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Influence of Al doping on the superconducting properties of magnesium diboride single crystals

for an Invited Paper for the MAR07 Meeting of The American Physical Society Influence of Al doping on the superconducting properties of magnesium diboride single crystals THIERRY KLEIN, Institut Neel, CNRS-BP 166, 38042 Grenoble Cedex We present a systematic study of the evolution of the lower (Hc1) and upper (Hc2) critical fields, gap values and Sommerfeld coefficient of Mg1−xAlxB2 single crystals (for x = 0, 0.1 and ≥ 0.2). We show that Hc1 and Hc2 deduced from Hall probe magnetization and specific heat measurements, respectively are both decreasing with increasing doping content. The corresponding anisotropy parameter ΓHc2(0) = H ab c2 (0)/H c c2(0) value also decreases from ∼ 5 in pure MgB2 samples down to ∼ 1.5 for x ≥ 0.2 whereas ΓHc1(0) = Hc c1(0)/H c1 (0) remains on the order of 1 in all samples. The magnetic field dependence of the anisotropy parameter ΓHc1 ≤ Γ(H) ≤ ΓHc2 has then been deduced from a detailled analysis of the angular dependence of the Sommerfeld coefficient for different values of the applied fields. The small and large gap values have been obtained both by fitting the temperature dependence of the zero field electronic contribution to the specific heat to the two gap model and by point contact spectroscopy measurements. Both measurements led to very similar values and the evolution of those gaps with Al concentration suggests that both band filling and interband scattering effects are present. This work has been performed in collaboration with : L.Lyard, C.Marcenat, J.Marcus, Z.Holanova, P.Szabo, P.Samuely, B.W.Kang, H-J.Kim, H-S.Lee, H-K.Lee,S-I.Lee, S.Tajima and S.Lee.