Infrared properties of Mg1-x Alx (B1-y Cy) 2 single crystals in the normal and superconducting state

The reflectivity R(omega) of ab-oriented Mg1-xAlx(B1-yCy)(2) single crystals has been measured by means of infrared microspectroscopy for 1300 <omega < 17 000 cm(-1). An increase with doping of the scattering rates in the pi and sigma bands is observed, being more pronounced in the C-doped crystals. The sigma band plasma frequency also changes with doping due to the electron doping, while the pi band one is almost unchanged. Moreover, a sigma ->sigma interband excitation, predicted by theory, is observed at omega(IB)similar or equal to 0.47 eV in the undoped sample, and shifts to lower energies with doping. By performing theoretical calculation of the doping dependence omega(IB), the experimental observations can be explained with the increase with electron doping of the Fermi energy of the holes in the sigma band. On the other hand, the sigma band density of states seems not to change substantially, suggesting that a relevant role, for what regards the reduction of T-c, can be played also by the disorder, at least for the doping level studied here. The superconducting state has been also probed by infrared synchrotron radiation for 30 <omega < 150 cm(-1) in one pure and one C-doped sample. In the undoped sample (T-c=38.5 K) a signature of the pi gap only is observed. At y=0.08 (T-c=31.9 K), the presence of the contribution of the sigma gap indicates dirty-limit superconductivity in both bands.