FORMATION OF LARGE BIPOLARONS

The ground-state energy of the large bipolaron is investigated by use of a variational approach and a canonical transformation, which was in troduced in earlier studies of the bound polaron. For the bipolaron th e transformation reproduces both the oscillator-type strong-coupling r esults and the weak-coupling limit. The stability region for the bipol aron is compared with existing theories. The main advantage of the pre sent method is the transparency of the variational results. To the bes t of our knowledge, the present Ansatz yields the lowest bounds predic ted so far for two dimensions in the case of alpha > 4.5. The theory i s analyzed with reference to materials with high (epsilon(0)/epsilon i nfinity) ratio.