Ultrafast Electron and Molecular Dynamics in Photoinduced and Electric-Field-Induced Neutral–Ionic Transitions

Takeshi Morimoto,Hiroshi Okamoto,Tatsuya Miyamoto

doi:10.3390/cryst7050132

Takeshi Morimoto, Hiroshi Okamoto + Show 1 more

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https://doi.org/10.3390/cryst7050132

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Abstract

Mixed-stacked organic molecular compounds near the neutral–ionic phase boundary, represented by tetrathiafulvalene-p-chloranil (TTF-CA), show a unique phase transition from a paraelectric neutral (N) phase to a ferroelectric ionic (I) phase when subjected to decreasing temperature or applied pressure, which is called an NI transition. This NI transition can also be induced by photoirradiation, in which case it is known as a prototypical ‘photoinduced phase transition’. In this paper, we focus on the ultrafast electron and molecular dynamics in the transition between the N and I states induced by irradiation by a femtosecond laser pulse and a terahertz electric-field pulse in TTF-CA. In the first half of the paper, we review the photoinduced N-to-I transition in TTF-CA studied by femtosecond-pump-probe reflection spectroscopy. We show that in the early stage of the transition, collective charge transfers occur within 20 fs after the photoirradiation, and microscopic one-dimensional (1D) I domains are produced. These ultrafast I-domain formations are followed by molecular deformations and displacements, which play important roles in the stabilization of photogenerated I domains. In the photoinduced I-to-N transition, microscopic 1D N domains are also produced and stabilized by molecular deformations and displacements. However, the time characteristics of the photoinduced N-to-I and I-to-N transitions in the picosecond time domain are considerably different from each other. In the second half of this paper, we review two phenomena induced by a strong terahertz electric-field pulse in TTF-CA: the modulation of a ferroelectric polarization in the I phase and the generation of a large macroscopic polarization in the N phase.

Highlights

The control of an electronic phase and related macroscopic properties by photoirradiation has recently attracted much attention [1]
Pump-probe opticalAspectroscopy using spectroscopy such a tuneable femtosecond tuneable femtosecond laser system enables the measurement of transient changes in optical laser system enables the measurement of transient changes in optical absorption or reflectivity spectra absorption or reflectivity spectra induced by photoirradiation, from which we can extract the induced by photoirradiation, from which we can extract the electronic-state changes and the dynamics electronic-state changes and the dynamics of electron, spin, and lattice degrees of freedom during of electron, spin, and lattice degrees of freedom during and after the photoinduced phase transition (PIPT)
We review our studies of the photoinduced transitions from the N phase to the I phase and from the I phase to the N phase (IN transition) in TTF-CA based upon femtosecond-pump-probe spectroscopy

Summary

Introduction

The control of an electronic phase and related macroscopic properties by photoirradiation has recently attracted much attention [1]. This phenomenon is called a photoinduced phase transition (PIPT), and is important as a new phenomenon in the fields of solid-state physics and materials science, and as a useful mechanism applicable to future optical switching and memory devices. When we aim to realize a PIPT in the subpicosecond time scale, promising targets are correlated electron materials, in which a photoexcited state causes a change in the surrounding electron (spin) systems via strong electron–electron (e–e) interactions and gives rise to a conversion to another electronic phase.

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