Terahertz light driven coherent excitation of a zone-folded Raman-active phonon mode in the Spin-Ladder System $\alpha'$-NaV$_2$O$_5$

TL;DR

This study demonstrates that intense terahertz pulses can coherently excite a zone-folded Raman-active phonon in $ ext{NaV}_2 ext{O}_5$, revealing nonlinear phonon coupling and enabling selective control of lattice dynamics in quantum materials.

Contribution

It provides the first experimental evidence of THz-driven coherent excitation of zone-folded phonons via nonlinear coupling in a spin-ladder system.

Findings

THz pulses induce coherent phonon oscillations at 1.85 THz.

NIR pulses do not produce measurable vibrational dynamics.

Nonlinear coupling between Raman and IR-active phonons is responsible.

Abstract

We investigate the out-of-equilibrium lattice dynamics in the spin-ladder system $\alpha'$-NaV$_2$O$_5$ using intense terahertz (THz) pump and near-infrared (NIR) probe spectroscopy. When quasi-single-cycle THz pulses interact with $\alpha'$-NaV$_2$O$_5$ in its low-temperature, dimerized charge-ordered phase, they induce coherent oscillations in the time domain at the zone-folded Raman-active phonon frequency of 1.85 THz. By combining pump-probe measurements with lattice dynamics modeling based on equation-of-motion approach, we propose that these oscillations arise from a nonlinear coupling between Raman-active and infrared (IR)-active phonon modes, with the latter being resonantly excited by the THz pulses. In contrast, excitation with NIR femtosecond laser pulses does not produce measurable vibrational dynamics, highlighting the unique potential of THz-driven, nonlinear light-matter interactions for the coherent and selective control of structural dynamics in quantum materials.