Collective modes and Raman response in Ta$_2$NiSe$_5$

TL;DR

This paper investigates the collective excitations in the excitonic insulator phase of Ta$_2$NiSe$_5$, revealing unexpected behaviors of amplitude and phase modes through a theoretical model.

Contribution

It introduces a semirealistic model and a time-dependent Hartree-Fock approach to analyze collective modes and Raman response in Ta$_2$NiSe$_5$, highlighting novel features of excitonic condensate excitations.

Findings

Amplitude mode frequency exceeds the single-particle gap.

The phase mode is massive and becomes heavier with increased electron-lattice coupling.

Scenarios where the phase mode does not appear as a sharp in-gap resonance are discussed.

Abstract

We explore the collective response in an excitonic insulator phase in Ta$_2$NiSe$_5$ using a semirealistic model including relevant lattice and electronic instabilities. We calculate order-parameter susceptibility and Raman response within a time-dependent Hartree-Fock approach. Contrary to the standard expectations, the amplitude mode frequency does not coincide with the single-particle gap but has a higher frequency. We find a phase mode that is massive because the excitonic condensation breaks a discrete symmetry only and that becomes heavier as the electron-lattice coupling is increased. These features are expected to apply to generic realistic excitonic insulators. We discuss scenarios under which the phase mode does not appear as a sharp in-gap resonance.