Mg-B site doping effects on the upper critical field temperature dependence of MgB2 samples with CaCO3 additions

A. SERQUIS; G. SERRANO; L. CIVALE; B. MAIOROV; N. HABERKON; M. JAIME

La Haya

Conferencia; Superconducting Centennial Conference EUCAS-ISEC-ICMC 2011; 2011

EUCAS-ISEC-ICMC

Theoretical models predicted that the presence of two superconducting gaps in MgB2 could allow tuning different upper critical fields by controlling diverse defect sublattices relative to orthogonal hybrid bands (s and pi). These models calculate a significant Hc2 enhancement in the dirty limit and an anomalous Hc2(T) upward curvature. Several experimental works already confirmed these unique phenomena that depend on intraband and interband scattering between the two bands, which can be modified by chemical substitution or irradiation. However, most studies only analyze the effect of B-site substitution (e.g. by oxygen or carbon) and very few report on the effect of Mg‐site substitution (e.g. by aluminum). We present a study on the effect of CaCO3 additions in the Hc2(T) dependence of MgB2 bulk samples. The microstructure is analyzed by x‐ray diffraction and electron microscopy to confirm the possible simultaneous substitution of Ca replacing Mg and C replacing B in the MgB2 structure. The superconducting properties are determined by transport measurements. The Hc2(T) dependence is obtained from four‐probe transport measurements in pulsed magnetic fields up to 65 T at NHMFL‐LANL, performed at temperatures between 1.4 and 35 K. The obtained results will be discussed under the frame of the Usadel equations for a dirty two gaps superconductor (from the models proposed by Gurevich and Golubov) to describe the Hc2 dependence on temperature observed in the studied samples.