Raman study of element doping effects on the superconductivity ofMgB2

Abstract

The influences of phonon frequency and unit cell volume on the superconductivity of element-doped ${\mathrm{MgB}}_{2}$ are discussed with reference to a Raman study on $\mathrm{SiC}$, C, Mn, and $\mathrm{Al}\text{\ensuremath{-}}\mathrm{Ag}$-doped $\mathrm{Mg}\text{\ensuremath{-}}\mathrm{B}$ materials. A phenomenon has been found in the doped samples, in that the phonon frequency changes to counteract the crystal lattice variation to keep the system stable within a Gr\uneisen parameter of 2.0\char21{}4.0. The chemical doping effects on phonon frequency and unit cell volume can be explained by the harmonicity-anharmonicity competition in the compounds. A decreased electronic density of states is responsible for the depression of superconductivity that is seen in doped ${\mathrm{MgB}}_{2}$. The possibility of a high critical temperature, ${T}_{c}$, in the $\mathrm{Mg}\text{\ensuremath{-}}\mathrm{B}$ system exists if the material can possess both a high phonon frequency and a big unit cell volume at the same time, as indicated by the isotope effect and hydrogenation experiments.