Upper critical fields up to 60 T in dirty magnesium diboride thin films

Upper critical fields of several magnesium diboride thin films were measured up to 28 T at the Grenoble High Magnetic Field Laboratory (GHMFL) in Grenoble and up to 60 T at the Laboratoire National des Champs Magne´tiques Pulse´s (LNCMP) in Toulouse. The samples were prepared both by pulsed laser deposition (PLD) and hybrid physical chemical vapor deposition (HPCVD) technique; they have critical temperatures between 29 and 39 K and normal state resistivities between 5 and 250 μΩ cm; one of them has been intentionally doped with carbon. The measured critical fields, μ₀H_c2, were exceptionally high; we obtained the record value of 52 T at 4.2 K in the parallel orientation. In contrast with the BCS predictions, no saturation in H_c2 at low temperature was observed. Furthermore, films with a wide range of resistivity values showed similar critical fields, suggesting that in a two band system resistivity and H_c2 are not trivially linked. The high H_c2 values seem to be related with the expanded c-axis. The structure of one of the samples was carefully investigated with X-ray diffraction at European Synchrotron Radiation Facility (ESRF) in Grenoble.