Superlight small bipolarons

Abstract

Recent angle-resolved photoemission spectroscopy (ARPES) has identified that a finite-range Fröhlich electron-phonon interaction (EPI) with c-axis polarized optical phonons is important in cuprate superconductors, in agreement with an earlier proposal by Alexandrov and Kornilovitch. The estimated unscreened EPI is so strong that it could easily transform doped holes into mobile lattice bipolarons in narrow-band Mott insulators such as cuprates. Applying a continuous-time quantum Monte-Carlo algorithm (CTQMC) we compute the total energy, effective mass, pair radius, number of phonons and isotope exponent of lattice bipolarons in the region of parameters where any approximation might fail taking into account the Coulomb repulsion and the finite-range EPI. The effects of modifying the interaction range and different lattice geometries are discussed with regards to analytical strong-coupling/non-adiabatic results. We demonstrate that bipolarons can be simultaneously small and light, provided suitable conditions on the electron-phonon and electron-electron interaction are satisfied. Such light small bipolarons are a necessary precursor to high-temperature Bose-Einstein condensation in solids. The light bipolaron mass is shown to be universal in systems made of triangular plaquettes, due to a novel crab-like motion. Another surprising result is that the triplet-singlet exchange energy is of the first order in the hopping integral and triplet bipolarons are heavier than singlets in certain lattice structures at variance with intuitive expectations. Finally, we identify a range of lattices where superlight small bipolarons may be formed, and give estimates for their masses in the anti-adiabatic approximation.

31 pages. To appear in J. Phys.: Condens. Matter, Special Issue 'Mott's Physics'

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• Electron-phonon interaction in Strongly Correlated Systems
• Trapping of lattice polarons by impurities
• Bipolarons from long range interactions: Singlet and triplet pairs in the screened Hubbard-Froehlich model on the chain
• Light and stable triplet bipolarons on square and triangular lattices
• "Quantum bipolaron" superconductivity from quadratic electron-phonon coupling
• Mobile small bipolarons on a three-dimensional cubic lattice
• Enhancement of gaps in thin graphitic films for heterostructure formation
• Aspects of strong electron-phonon coupling in superconductivity of compressed metal hydrides MH6 with Im-3m structure
• Optimal interlayer hopping and high temperature Bose-Einstein condensation of local pairs in quasi 2D superconductors
• Ubiquitous light real-space pairing from long-range hopping and interactions
• Mass of a lattice polaron from extended Holstein model using Yukawa potential
• Bipolarons and polarons in the Holstein-Hubbard model: Analogies and differences
• Superlight pairs in face-centred-cubic extended Hubbard models with strong Coulomb repulsion
• Strictly localised triplet dimers on one- and two-dimensional lattices
• Real-space d-wave superconductivity from weak attraction