The charge transfer induced by Cr doping in MgB 2

Abstract

Mg 1− x Cr x B 2 polycrystal bulk samples with 0 ⩽ x ⩽ 5% have been synthesized by a solid-state reaction and studied by X-ray diffraction, SEM and Raman spectrum. It is found that the c-axis of the lattice decreases as the Cr content increases, while the a-axis remains unchanged. Moreover, crystal grain size increases apparently with Cr doping concentration increase. The normal-state resistivity increases and the superconducting transition temperature ( T c) decreases from 38.2 K ( x = 0) to 35.1 K ( x = 0.03) with the increase of Cr content. It is suggested that the charge transfer between the Mg-layer and the B-layer causes the decrease of the charge carrier concentration and induces the changes of T c and normal-state resistivity. On the other hand, by the Raman scattering study, it is found that the linewidth of Raman spectrum increases with the increase of Cr content, which is resulted by the competition between the electron–phonon interaction and substitution-induced disorder. The Raman peak has no evident shift due to the countervailing between the effects of the electron–phonon coupling and the grain size.