Intermediate-Coupling Polaron Effective Mass

We have calculated the polaron effective mass $m^{*}$ by extending the variational ansatz of a previous paper, where, in the Lee-Low-Pines representation, pair correlations between wave vectors of virtually emitted phonons are taken into account. Our results suggest that (1) effective masses calculated by fourth-order perturbation theory lie below the true values but are quite accurate for $\alpha <~3$ and (2) the Feynman theory gives effective masses which are somewhat too high in the intermediate-coupling region. For $\alpha \stackrel{\sim }{<}4.5$ we propose the estimate $\frac{m^{*}}{m}=1+\frac{\alpha }{6}+0.02363\mathrm{}{\alpha }^2+0.0014\mathrm{}{\alpha }^3$.