Nonlinear order parameter oscillations and lattice coupling in strongly-driven charge-density-wave systems

TL;DR

This study reveals anharmonic nonlinear oscillations of charge-density wave order and phonons in strongly driven 1T-TaS_2 and TbTe_3, showing mode coupling and modulation effects analyzed via time-dependent Ginzburg-Landau theory.

Contribution

It demonstrates the anharmonic response and mode coupling in CDW systems under strong laser excitation, a novel observation in these materials.

Findings

Periodic modulation of amplitude mode excitation efficiency

Anharmonic interaction between phonons and amplitude mode

Mode potential anharmonicity explained by broken symmetry state

Abstract

The anharmonic response of charge-density wave (CDW) order to strong laser-pulse perturbations in 1T-TaS_2 and TbTe_3 is investigated by means of a multiple-pump-pulse time-resolved femtosecond optical spectroscopy. We observe remarkable anharmonic effects hitherto undetected in the systems exhibiting collective charge ordering. The efficiency for additional excitation of the amplitude mode by a laser pulse becomes periodically modulated after the mode is strongly excited into a coherently oscillating state. A similar effect is observed also for some other phonons, where the cross-modulation at the amplitude-mode frequency indicates anharmonic interaction of those phonons with the amplitude mode. By analyzing the observed phenomena in the framework of time-dependent Ginzburg-Landau theory we attribute the effects to the anharmonicity of the mode potentials inherent to the broken symmetry state of the CDW systems.