XPS study of the electronic density of states in the superconducting Mo₂B and Mo₂BC compounds

Abstract

The electronic structure of the Mo$_2$BC and Mo$_2$B compounds was investigated by X-ray photoelectron spectroscopy. The Mo 3d, C 1s, and B 1s core levels are identified. For the Mo$_2$BC, the core-level binding energies corresponding to Mo 3d$_{5/2}$, B 1s, and C 1s are localized at 227.90, 187.94, and 282.95 eV, respectively, whereas for the Mo$_2$B, the Mo 3d$_{5/2}$, and B 1s are localized at 228.09 and 188.06 eV, respectively. Core-level binding energies shifts are observed in both compounds using the charge-potential model. The electronic density of states was calculated for Mo$_2$B and Mo$_2$BC using GGA approximation. Our results show that the electronic density of states at the Fermi level in the Mo$_2$B is higher than that in the Mo$_2$BC. The dominance of the Mo 4d states down to 8 eV below the Fermi level is found. The calculated total DOS was consistent with the XPS valence band spectra. Finally, within the BCS theory framework, the presence of superconductivity in both compounds can not be explained only as a function of the electronic density of states at the Fermi level. The electron-phonon coupling constant ($\lambda$) was calculated using the McMillan equation; the obtained values were 0.75 for Mo$_{2}$BC and 0.70 for Mo$_{2}$B. These values indicate that both compounds are intermediate coupled superconductors.