Strong interband interaction in the excitonic insulator phase of Ta2NiSe5

Abstract

Excitonic insulator (EI) was proposed in 60's as a distinct insulating state originating from pure electronic interaction, but its material realization has been elusive with extremely few material candidates and with only limited evidence such as anomalies in transport properties, band dispersions, or optical transitions. We investigate the real-space electronic states of the low temperature phase in Ta2NiSe5 with an atomic resolution to clearly identify the quasiparticle energy gap together with the strong electron-hole band renormalization using scanning tunneling microscopy (STM) and spectroscopy (STS). These results are in good agreement with the EI transition scenario in Ta2NiSe5. Our spatially-resolved STS data and theoretical calculations reveal further the orbital inversion at band edges, which indicates the exciton condensation close to the Bardeen-Cooper-Schrieffer regime. arXiv:1811.01552v1 [cond-mat.mtrl-sci]

Band hybridization at the semimetal-semiconductor transition of Ta2NiSe5 enabled by mirror-symmetry breaking

“…We estimate this to be 0.13 eV, otherwise hints of the conduction band would have been visible at the elevated temperatures here. These observations are consistent with the optical gap of 0.16 eV 12,18 , and with analysis of scanning tunneling spectra which show a similar gap size, with the chemical potential pinned closer to the conduction band minimum than the valence band maximum 15 .…”

“…It is therefore attractive to consider EI candidates with a small direct band gap/overlap. Ta 2 NiSe 5 is currently the most prominent material in this category, and a number of its physical properties are consistent with an EI-like transition [12][13][14][15][16][17] . Transport and optical spectroscopy indicate a near-zero band gap at high temperatures but the opening of a band gap at low temperatures 12,18 , with a clear resistivity anomaly at T c =327 K 19 .…”

Mapping the unoccupied state dispersions in Ta2NiSe5 with resonant inelastic x-ray scattering

“…There have been various DFT studies of Ta 2 NiSe 5 since 2012, and mainly two difficulties were encountered for this complicated material. First, the description of its normal phase band structure above T c is delicate because the electronic structure of Ta 2 NiSe 5 is close to being either a semimetal or a semiconductor, depending on the fine details of the DFT calculation, especially the choice of an appropriate functional for describing this material [11,12,17,18,33]. Second, the evaluation of the band gap in the low-temperature semiconducting phase is a difficult task because it is a combination of a hybridization band gap due to the low-symmetry monoclinic phase and a correlation gap due to the excitonic insulator phase.…”

“…At T c , the valence band maximum measured by ARPES at continuously shifts to higher binding energy up to about 0.18 eV [6,7], and the optical band gap increases in a similar way up to about 0.16 to 0.22 eV [8,9]. The nature of the low-energy electronic structure above T c is currently heavily debated and argued to be a semimetal [10][11][12], a zero-gap semiconductor [13], or a semiconductor [6,7,14]. The difficulty of classifying its electronic structure arises from the small size of the band gap at the Fermi level and the occurrence of fluctuations of the low-temperature phase [7], potentially hiding the true semimetallic nature of Ta 2 NiSe 5 [16].…”

“…In a configuration interaction picture, Ta 2 NiSe 5 has been shown to be a negative charge transfer material, and its Ni site has a mainly d 9 L ground state with some d 10 L 2 contribution [2]. In parallel, studies based on density functional theory (DFT) calculations disagree on its capability to capture the normal state of Ta 2 NiSe 5 , notably due to the difficulty of finding an appropriate functional for describing this material [11,12,17,18]. In addition, time-resolved studies based on pump-probe techniques support the existence of an excitonic insulator ground state at low temperature [14,[19][20][21][22].…”