rho \left(t\right)\left[\mathrm{\Delta }{P}_{\mathrm{el}}\left(t\right)\right]$ is the charge transfer (electric polarization) variation induced by the THz pulse and $E\left(t\right)$ is the electric field of the pulse at time $t$. The absolute value of $\mathrm{\Delta }\rho \left(t\right)$ 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.

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Abstract-Density dependence of the terahertz absorption spectra in optically excited semiconductors

1. Yen Thi Hai Le and Takeshi J. Inagaki^*

Article first published online: 25 NOV 2014

DOI: 10.1002/pssb.201451191

http://onlinelibrary.wiley.com/doi/10.1002/pssb.201451191/abstract

We theoretically investigate the terahertz (THz) absorption spectra in optically excited direct bandgap bulk semiconductors for various electron–hole (e–h) pair densities with analyzing Coulomb interacting two-band e–h systems. The exciton Mott transition (the density ionization) can clearly be studied because, above the critical density, the exciton structures originating from the intraexciton transitions disappear and only the Drude component is obtained. The present theory also predicts that the transition energy from 1s to 2p states is almost independent of density below the exciton Mott density. We also study the interplay between the density ionization of excitons and the abrupt increase in the ionization ratio and in the e–h quasi-chemical potential owing to the quantum dissociation.