Field and angular dependence of the Sommerfeld coefficient in Al-doped Mg B2 single crystals

The angular and field dependence of the Sommerfeld coefficient γ= lim Cel T T→0 (Cel being the electronic contribution to the specific heat) has been measured in Al-doped Mg B2 single crystals (for x=0, x=0.1, and x∼0.2). We show that the decomposition previously introduced to describe γ (H,θ) (where θ is the angle between the applied field and the c axis) in pure samples [Phys. Rev. Lett. 98, 137001 (2007)] is well adapted to doped samples: e.g., the contribution of the σ band to the specific heat is proportional to B Bc2 (θ) whereas the contribution of the π band is isotropic but highly nonlinear in field. We hence present the evolution of the coherence lengths of the two bands and corresponding Fermi velocities with doping.