Photoinduced High-Frequency Charge Oscillations in Dimerized Systems

TL;DR

This study investigates photoinduced charge oscillations in dimerized systems, revealing a transition from multi-frequency to single-frequency oscillations with a new high-energy peak, linked to electronic breathing modes and experimental reflectivity features.

Contribution

It demonstrates how strong photoexcitation induces high-frequency charge oscillations resembling electronic breathing modes in dimerized systems, connecting theoretical predictions with experimental observations.

Findings

Weak fields produce oscillations matching optical conductivity spectra.

Strong fields induce a dominant high-energy oscillation peak.

The high-energy peak relates to charge-transfer processes and electronic breathing modes.

Abstract

Photoinduced charge dynamics in dimerized systems is studied on the basis of the exact diagonalization method and the time-dependent Schr\"odinger equation for a one-dimensional spinless-fermion model at half filling and a two-dimensional model for $\kappa$-(bis[ethylenedithio]tetrathiafulvalene)$_2$X [$\kappa$-(BEDT-TTF)$_2$X] at three-quarter filling. After the application of a one-cycle pulse of a specifically polarized electric field, the charge densities at half of the sites of the system oscillate in the same phase and those at the other half oscillate in the opposite phase. For weak fields, the Fourier transform of the time profile of the charge density at any site after photoexcitation has peaks for finite-sized systems that correspond to those of the steady-state optical conductivity spectrum. For strong fields, these peaks are suppressed and a new peak appears on the high-energy side, that is, the charge densities mainly oscillate with a single frequency, although the oscillation is eventually damped. In the two-dimensional case without intersite repulsion and in the one-dimensional case, this frequency corresponds to charge-transfer processes by which all the bonds connecting the two classes of sites are exploited. Thus, this oscillation behaves as an electronic breathing mode. The relevance of the new peak to a recently found reflectivity peak in $\kappa$-(BEDT-TTF)$_2$X after photoexcitation is discussed.