Decrease of Tc and persistent two gaps upon enhancement of the Ca doping in MgB2 superconductor

Superconducting properties have been investigated in the Ca-doped MgB2 (Mg1-xCaxB2) superconductors, which have been synthesized under high temperature and pressure. With increasing doping level x, a linear drop of the critical temperature T-c is observed, and the gap Delta(sigma) keeps nearly fixed and the gap Delta(pi) deceases, giving rise to the persistent two-gap feature in the Mg1-xCaxB2 samples for x up to 0.07. Both the Sommerfeld constant gamma(n) and the relative weight gamma(pi)/gamma(n) are found to decrease with raising x, indicative of the decrease of N-pi(0).