Insights into superconductivity of LaO from experiments and first-principles calculations

To explore the mechanism of superconductivity in lanthanum monoxide (LaO) film, using first-principles calculations we investigated the electronic properties, phonon vibration, and electron-phonon coupling characteristics in LaO with a tetragonally distorted rocksalt structure in accordance with our experiments. It is demonstrated that LaO shows coexisting metallic and ionic characteristics, and the electronic density of states at the Fermi level is primarily from La-$d$ and O-$p$ orbitals. The low-frequency phonons from La atoms dominate 61% of the electron-phonon coupling constant, and the high-frequency phonons derived from O atoms contribute the rest. The calculated ${T}_{\mathrm{c}}$ of about 5.37 K is consistent with the experimental result, indicating that the superconductivity in LaO can be clearly illustrated by the phonon-mediated mechanism and McMillan's formula. The calculated results reveal that comparing LaO with other lanthanum monochalcogenides, the electronic properties vary more significantly than the phonon properties and lead to a stronger electron-phonon coupling in LaO, which is responsible for its higher ${T}_{\mathrm{c}}$. The theoretical and experimental investigations provide deep insights into the mechanism of superconductivity in the LaO system.