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

The mechanism of high superconducting transition temperatures (Tc) 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 2Δ0/kBTc=3.51±0.05, and strong electron-phonon interactions with a coupling constant λ∼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 Tc. Such effects will also be critical for finding new superconductors.Received 16 March 2018Revised 8 July 2018DOI:https://doi.org/10.1103/PhysRevLett.121.117002© 2018 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectron-phonon couplingElectronic structureSuperconducting gapSuperconducting order parameterSuperconductor-insulator transitionPhysical SystemsHigh-temperature superconductorsOxidesTechniquesAngle-resolved photoemission spectroscopyElectron-correlation calculationsCondensed Matter, Materials & Applied Physics