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Density functional theory methods are applied to investigate the properties of the new superconductor $beta$-YbAlB$_4$ and its polymorph $alpha$-YbAlB$_4$. 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 $beta$-YbAlB$_4$ and $alpha$-YbAlB$_4$. 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 4$f$ electrons hybridize strongly with the conduction sea when compared to the related materials YbRh$_{2}$Si$_{2}$ and YbB$_{2}$. Comparing $beta$-YbAlB$_4$ to the electronic structure of related crystal structures indicates a key role of the 7-member boron coordination of the Yb ion in $beta$-YbAlB$_4$ 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.
We analyze the key origin of quantum valence criticality in the heavy electron metal $beta$-YbAlB$_4$ evidenced in the sister compound $alpha$-YbAl$_{0.986}$Fe$_{0.014}$B$_4$. By constructing a realistic canonical model for $beta$-YbAlB$_4$, we evalu
$alpha$-YbAlB$_4$ is the locally isostructural polymorph of $beta$-YbAlB$_4$, the first example of an Yb-based heavy fermion superconductor which exhibits pronounced non-Fermi-liquid behavior above $T_{rm c}$. Interestingly, both $alpha$-YbAlB$_4$ an
We report an angle-resolved photoemission (ARPES) study of $beta$-YbAlB$_4$, which is known to harbor unconventional quantum criticality (QC) without any tuning. We directly observe a quasiparticle peak (QP), emerging from hybridization, characterize
$beta$-YbAlB$_4$ is the first Yb-based heavy fermion superconductor with $T_{rm c} = 80$ mK. We measured low temperature magnetization of high-purity single crystals down to $T=$ 25 mK. The measurements have revealed a considerable amount of volume f
Measurements of the electric resistivity $rho(T)$ under pressure up to 8 GPa were performed on high-quality single-crystals of the Yb-based heavy fermion system $beta$-YbAlB$_4$ in the temperature range $2<T<$ 300 K. In the resistivity data, we obser