ar X iv : c on d - m at / 0 30 44 78 22 A pr 2 00 3 Upper critical fields of MgB 2 thin films

Critical fields of four MgB 2 thin films with a normal state resistivity ranging from 5 to 50 µΩcm and T c from 29.5 to 38.8 K were measured up to 28 T. H c2 (T) curves present a linear behavior towards low temperatures. Very high critical field values have been found, up to 24 T along the c-axis and 57 T in the basal plane not depending on the normal state resistivity values. In this paper, critical fields will be analyzed taking into account the multiband nature of MgB 2 ; we will show that resistivity and upper critical fields can be ascribed to different scattering mechanisms.