VARIATIONAL CALCULATION FOR LARGE BIPOLARONS IN THE STRONG-COUPLING LIMIT

A variational calculation is presented for large bipolarons in the str ong-coupling limit. By means of the phonon coherent state, two coupled self-consistent integrodifferential equations for the center-of-mass and relative motions have been derived. Compared with previous works, lower binding energies for large bipolarons in three dimensions (3D) a nd two dimensions (2D) have been calculated. As a result, the values o f lower bounds to critical ratios between dielectric constants below w hich bipolarons can exist are obtained as eta(c) = 1.23 in 3D and eta( c) = 1.64 in 2D.