Finite-temperature photoemission in the extended Falicov-Kimball model: a case study for Ta$_2$NiSe$_5$

TL;DR

This study uses advanced numerical techniques to analyze the photoemission spectra of the extended Falicov-Kimball model at various temperatures, revealing features indicative of an excitonic insulator state in Ta$_2$NiSe$_5$ and explaining experimental observations.

Contribution

It applies the time-dependent density-matrix renormalization group method with infinite boundary conditions to directly simulate dynamical correlations in the thermodynamic limit for the first time in this context.

Findings

Detection of a two-peak structure near Fermi momenta suggesting BCS-like excitonic insulator

Observation of conduction band leakage at higher temperatures

Explanation of non-interacting band features in photoemission experiments

Abstract

Utilizing the unbiased time-dependent density-matrix renormalization group technique, we examine the photoemission spectra in the extended Falicov-Kimball model at zero and finite temperatures, particularly with regard to the excitonic insulator state most likely observed in the quasi-one-dimensional material Ta$_2$NiSe$_5$. Working with infinite boundary conditions, we are able to simulate all dynamical correlation functions directly in the thermodynamic limit. For model parameters best suited for Ta$_2$NiSe$_5$ the photoemission spectra show a weak but clearly visible two-peak structure, around the Fermi momenta $k\simeq\pm k_{\rm F}$, which suggests that Ta$_2$NiSe$_5$ develops an excitonic insulator of BCS-like type. At higher temperatures, the leakage of the conduction-electron band beyond the Fermi energy becomes distinct, which provides a possible explanation for the bare non-interacting band structure seen in time- and angle-resolved photoemission spectroscopy experiments.