Terahertz-field-induced polar charge order in electronic-type dielectrics

Abstract

Ultrafast electronic-phase change in solids by light, called photoinduced phase transition, is a central issue in the field of non-equilibrium quantum physics, which has been developed very recently. In most of those phenomena, charge or spin orders in an original phase are melted by photocarrier generations, while an ordered state is usually difficult to be created from a non-ordered state by a photoexcitation. Here, we demonstrate that a strong terahertz electric-field pulse changes a Mott insulator of an organic molecular compound in κ-(ET)2Cu[N(CN)2]Cl (ET = bis(ethylenedithio)tetrathiafulvalene), to a macroscopically polarized charge-order state; herein, electronic ferroelectricity is induced by the collective intermolecular charge transfers in each dimer. In contrast, in an isostructural compound, κ-(ET)2Cu2(CN)3, which shows the spin-liquid state at low temperatures, a similar polar charge order is not stabilized by the same terahertz pulse. From the comparative studies of terahertz-field-induced second-harmonic-generation and reflectivity changes in the two compounds, we suggest the possibility that a coupling of charge and spin degrees of freedom would play important roles in the stabilization of polar charge order.

Control of electronic polarization via charge ordering and electron transfer: electronic ferroelectrics and electronic pyroelectrics

“… 48 In this compound, a THz field-induced macroscopically polarized charge-order state was observed. 49 …”

Recent advances in molecular ferroelectrics

“…In the model molecular ferroelectric croconic acid, a large polarization modulation in the sub-picosecond timescale has been achieved by using a terahertz electric field pulse obtained by optical excitation [64]. Similarly, a strong terahertz electric field pulse changes a Mott insulating state in the organic molecular compound κ-(ET) 2 Cu[N(CN) 2 ]Cl (ET = bis(ethylenedithio)tetrathiafulvalene) to a macroscopically ferroelectric CO state via a dissipationless electron transfer process [65]. Such complex experiments are useful for the understanding the hidden ferroelectric nature of correlated electron materials, but applicability in devices is at the laboratory level.…”

Terahertz radiation by quantum interference of excitons in a one-dimensional Mott insulator

“…Recent developments in femtosecond laser technology have enabled the generation of nearly monocyclic terahertz pulses with electric-field amplitudes far exceeding 100 kV/cm 1 – 4 , which originated from optical rectification (OR) in second-order nonlinear optical crystals excited with a visible femtosecond pulse. The intense terahertz pulse has been used as an excitation pulse to control the electronic state properties such as electric conductivity 5 – 8 , superconductivity 9 – 11 , dielectricity 12 – 18 , magnetism 19 – 23 , and topology 24 . Terahertz pulses with variable phase, frequency and amplitude are useful for controlling the electronic states of matter 25 – 28 but are challenging to achieve in the simple OR framework in which a second-order nonlinear optical crystal is excited with a femtosecond laser pulse in the transparent region.…”