Melting of excitonic insulator phase by an intense terahertz pulse in Ta$_2$NiSe$_5$

TL;DR

This paper investigates how intense terahertz pulses can induce the melting of the excitonic insulator phase in Ta$_2$NiSe$_5$ by causing exciton dissociation and metallization, revealing threshold behavior and different melting mechanisms.

Contribution

It demonstrates the electric-field-induced melting of the excitonic phase via quantum tunneling, distinguishing it from optical excitation effects in Ta$_2$NiSe$_5$.

Findings

Weak terahertz pulses reveal excitonic spectral features.

Strong terahertz pulses induce exciton dissociation and metallization.

Threshold electric field for exciton dissociation via quantum tunneling.

Abstract

In this study, the optical response to a terahertz pulse was investigated in the transition metal chalcogenide Ta$_2$NiSe$_5$, a candidate excitonic insulator. First, by irradiating a terahertz pulse with a relatively weak electric field (0.3 MV/cm), the spectral changes in reflectivity near the absorption edge due to third-order optical nonlinearity were measured and the absorption peak characteristic of the excitonic phase just below the interband transition was identified. Next, by irradiating a strong terahertz pulse with a strong electric field of 1.65 MV/cm, the absorption of the excitonic phase was found to be reduced, and a Drude-like response appeared in the mid-infrared region. These responses can be interpreted as carrier generation by exciton dissociation induced by the electric field, resulting in the partial melting of the excitonic phase and metallization. The presence of a distinct threshold electric field for carrier generation indicates exciton dissociation via quantum-tunnelling processes. The spectral change due to metallization by the electric field is significantly different from that due to the strong optical excitation across the gap, which can be explained by the different melting mechanisms of the excitonic phase in the two types of excitations.