  We performed an extensive investigation on the correlations among
superconductivity, structural instability and band filling in Nb1-xB2
materials. Structural measurements reveal that a notable phase transformation
occurs at x=0.2, corresponding to the Fermi level (EF) in the pseudogap with
the minimum total density of states (DOS) as demonstrated by the
first-principles calculations. Superconductivity in Nb1-xB2 generally becomes
visible in the Nb-deficient materials with x=0.2. Electron energy-loss
spectroscopy (EELS) measurements on B K-edge directly demonstrated the presence
of a chemical shift arising from the structural transformation. Our
systematical experimental results in combination with theoretical analysis
suggest that the emergence of hole states in the sigma-bands plays an important
role for understanding the superconductivity and structural transition in
Nb1-xB2.
