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_22NiSe_55. 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_22NiSe_55 the photoemission spectra show a weak but clearly visible two-peak structure, around the Fermi momenta k\simeq\pm k_{F}k≃±kF, which suggests that Ta_22NiSe_55 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.
Highlights
Results are obtained by t-density-matrix renormalization group method (DMRG) using infinite boundary conditions (IBC) and a window size LW = 200 to keep the values of the dynamical correlation functions on the boundaries less than 10−8 up to the target time tfin · tc = 16
It should be noted at this point that simple BCS-like mean-field approaches will cover the gap opening effect and basically reproduce the dispersions of the coherent part of the spectral functions but fail in giving the correct distribution of the spectral weight and describing the incoherent contributions to the spectra
We examined the ground-state and spectral properties of the half-filled extended Falicov-Kimball model (EFKM) in one spatial dimension where mean-field-like approaches usually fail
Summary
Superconductivity is a classic example of emerging quantum coherence on a macroscopic scale. Valence-band holes and conducting-band electrons may form excitonic pairs in semimetals with small band overlap or in semiconductors with a small band gap, triggered by their Coulomb attraction, where the excitons are expected to “condense” in a macroscopic coherent state at low temperatures under restrictive conditions [1,2,3,4,5] This state is called excitonic insulator (EI) because it exhibits no “super-transport" properties. [23] attributed this low-energy peak to the giant oscillator strength of spatially extended exciton-phonon bound states, while the theoretical work [24] concluded that the interorbital Coulomb interaction between valence and conduction bands should be sufficient to explain this peak Another important issue is whether the formation of the EI in Ta2NiSe5 follows a BCS or a BEC scenario. Appendix B provides results for the photoemission spectra in the 1D half-filled Hubbard model
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