Muon spin relaxation and magnetic measurements on Ba0.63K0.37BiO3:: Evidence for polaronic strong-coupling phonon-mediated pairing

A penetration depth measurement was carried out on the optimally doped bismuthate superconductor Ba0.63K0.37BiO3 (T-c=29.2 K) using the muon-spin-relaxation technique. We find that the temperature dependence of the penetration depth lambda(T) of this compound is in excellent agreement with strong-coupling phonon-mediated superconductivity with a reduced energy gap of 2 Delta(0)/k(B)T(c)=4.4 and a retarded electron-phonon coupling constant lambda(e-p)=1.4. The observed large reduced energy gap rules out the possibility of pairing mechanisms based on coupling to high-energy electronic excitations. Quantitative data analyses indicate that high-temperature superconductivity in bismuthates arises from the Cooper pairing of polaronic charge carriers.