Pressure dependence of and of a dirty two-gap superconductor, carbon-doped MgB2

We have measured T c and H c 2 ( T ) of carbon-doped MgB2 under hydrostatic pressures up to 15.6 kbar. d T c / d P is determined to be −0.20  K/kbar and H c 2 ( T = 0 ) decreases with pressure at a rate that is consistent with the theoretical value for pure MgB2, d H c 2 / d P = − 0.036 T/kbar . By analyzing our results within the theoretical framework of a dirty two-gap supersonductor, we determine values for the interband coupling and the ratio between the diffusivities associated with the two bands at three different pressures. We also extract the diffusivities and coherence lengths associated with each band. Finally, we estimate the pressure dependence of the charge carrier concentration in the σ band to be d ln n / d P = − 0.013 / kbar .