Lattice Effects in Diborides

Abstract

The lattice effects in diborides have been investigated by Raman spectroscopy using several atomic substitutions for Mg and B. The doping dependence of T c can be associated with the gradual filling of the σ- and the π-bands and the modifications in the Fermi surface. The data show that C doping for B induces a much faster reduction in T c than Al for Mg, while the simple Li substitution has a minor effect. On the other hand, the double Li-C substitution reduces T c faster than the simple C doping, which can be attributed to the different role of hole and electron doping for the π- and σ-bands. Finally, Mn ions have a drastic effect on superconductivity apparently acting as magnetic pair breaking centers. In the Raman spectra of the mixed compounds, two modes of E2g symmetry appear at ∼600 cm−1 and ∼850 cm−1 (defined as the soft and hard modes, respectively), which do not evolve one to the other with doping, but they coexist in a typical two-mode behavior for both Mg(B1−x C x )2 and Mg1−x Al x B2 sets of compounds. MgB2 appears to be metastable with intrinsic structural and electronic phase competition, close to topological electronic transitions of the Fermi surface that can be tuned by doping or internal strain from the atomic substitutions. The analysis of the phonon frequency dependence on doping indicates that the behavior of the renormalized mode cannot explain the reduction of T c in the phase coexistence level. It appears that other effects play also an important role in the superconductivity of diborides.