Density functional theory methods are applied to investigate the properties
of the new superconductor β-YbAlB4​ and its polymorph
α-YbAlB4​. We utilize the generalized gradient approximation + Hubbard
U (GGA+U) approach with spin-orbit(SO) coupling to approximate the effects of
the strong correlations due to the open 4f shell of Yb. We examine closely
the differences in crystal bonding and symmetry of β-YbAlB4​ and
α-YbAlB4​. The in-plane bonding structure amongst the dominant
itinerant electrons in the boron sheets is shown to differ significantly. Our
calculations indicate that, in both polymorphs, the localized 4f electrons
hybridize strongly with the conduction sea when compared to the related
materials YbRh2​Si2​ and YbB2​. Comparing β-YbAlB4​ to the
electronic structure of related crystal structures indicates a key role of the
7-member boron coordination of the Yb ion in β-YbAlB4​ in producing its
enhanced Kondo scale and superconductivity. The Kondo scale is shown to depend
strongly on the angle between the B neighbors and the Yb ion, relative to the
x−y plane, which relates some of the physical behavior to structural
characteristics.Comment: 9 pages, 9 figures, 2 table