Specific heat and magnetic relaxation analysis of MgB₂ bulk samples with and without additives

The effects of SiC and Carbon doping on the superconducting properties of MgB₂ polycrystalline samples have been analysed by means of specific heat and magnetic relaxation measurements. It is known that the addition of nanometric powders of SiC and C leads to the enhancement of B_irr and J_c. However, the underlying physical mechanism is not completely understood. Magnetic relaxation measurements did not show detectable effects of both the additions on the pinning properties of MgB₂. It follows that doping acts mainly introducing disorder into the superconductor and thus raising B_c2. In the case of MgB_1.9 C_0.1, specific heat measurements show that the C substitution on the B sites modifies the low temperature shoulder related to the second gap. This effect is not visible in the sample doped with SiC. From the distribution of T_c determined from the deconvolution of the calorimetric data, we argue that SiC leads to an inhomogeneous distribution of C.