Diatomic substitutionals in superconducting Nb(i-x)B2

• Published in: Physica. C, Superconductivity Vol. 449; no. 1; pp. 1 - 8
• Main Authors: FARRELL, H. H, LAVIOLETTE, Randall A, LILLO, T. M
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Science 01-11-2006
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Effects of material synthesis, crystal structure, and chemical composition; Exact sciences and technology; Physics; Superconductivity; Transition temperature variations; Stoichiometry; Vacancies; Superconducting transitions; Superconductivity; Superconducting phonons; Electronic density of states; 74.25.Jb; 74.25.Kc; Niobium borides; Superconducting materials; Superconductors; Fermi level; Binding energy; 74.62.Dh; Superconducting electronic structure; Superconductivity transition temperature variations; Phonon mode; Density functional method; 74.70.-b; Defect formation
• ISSN: 0921-4534; 1873-2143
• DOI: 10.1016/j.physc.2006.05.104

Using first-principles, density functional theory calculations, we have shown that the increase in superconducting critical temperature to above 9K experimentally observed for B-rich, off-stoichiometry NbB2 material is associated with the formation of B-dimers on Nb sites rather than with the formation of Nb vacancies, as previously proposed. Our calculations show that certain of these B-dimer configurations have lower binding energies than do the vacancy structures. This result is pressure independent in the range between 0 and 10GPa, and above. Further, the dimer-containing materials have a higher electronic density of states at the Fermi level than do the vacancy structures. Finally, the presence of B-dimers results in the splitting off of phonon normal modes. These last two factors are relevant to the improved superconducting properties.