Fig. 2.

(A) Binding energy of spin singlet (diamonds) and triplet (circle) pairs, obtained by solving the lattice two-electron problem for $t_0/\mathrm{\Delta }=0.1$ and $U_B=3V_0$. The lowest two-particle bound state has spin-1, and its energy is perfectly captured by the effective continuum model in Eq. 8 (gray line). We also show the (B) triplet and (C and D) degenerate singlet bound state wave functions as a function of the relative distance between the two doped charges, with $V_0=2\mathrm{\Delta }$. The radius and color of the circles indicate the amplitude and phase, respectively, of the wave function on each site.