A comprehensive study of bond bipolaron superconductivity in 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 adiabatic regime ($\omega/t < 1.0$), single polarons remain light with a small effective mass, while bipolarons remain compact and lightweight, resulting in a high superfluid transition temperature $T_c$. We systematically study the dependence of $T_c$ on electron-phonon coupling strength and on-site interaction in the bond bipolaron model. Our results show that a moderate on-site repulsion enhances $T_c$ by stabilizing compact yet lightweight bipolarons, leading to high-$T_c$ superconductivity in the dilute limit. Notably, the triangular lattice sustains relatively high $T_c$ 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-$T_c$ bipolaronic superconductivity.