Effects of Al doping on the normal and superconducting properties of MgB2: A specific heat study

The effects of Mg substitution by Al on the specific heat of MgB2 have been studied in the normal and in the superconducting state. The Sommerfeld coefficient, evaluated by measurements in magnetic field up to 7 T, progressively decreases from gamma=3.0+/-0.2 mJ/mol K-2 for undoped MgB2 down to gamma=1.9+/-0.2 mJ/mol K-2 for Mg0.6Al0.4B2 as a result of the change of the density of states and of the electron-phonon coupling constants. The superconducting contribution to the specific heat c(sc) clearly shows an excess at low temperature compared with conventional single-gap BCS behavior also for doped samples with x up to 0.3. The two-band model has been used to fit the temperature dependence of c(sc) and the amplitude of the two gaps has been evaluated for different Al concentrations. The changes of the energy gaps are in a rather poor agreement with those predicted by taking into account changes in the electronic and phononic structure only. We suggest that disorder also plays an important role when Al substitutes Mg.