Correlation between transition temperature and crystal structure of niobium, vanadium, tantalum and mercury superconductors

The bond length ( x) of transition metals Nb, V, Ta and Hg represents the shortest atomic separation in a crystal unit cell. It is suggested that there exists a strong correlation between ( x) and the inverse of the critical superconducting transition temperature T c in the form (2 x) 2 N eff = m 2 1/ T c . Here N eff is the number of electrons in the outermost s-shell. The slope of the fitted straight line has a value of m 2≈3.0×10 −18 m 2 K.