Correlations among superconductivity, structural instability, and band filling in Nb_(1-x)B_2 at the critical point x ≈ 0.2

摘要

We performed an extensive investigation on the correlations among superconductivity, structural instability, and band filling in Nb_(1-x)B_2 materials. Structural measurements reveal that a notable phase transformation occurs at x ≈ 0.2, corresponding to the Fermi level (E_F) in the pseudogap with the minimum total density of states (DOS) as demonstrated by the first-principles calculations. Superconductivity in Nb_(1-x)B_2 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 σ bands plays an important role for understanding the superconductivity and structural transition in Nb_(1-x)B_2.