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arXiv:2204.09085 · Physical review. B./Physical review. B

Photoinduced metallization of excitonic insulators

Satoshi Ejima, Florian Lange, Holger Fehske

Published Jun 21, 2022• 12 citations
arXivHTMLPDF

Abstract

Utilizing the time-dependent density-matrix renormalization-group technique, we numerically prove photoinduced pairing states in the extended Falicov-Kimball model (EFKM) at half-filling, both with and without internal SU(2) symmetry. In the time-dependent photoemission spectra, an extra band appears above the Fermi energy after pulse irradiation, indicating an insulator-to-metal transition. Even in the absence of the SU(2) structure, the pair correlations are enhanced during the pump, and afterwards they decrease over time. This implies the possible metallization of ${\mathrm{Ta}}_{2}\mathrm{Ni}{\mathrm{Se}}_{5}$, a strong candidate for an excitonic insulator material, for which the EFKM is considered to be the minimal theoretical model. Simulating the photoemission with optimized pulse parameters, we demonstrate a photoinduced quantum phase transition, in accord with recent findings in time- and angle-resolved photoemission spectroscopy experiments on ${\mathrm{Ta}}_{2}\mathrm{Ni}{\mathrm{Se}}_{5}$.

DOI
10.1103/physrevb.105.245126

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Finite-temperature photoemission in the extended Falicov-Kimball model: a case study for Ta2_22​NiSe5_55​
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 2 NiSe _5 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 2 NiSe _5 5 the photoemission spectra show a weak but clearly visible two-peak structure, around the Fermi momenta k\simeq\pm k_{F} k ≃ ± k F , which suggests that Ta _2 2 NiSe _5 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.
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We consider an extended spinless Falicov-Kimball model at an arbitrary doping level, focusing on the range of parameter values where a uniform excitonic insulator is stabilized at half-filling. In the two-dimensional case, we compare the properties of possible uniform phases and construct the Hartree--Fock phase diagrams, which include sizable phase separation regions. It is seen that the excitonic insulator can appear as a component phase in a mixed-phase state in a broad interval of doping levels. In addition, in a certain range of parameter values the excitonic metal (doped excitonic insulator) is identified as the lowest-energy uniform phase. We suggest that this phase, which is unstable with respect to phase separation, may be stabilized when the phase separation is suppressed by the long-range Coulomb interaction. Overall, we find that excitonic correlations can affect the behavior of the system relatively far away from half-filling.