Giant exciton Fano resonance in quasi-one-dimensional Ta2NiSe5
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We report the complex dielectric function of the quasi-one-dimensional chalcogenide Ta2NiSe5, which undergoes a structural phase transition presumably associated with exciton condensation below Tc = 326 K [Y. Wakisaka et al., Phys. Rev. Lett. 103, 026402 (2009); Y. F. Lu et al., Nat. Commun. 8, 14408 (2017)], and of the isostructural Ta2NiSe5, which does not exhibit such a transition. Using spectroscopic ellipsometry, we have detected exciton doublets with pronounced Fano line shapes in both the compounds. The exciton Fano resonances in Ta2NiSe5 display an order-of-magnitude higher intensity than those in Ta2NiSe5. In conjunction with prior theoretical work [E. Rashba, Sov. Phys. Semicond. 8, 807 (1975)], 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 Ta2NiSe5 and Ta2NiSe5 is confirmed by the pronounced temperature dependence of sharp interband transitions in the optical spectra, the peak energies and widths of which 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 exciton insulator ground state.
Larkin , T I , Yaresko , A N , Proepper , D , Kikoin , K A , Lu , Y F , Takayama , T , Mathis , Y -L , Rost , A W , Takagi , H , Keimer , B & Boris , A V 2017 , ' Giant exciton Fano resonance in quasi-one-dimensional Ta 2 NiSe 5 ' Physical Review. B, Condensed matter and materials physics , vol 95 , no. 19 , 195144 . DOI: 10.1103/PhysRevB.95.195144
Physical Review. B, Condensed matter and materials physics
© 2017 American Physical Society. This work has been made available online in accordance with the publisher’s policies. This is the author created, accepted version manuscript following peer review and may differ slightly from the final published version. The final published version of this work is available at https://doi.org/10.1103/PhysRevB.95.195144
This work was partly supported by JSPS KAKENHI Grants No. 24224010, No. 15H05852, and No. 17H01140.
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