  The evolution of the superconducting properties of the carbon-doped MgB2
superconductors, MgB(2-x)Cx (x= 0.02, 0.04, 0.06) have been investigated by the
transverse-field muon spin rotation (TF-muSR) technique. The low-temperature
depolarisation rate, sigma(0) at 0.6 T which is proportional to the second
moment of the field distribution of the vortex lattice decreases monotonically
with increasing electron doping and decreasing Tc. In addition, the temperature
dependence of sigma(T) has been analysed in terms of a two-gap model. The size
of the two superconducting gaps decreases linearly as the carbon content
increases, while the doping effect is more pronounced for the smaller gap
related to the 3D pi-sheets of the Fermi surface.
