Electronic Structures and Phonon Spectra in Boronitride Superconductors LaMBN (M= Ni, Pt)

We have investigated electronic structures and phonon spectra of newly discovered isostructural superconductors LaNiBN (T_c = 4.1 K) and LaPtBN (T_c = 6.7 K). We have found that their electronic structures are substantially three-dimensional, leading to metallicity both in NiB (PtB) and the intervening LaN layers. Our ab initio phonon calculations show that almost all phonon modes contribute to the electron-phonon coupling (EPC) mechanism, reflecting that both layers are involved in the superconductivity. For LaNiBN, we obtain an EPC strength of \lambda = 0.52 and a logarithmically averaged characteristic phonon frequency of \omega_{log} = 376 K, leading to T_c = 3.9 K. Compared with the Ni B_{1g} mode in LaNiBN, the Pt B_{1g} mode in LaPtBN is reduced by ~70%, leading to a slightly enhanced \lambda = 0.56 and an ~20 % reduced \omega_{log}. The estimated T_c is 5.4 K for LaPtBN, in good agreement with the experiment. We do not find any indication of magnetic instability for either LaNiBN or LaPtBN, which implies that both systems are EPC mediated superconductors. Further, we have found an interesting trend of monotonic increase of T_c with respect to the boron height in the NiB (PtB) layer of both borocarbide and boronitride superconductors, which suggests a possible way to enhance T_c in these systems.