beta irradiation borosilicate glasses: the role of the mixed alkali effect
Résumé
The first penetration field (H-p) has been deduced from local magnetization and specific heat measurements in magnesium diboride single crystals. For H-a(parallel toc), the geometrical barriers (GB) play a dominant role in the irreversibility mechanism. In thin samples, neglecting the GB in this direction would then lead to a large overestimation of H-c1 deduced from H-p through the standard elliptical formula. The lower critical field is found to be isotropic at low temperature (similar to0.11+/-0.01 T).