Orbital-selective superconductivity in $γ$-BiPd: An {\it ab initio} study}

Abstract: We investigate the superconducting (SC) properties of experimentally realized $\gamma$-BiPd by solving the Migdal-Eliashberg equations. Our study includes calculations of the SC gap $\Delta_{{\bf{k}}}$, the electron-phonon coupling strength $\lambda_{{\bf{k}}}$, the superconducting quasiparticle density of states ($N_{s}$), and the critical temperature $T_{c}$. $\gamma$-BiPd posses a complex FS, consisting of four Fermi sheets: two electron pockets and two hole pockets, each characterized by distinct atomic orbitals. Our key finding is that superconductivity in $\gamma$-BiPd is primarily orbital-selective, with significant contributions in $\Delta_{{\bf{k}}}$ and $\lambda_{{\bf{k}}}$ from the Bi $p_z$-orbital at the $K$-point, associated with the neck of electron pocket $E2$ on the FS. While our results reveal an anisotropic nature of $\Delta_{{\bf{k}}}$ and $\lambda_{{\bf{k}}}$ across the FS, we observe a single peak in $N_s$, consistent with experimental observations of single-gapped BCS superconductivity in this material. We also examine the influence of spin-orbit coupling (SOC) and find strong impact on both normal and superconducting properties, despite $\gamma$-BiPd being centrosymmetric. Including SOC results in the disappearance of the hole pocket $H2$ from the FS, leading to modification of $\lambda_{{\bf{k}}}$, $\Delta$ and $T_c$. Our calculated $T_c$ values are $\sim$1.26 K without SOC and 0.8 K with SOC, aligning well in order of magnitude with the experimental value of about 3.3 K.