First-principles electron-phonon interactions and polarons in the parent cuprate La₂CuO₄

Abstract

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, La₂⁢CuO₄ (LCO), by means of first-principles calculations. Leveraging parameter-free Hubbard-corrected density functional theory, we obtain a ground state with the 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 bond stretching and bond bending in the Cu-O plane 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 doped lanthanum cuprates is also present in the parent compound, and elucidate its microscopic origin.

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