2D coherent spectroscopy signatures of exciton condensation in Ta$_2$NiSe$_5$

TL;DR

This paper demonstrates that two-dimensional coherent spectroscopy can effectively distinguish excitonic order from lattice-driven effects in Ta$_2$NiSe$_5$, revealing detailed insights into exciton condensation and collective modes.

Contribution

It introduces a method to use 2DCS to identify excitonic condensates and their collective modes, differentiating them from lattice effects in a realistic material model.

Findings

2DCS signals are strongly enhanced by condensate modes in the excitonic regime.

Single-particle and collective contributions overlap in linear response, but are distinguishable in 2DCS.

Amplitude mode contribution diminishes with increasing electron-phonon coupling.

Abstract

We show that the nonlinear optical response probed by two-dimensional coherent spectroscopy (2DCS) can discriminate between excitonic and lattice driven order. In the excitonic regime of a realistic model of Ta$_2$NiSe$_5$, the third order 2DCS signals are strongly enhanced by the condensate's amplitude and phase modes, with negligible contributions from single-particle excitations. In the linear optical response, in contrast, single-particle and collective-mode contributions overlap. With increasing electron-phonon coupling, the amplitude mode contribution to 2DCS initially remains robust, but then drops rapidly and remains small in the phonon-dominated regime -- even in systems with large order parameter. 2DCS also aids the detection of the massive relative phase mode, which is analogous to the Leggett mode in superconductors. Our analysis, based on the time-dependent Hartree-Fock approach, demonstrates that 2DCS can track the emergence of the symmetry-broken state and the crossover from Coulomb-driven to phonon-driven order.