First-Principles Electron-Phonon Interactions and Polarons in the Parent Cuprate La_2CuO_4
arxiv(2024)
摘要
Understanding electronic interactions in high-temperature superconductors is
an outstanding challenge. In the widely studied cuprate materials, experimental
evidence points to strong electron-phonon (e-ph) coupling and broad
photoemission spectra. Yet, the microscopic origin of this behavior is not
fully understood. Here we study e-ph interactions and polarons in a
prototypical parent (undoped) cuprate, La_2CuO_4 (LCO), by means of
first-principles calculations. Leveraging parameter-free Hubbard-corrected
density functional theory, we obtain a ground state with band gap and Cu
magnetic moment in nearly exact agreement with experiments. This enables a
quantitative characterization of e-ph interactions. Our calculations reveal
two classes of longitudinal optical (LO) phonons with strong e-ph coupling to
hole states. These modes consist of Cu-O plane bond-stretching and bond-bending
as well as vibrations of apical O atoms. The hole spectral functions, obtained
with a cumulant method that can capture strong e-ph coupling, exhibit broad
quasiparticle peaks with a small spectral weight (Z≈0.25) and
pronounced LO-phonon sidebands characteristic of polaron effects. Our
calculations predict features observed in photoemission spectra, including a
40-meV peak in the e-ph coupling distribution function not explained by
existing models. These results show that the universal strong e-ph coupling
found experimentally in lanthanum cuprates is an intrinsic feature of the
parent compound, and elucidates its microscopic origin.
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