Effects of Lattice Anisotropy on Bipolaron Symmetry

We study the pairing symmetry of large bipolarons as a function of lattice anisotropy and the binding energy of a small polaron, using variational wavefunctions. We find that an $s$-wave state is stable over a wide range of parameters. However below a threshold binding energy, we find evidence for a singlet to triplet pairing transition. We find that $p$-wave states are stable for sufficient anisotropy, with lobes oriented perpendicular to the easy hopping direction.