Systematics in the superconducting and normal state properties in chemically substituted MgB₂

Original · EN

The superconducting transition temperature, TC, the residual resistivity ρ₀ and the slope of resistivity curve at high temperature, dρ/dT, have been measured in a series of MgB₂ samples that have been chemically substituted to varying degree with Li or Cu at the Mg-site and by Li or Cu at the Mg-site along with C substitution at the B-site. DC resistivity and ac susceptibility measurements were employed to extract the above parameters. TC versus the electron count (estimated from simple chemical valence count arguments) shows a universal behaviour, with TC being constant at the MgB₂ value for electron counts lower than in MgB₂ but rapidly decreasing for larger electron counts. The temperature dependence of resistivity in the normal state fits to the Bloch- Gruneisen formula, from which the Debye temperature, θD, and the ρ₀ are extracted. θD variation with TC is not systematic, whereas ρ₀ versus TC shows a systematic variation that depends on the type of the chemical substituent. This dependence has a signature of the nature of the intraband/interband scattering affected by the chemical substitutions. dρ/dT increases with C substitution, but decreases with Li and Cu substitution, implying that C substitution leads to the domination of conductivity by the σ band, while in the Li/Cu substituted samples the π band dominates conduction.