Giant exciton Fano resonance in quasi-one-dimensional Ta2NiSe5

Abstract

We report the complex dielectric function of the quasi-one-dimensional chalcogenide Ta2NiSe5, which exhibits a structural phase transition that has been attributed to exciton condensation below Tc = 326 K [1, 2], and of the isostructural Ta2NiS5 which does not exhibit such a transition. Using spectroscopic ellipsometry, we have detected exciton doublets with pronounced Fano lineshapes in both the compounds. The exciton Fano resonances in Ta2NiSe5 display an order of magnitude higher intensity than those in Ta2NiS5. In conjunction with prior theoretical work [3], we attribute this observation to the giant oscillator strength of spatially extended exciton-phonon bound states in Ta2NiSe5. The formation of exciton-phonon complexes in Ta2NiS5 and Ta2NiSe5 is confirmed by the pronounced temperature dependence of sharp interband transitions in the optical spectra, whose peak energies and widths scale with the thermal population of optical phonon modes. The description of the optically excited states in terms of strongly overlapping exciton complexes is in good agreement with the hypothesis of an EI ground state. arXiv:1702.05953v1 [cond-mat.str-el]

Temperature-dependent excitonic superfluid plasma frequency evolution in an excitonic insulator, Ta2NiSe5

“…This peak centered near 3050 cm −1 seems to be closely related to the flat valence band (or the proposed excitonic condensation feature) near the Γ-point in the Brillouin zone, which was observed via ARPES studies 9,10,13 , since its temperature-dependent behavior and energy scale are similar to those of the flat valence band. The same sharp interband transition has been reported in a recent study of Ta 2 NiSe 5 probed using spectroscopic ellipsometry 15,16 . Interestingly, we also observe some more sharp peaks in the optical conductivity (associated with interband transitions) in a higher energy region above 5000 cm −1 , with these peaks displaying a temperature-dependence behavior similar to that of the 3050 cm −1 peak.…”

“…A recent study also shows that Ta 2 NiSe 5 is a zero-gap semiconductor, with a transitions to an EI occurring near 326 K (referred as the onset temperature ()) 15 . There was another very recent ellipsometry spectroscopic study on Ta 2 NiSe 5 16 ; the authors claimed that exciton-phonon complexes in Ta 2 NiS 5 and Ta 2 NiSe 5 are confirmed and their observation agrees with the hypothesis of an excitonic insulator ground state. In this article, we provide a new set of anisotropic optical data of Ta 2 NiSe 5 obtained using a different optical spectroscopy technique from the ellipsometry technique.…”

Photo-excited dynamics in the excitonic insulator Ta2NiSe5

“…At large time delays the gap has consequently already recovered and the deviation from the fit is therefore not present in the temperature dependence of the amplitude A ∞ . The value of Δ EI ≈ 100 meV obtained from the fits to both A 1 and A ∞ agrees roughly with the value of 2Δ ≈ 160-180 meV determined from ARPES and optical conductivity measurements at low temperatures [12,21] and an approximate transport gap of about 100 meV [19]. On the one hand the discrepancies can possibly be attributed to the fact that the time resolved measurements are probing an excited state and not the ground state itself: In this excited state a significant number of excitons are heated out of the ground state.…”

“…However we note that optical probes do show a gradual fill-in of the gap on increasing temperature [12]. i Recently we already showed that the coupling to the exciton condensate is very high at parallel polarization to the Ni-and Ta-chains, which leads to a complete depletion of the condensate even at low excitation densities [22].…”

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 .…”