  Based on the framework of an extended multiphase transport model with
mean-field potentials in both the partonic phase and the hadronic phase, we
explain the elliptic flow difference between $\pi^+$ and $\pi^-$ in the
beam-energy scan program at the relativistic heavy-ion collider by
incorporating the vector-isovector potential for quarks and antiquarks with
different isospins. It is found that the isospin splitting of pion elliptic
flow favors a strong vector-isovector interaction, and thus serves as a probe
of the quark matter equation of state as well as the QCD phase structure at
finite baryon and isospin chemical potentials.
