  Within the framework of a multiphase transport (AMPT) model that includes
both initial partonic and final hadronic interactions, we show that including
mean-field potentials in the hadronic phase leads to a splitting of the
elliptic flows of particles and their antiparticles, providing thus a plausible
explanation of the different elliptic flows between $p$ and ${\bar p}$, $K^+$
and $K^-$, and $\pi^+$ and $\pi^-$ observed in recent Beam Energy Scan (BES)
program at the Relativistic Heavy-Ion Collider (RHIC).
