Signatures of ultrafast reversal of excitonic order in Ta$_2$NiSe$_5$

TL;DR

This paper demonstrates that the excitonic order parameter in Ta$_2$NiSe$_5$ can be ultrafastly reversed using laser pulses, combining theoretical modeling with experimental evidence from time-resolved reflectivity.

Contribution

It introduces a method to controllably reverse the excitonic order in Ta$_2$NiSe$_5$ on ultrashort timescales, expanding ultrafast control techniques beyond spin and charge orders.

Findings

Evidence of excitonic order parameter reversal in Ta$_2$NiSe$_5$

Observation of anomalous phonon behavior linked to order reversal

Theoretical derivation of equations of motion for the order parameter

Abstract

In the presence of electron-phonon coupling, an excitonic insulator harbors two degenerate ground states described by an Ising-type order parameter. Starting from a microscopic Hamiltonian, we derive the equations of motion for the Ising order parameter in the phonon coupled excitonic insulator Ta$_2$NiSe$_5$ and show that it can be controllably reversed on ultrashort timescales using appropriate laser pulse sequences. Using a combination of theory and time-resolved optical reflectivity measurements, we report evidence of such order parameter reversal in Ta$_2$NiSe$_5$ based on the anomalous behavior of its coherently excited order-parameter-coupled phonons. Our work expands the field of ultrafast order parameter control beyond spin and charge ordered materials.