Inhibition of the photoinduced structural phase transition in the excitonic insulator Ta$_2$NiSe$_5$

TL;DR

This study uses femtosecond spectroscopy to show that optical excitation in Ta$_2$NiSe$_5$ cannot induce a structural phase transition due to saturation effects, revealing a blocking mechanism in the excitonic insulator.

Contribution

It demonstrates a saturation-induced blocking mechanism preventing photoinduced structural phase transition in Ta$_2$NiSe$_5$, highlighting limits of optical control in excitonic insulators.

Findings

Optical response saturates at a critical fluence of 0.30 mJ/cm$^2$

Phonon dynamics strongly couple to free carrier excitation

No structural phase transition occurs under strong optical excitation

Abstract

Femtosecond time-resolved mid-infrared reflectivity is used to investigate the electron and phonon dynamics occurring at the direct band gap of the excitonic insulator Ta$_2$NiSe$_5$ below the critical temperature of its structural phase transition. We find that the phonon dynamics show a strong coupling to the excitation of free carriers at the \Gamma\ point of the Brillouin zone. The optical response saturates at a critical excitation fluence $F_C = 0.30~\pm~0.08$~mJ/cm$^2$ due to optical absorption saturation. This limits the optical excitation density in Ta$_2$NiSe$_5$ so that the system cannot be pumped sufficiently strongly to undergo the structural change to the high-temperature phase. We thereby demonstrate that Ta$_2$NiSe$_5$ exhibits a blocking mechanism when pumped in the near-infrared regime, preventing a nonthermal structural phase transition.