Band Modification Effect on the Normal and Superconducting State Properties of Bulk MgB2

Modification of sigma and pi bands was studied in MgB2 by doping 3, 6 and 9 wt% of C and Fe, respectively. The samples synthesized by a solid-state route were characterized by XRD, and magnetization (M) and resistivity (rho) measurements were in the temperature range (T) 4.2-300 K and magnetic field range (B) 0-12 T, respectively. The decrease (increase) of the lattice parameter a with C (Fe) doping, consistent with B (Mg) site substitution, confirms the expected changes in sigma (pi) bands. This is supported by the fact that normal-state rho(T) of all the samples can be fitted by a two-band model and the scattering rates in both the bands are found to be dependent on the dopant. The influence of C and Fe doping on various superconducting properties of the host MgB2 is also found to be significantly different. For instance, in the presence of magnetic field, Fe doping shows a much larger broadening of the superconducting transition when compared to C doping. The critical current density (J (C)(B)) at 4.2 K vanishes for C (Fe) doping at around T similar to 12 (similar to 3). It is shown that the band modification and the superconducting properties are correlated