Valence band XPS and UPS studies of non-stoichiometric superconducting NbB2+x

The electronic structure of non-stoichiometric superconducting NbB2+x has been investigated by x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The analysis of the valence band using XPS and UPS reveals that the boron doping induces a systematic decrease in the density of states at Fermi level N(E-F) similar to that observed in the Nb1-xB2 system. In particular, N(E-F) is lower for the superconducting samples than for the non-superconducting ones. In the superconducting samples, we confirm that the T-c is inversely proportional to N(E-F). Therefore, the presence of superconductivity in these samples cannot be explained only as a function of the N(E-F). Thus, the presence of superconductivity in our samples might be explained by increases in the number of holes in-plane conduction, due to an increase in the number of niobium vacancies as boron content is increased.