Specific Heat and Magnetic Relaxation Analysis of ${\rm MgB}_{2}$ Bulk Samples With and Without Additives

Abstract

The effects of SiC and Carbon doping on the superconducting properties of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">irr</sub> and J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> . 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 MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . It follows that doping acts mainly introducing disorder into the superconductor and thus raising B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</sub> . In the case of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.9</sub> C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> , 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> determined from the deconvolution of the calorimetric data, we argue that SiC leads to an inhomogeneous distribution of C.