Chemical bonding dictates drastic critical temperature difference in two seemingly identical superconductors.

Though YB and LaB share the same crystal structure, atomic valence electron configuration, and phonon modes, they exhibit drastically different phonon-mediated superconductivity. YB superconducts below 8.4 K, giving it the second-highest critical temperature of known borides, second only to MgB. LaB does not superconduct until near-absolute zero temperatures (below 0.45 K), however. Though previous studies have quantified the canonical superconductivity descriptors of YB's greater Fermi-level (E) density of states and higher electron-phonon coupling (EPC), the root of this difference has not been assessed with full detail of the electronic structure. Through chemical bonding, we determine low-lying, unoccupied 4f atomic orbitals in lanthanum to be the key difference between these superconductors. These orbitals, which are not accessible in YB, hybridize with π B-B bonds and bring this π-system lower in energy than the σ B-B bonds otherwise at E. This inversion of bands is crucial: the optical phonon modes we show responsible for superconductivity cause the σ-orbitals of YB to change drastically in overlap, but couple weakly to the π-orbitals of LaB. These phonons in YB even access a crossing of electronic states, indicating strong EPC. No such crossing in LaB is observed. Finally, a supercell (the M k-point) is shown to undergo Peierls-like effects in YB, introducing additional EPC from both softened acoustic phonons and the same electron-coupled optical modes as in the unit cell. Overall, we find that LaB and YB have fundamentally different mechanisms of superconductivity, despite their otherwise near-identity.