Charge and dielectric response to terahertz pulses in the charge-ordered phase of α−(BEDT−TTF)2I3

Abstract

We investigated the dielectric properties of the charge-ordered phase of α-(BEDT-TTF) 2 I 3 using exact numerical calculations of an extended Hubbard model. The electronic contribution to the electric polarization (electronic polarization)P of the charge-ordered ground state is obtained by directly calculating the current when transfer integrals were changed adiabatically from symmetric integrals to integrals for the charge-ordered phase without inversion symmetry. The angle of P from the positive b−axis is 36 • , which is consistent with experimental results and previous theoretical results based on density functional theory. Furthermore, we numerically calculated the dynamics induced by terahertz (THz) pulse excitation. Both the THz-pulse induced variation of the electronic polarization magnitude and that of the charge disproportionation that shows the charge-order amplitude, are largest when the electric field of the THz pulse andP have almost the same direction. This originates from the charge transfer through bond b2 ′ being dominant in both the adiabatic flow of current and THz pulse excitation. These results reproduce important features of experimental results of THz-pulse induced dynamics.Ferroelectric materials are widely used in various devices, such as random-access memory devices, capacitors, sensors, piezoelectric actuators, and optical devices. 1-3 In conventional ferroelectrics, electric polarization is governed by the rotation of polar molecules (orderdisorder type) or the displacement of ions (displacive type), and the typical time constants of polarization change vary from microseconds to milliseconds. If the ferroelectric polarization could be controlled in the picosecond time domain, ferroelectric materials could be used for advanced switching devices. Recently, ferroelectricity that arises from electron transfer, which is termed electronic ferroelectricity, 4-6 has been observed in various materials, such as multiferroics, 7-14 transition metal oxides, 15-17 and organic molecular compounds, 18-37 and much faster polarization switching is expected for the new type of ferroelectricity. 5,6 This paper focuses on α-(BEDT-TTF) 2 I 3 (BEDT-TTF: bis[ethylenedithio]tetrathiafulvalene) among various electronic ferroelectrics. The charge-transfer salts (BEDT-TTF) 2 X (X: a monovalent anion) can be described as quasi-two-dimensional strongly correlated electron systems with a quarter-filled valence band in the hole picture.As a result of the strong Coulomb interaction, α-(BEDT-TTF) 2 I 3 exhibits charge-ordering transition and a horizontal charge order forms below the transition temperature. [26][27][28][38][39][40][41][42][43][44][45][46] We show the lattice structure of α-(BEDT-TTF) 2 I 3 in Fig. 1. In the charge-ordered phase, the crystal symmetry is P 1 with no inversion symmetry, and there are crystallographically 4 nonequivalent sites and 12 nonequivalent bonds. They are labeled as indicated in Fig. 1.Sites A and B (A ′ and C) are charge rich (charge poor) in the horizontal charge-ordered state.Th...

Terahertz pulse induced transitions between ionic and neutral phases and electronic polarization reversal in TTF-CA

“…As shown in our previous paper, i n (t) is strongly affected by this finite-size effect when A(t) = 0. 54,55 We therefore consider the dynamics induced by the double half-cycle pulses.…”

“…Terahertz (THz) pulses are powerful tools for investigating the ultrafast dielectric response of electronic ferroelectrics. [52][53][54][55] The THz-pump optical-probe and secondharmonicgeneration-probe measurements in TTF-CA have revealed the instantaneous re- sponse of charge and the electronic contribution to the electric polarization (electronic polarization). 52 It has been theoretically shown that the instantaneous response originates from the adiabatic nature of the THz-pulse excited state.…”