  Magnetic measurements have been carried out in the superconducting and normal
states of the optimally doped nonmagnetic bismuthate superconductor
Ba0.63K0.37BiO3. The magnetic data along with previous muSR, resistivity, and
tunneling data consistently show that there is a large polaronic enhancement in
the density of states and effective electron-phonon coupling constant. The
first-principle calculation within the density-functional theory indicates a
small electron-phonon coupling constant of about 0.3-0.4, which can only lead
to about 1 K superconductivity within the conventional phonon-mediated
mechanism. Remarkably, the polaronic effect increases the electron-phonon
coupling constant to about 1.4, which is large enough to leads to 32 K
superconductivity. The present work thus uncovers the mystery of
high-temperature superconductivity in bismuthate superconductors, which will
also provide important insight into the pairing mechanism of other
high-temperature superconductors.
