Unveiling the Superconducting Mechanism of Ba0.51K0.49BiO3

The mechanism of high superconducting transition temperatures (T-c) in bismuthates remains under debate despite more than 30 years of extensive research. Our angle-resolved photoemission spectroscopy studies on Ba0.51K0.49BiO3 reveal an unexpectedly 34% larger bandwidth than in conventional density functional theory calculations. This can be reproduced by calculations that fully account for long-range Coulomb interactions-the first direct demonstration of bandwidth expansion due to the Fock exchange term, a long-accepted and yet uncorroborated fundamental effect in many body physics. Furthermore, we observe an isotropic superconducting gap with 2 Delta(0)/k(B)T(c) = 3.51 +/- 0.05, and strong electron-phonon interactions with a coupling constant lambda similar to 1.3 +/- 0.2. These findings solve a long-standing mystery-Ba0.51K0.49BiO3 is an extraordinary Bardeen-Cooper-Schrieffer superconductor, where long-range Coulomb interactions expand the bandwidth, enhance electron-phonon coupling, and generate the high T-c. Such effects will also be critical for finding new superconductors.