Correlations among superconductivity, structural instability, and band filling in Nb1-xB2 at the critical point x=0.2

TL;DR

This study investigates how superconductivity, structural changes, and electronic band filling are interconnected in Nb1-xB2, revealing a critical composition at x=0.2 where a phase transition and superconductivity emerge.

Contribution

It provides new insights into the relationship between structural instability, band filling, and superconductivity at a specific critical point in Nb1-xB2.

Findings

Structural phase transformation at x=0.2

Superconductivity appears at x=0.2

Hole states in sigma-bands are crucial for superconductivity

Abstract

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.