Uncovering long-lived relaxation channel and exciton-phonon coupling in \textrm{Ta\textsubscript{2}NiSe\textsubscript{5}} via non-degenerate pump-probe spectroscopy

TL;DR

This study uses temperature-dependent pump-probe spectroscopy to reveal a long-lived exciton-phonon relaxation channel and distinct phonon modes in Ta2NiSe5, advancing understanding of its excitonic insulator phase.

Contribution

It uncovers a slow exciton-phonon relaxation process and distinguishes phonon modes with different couplings to the excitonic order, extending the temporal window of analysis.

Findings

Identified a long-lived relaxation component with a decay time of 280-600 ps.

Observed two coherent phonon modes at 1.0 and 2.9 THz with different coupling behaviors.

Demonstrated the importance of extended temporal detection in pump-probe spectroscopy.

Abstract

An excitonic insulator represents a quantum phase in which spontaneous condensation of excitons leads to novel many-body phenomena. Ta$_2$NSi$_5$ (TNSe), a layered narrow-gap semiconductor, has emerged as a model platform to probe these correlated excitonic phases and their underlying dynamics below 327 K. In this work, we investigate the nonequilibrium dynamics of TNSe using temperature-dependent, non-degenerate optical pump-probe spectroscopy with a 3.14 eV pump and a 1.57 eV probe, extending the accessible pump-probe delay window up to 500 ps. In addition to the well-established sub-picosecond relaxation channel ($\sim$ 0.7- 0.9 ps) associated with carrier cooling and recombination, accompanied by exciton reformation, we uncover a much slower recovery process with a decay time of $\sim$ 280-600~ps, significantly longer than previously reported. We attribute this unusually prolonged recovery to enhanced scattering between excitons and nonequilibrium phonons, which delays the re-establishment of equilibrium excitonic correlations. On top of this bi-exponential background, we observe two coherent phonon modes at 1.0 and 2.9 THz with distinctly different coupling behaviors. The 1.0 THz mode exhibits an order-parameter-like temperature dependence, consistent with strong coupling to the excitonic condensate in TNSe. In contrast, the 2.9 THz mode does not exhibit any discernible coupling to the excitonic order parameter, and appears to arise from anharmonic lattice dynamics associated with the structural phase transition. Together, these results elucidate the hierarchy of relaxation pathways in TNSe and highlight the importance of extending the temporal detection window in pump-probe measurements to fully capture long-lived exciton-phonon dynamics.