Comprehensive study of bond bipolaron superconductivity on the triangular lattice

Abstract

We employ the diagrammatic Monte Carlo method with a lattice path-integral formulation for both electron and phonon degrees of freedom to investigate the formation and properties of bond polarons and bipolarons on a two-dimensional triangular lattice. In the deep adiabatic regime (ω/t<1.0), single polarons remain light with a small effective mass, while bipolarons remain compact and lightweight, resulting in a high superfluid transition temperature Tc. We systematically study the dependence of Tc on electron-phonon coupling strength and on-site interaction in the bond bipolaron model. Our results show that a moderate on-site repulsion enhances Tc by stabilizing compact yet lightweight bipolarons, leading to high-Tc superconductivity in the dilute limit. Notably, the triangular lattice sustains relatively high Tc across a wide range of phonon frequencies, outperforming square lattice geometry. This enhancement arises from the higher coordination number and the bond-centered nature of the electron-phonon coupling. These findings suggest that triangular lattice geometry offers a promising platform for realizing high-Tc bipolaronic superconductivity.

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