Influence of Al doping on the critical fields and gap values in magnesium diboride single crystals

The lower (Hc1) and upper (Hc2) critical fields of Mg1-x Alx B2 single crystals (for x=0, 0.1, and 0.2) have been deduced from local magnetization and specific heat measurements, respectively. We show that Hc1 and Hc2 are both decreasing with increasing doping content. The corresponding anisotropy parameter Γ Hc2 (0)= H c2 ab (0) H c2 c (0) value also decreases from ∼5 in pure MgB2 samples down to ∼1.5 for x 0.2 whereas Γ Hc1 (0)= H c1 c (0) H c1 ab (0) remains on the order of 1 in all samples. The small and large gap values have been obtained by fitting the temperature dependence of the zero-field electronic contribution to the specific heat to the two-gap model for the three Al concentrations. Very similar values have also been obtained by point contact spectroscopy measurements. The evolution of those gaps with Al concentration suggests that both band filling and interband scattering effects are present.