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We perform a comprehensive study of charge excitations in a bilayer electron system in the presence of the long-range Coulomb interaction (LRC). Our major point is to derive formulas of the LRC that fully respect the bilayer lattice structure. This is an extension of the LRC obtained by Fetter in the electron-gas model 50 years ago and can now be applicable to any electron density. We then provide general formulas of the charge susceptibility in the random phase approximation and study them numerically. The charge ordering tendency is not found and instead we find two plasmon modes: ω+ and ω− modes. Our second major point is to elucidate their spectral weight distribution and the effect of electron tunneling between the layers. The spectral weight of the ω± modes does not have 2π periodicity along the qzc direction; qz and c are momentum and the lattice constant along the z direction, respectively. The ω+ mode loses spectral weight at in-plane momentum q∥ = (0,0) at qzc = 2nπ with n being integer whereas the ω− mode has no spectral weight at qzc = 0 for any q∥ but acquires sizable spectral weight at qzc = 2nπ with n ̸= 0. Both ω± modes are gapped at q∥ = (0,0). When qzc is away from 2nπ, the ω± modes show striking behavior. When the intrabilayer hopping tz is relatively small (large), the ω− (ω+) mode becomes gapless at q∥ = (0,0) whereas the ω+ (ω−) mode retains the gap. However, when the interbilayer hopping integral tz′ is taken into account, the gapless mode acquires a gap at q∥ = (0, 0) and both ω± modes are gapped at any qzc. To highlight the special feature of the LRC, we also clarify a difference to the case of a short-range interaction. While the strong electron correlation effects are not included, the present theory captures available data of the charge excitations observed by resonant inelastic x-ray scattering for Y-based cuprate superconductors.

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• In Copyright
  

  ©2025 American Physical Society

Keyword: 電荷ダイナミクス, プラズモン, 長距離クーロン力, 二層系, 銅酸化物高温超伝導体

Date published: 2025-02-19

Publisher: American Physical Society (APS)

Journal:

• Physical Review B (ISSN: 24699950) vol. 111 issue. 8 085138

Funding:

• Japan Society for the Promotion of Science JP20H01856
• Ministry of Education, Culture, Sports, Science and Technology

Manuscript type: Author's version (Accepted manuscript)

MDR DOI: https://doi.org/10.48505/nims.5350

First published URL: https://doi.org/10.1103/physrevb.111.085138

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Updated at: 2025-03-03 16:30:09 +0900

Published on MDR: 2025-03-03 16:30:10 +0900