Effects of phonon-phonon coupling on properties of pygmy resonance in ${}^{124-132}$Sn

Starting from an effective Skyrme interaction we study effects of phonon-phonon coupling on the low-energy electric dipole response in ${}^{124-132}$Sn. The QRPA calculations are performed within a finite rank separable approximation. The inclusion of two-phonon configurations gives a considerable contribution to low-lying strength. Comparison with available experimental data shows a reasonable agreement for the low-energy $E1$ strength distribution.