  We present a systematic study of the mass dependence of the low-energy
electric dipole strength (LEDS) in Sn isotopes in the range $ A = 111 - 124$
based on data obtained with the Oslo method and with relativistic Coulomb
excitation in forward-angle ($p,p^\prime$) scattering. The combined data cover
an energy range of $2 - 20$ MeV which permits, with minimal assumptions, a
decomposition of the total strength into the contribution from the low-energy
tail of the isovector giant dipole resonance (IVGDR) and possible
resonance-like structures on top of it. In all cases, a resonance peaked at
about 8.3 MeV is observed, exhausting an approximately constant fraction of the
Thomas-Reiche-Kuhn (TRK) sum rule with a local maximum at $^{120}$Sn which
might be related to shell structure effects. For heavier isotopes ($A \geq
118$) a consistent description of the data requires the inclusion of a second
resonance centered at 6.5 MeV, representing the isovector response of the pygmy
dipole resonance (PDR). Its strength corresponds to a small fraction of the
total LEDS only and shows an approximately linear dependence on mass number.
The experimental results are also compared to ab initio-based microscopic
calculations to investigate the importance of an inclusion of quasiparticle
vibration coupling (qPVC) for a realistic description of the LEDS.
