Upper Critical Fields up to 60T in Dirty Magnesium Diboride Thin Films

TL;DR

This study measures extremely high upper critical magnetic fields in magnesium diboride thin films, revealing that these fields exceed BCS predictions and are linked to structural properties like c-axis expansion.

Contribution

It reports record high critical fields up to 52 T in magnesium diboride thin films, challenging existing theories and showing resistivity is not directly correlated with Hc2 in two-band systems.

Findings

Critical fields up to 52 T at 4.2 K were achieved.

No saturation of Hc2 at low temperatures was observed.

High Hc2 values are associated with c-axis expansion.

Abstract

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 Magnetiques Pulses (LNCMP) in Toulouse. The samples were prepared both by pulsed laser deposition (PLD) and hybrid physical chemical vapour deposition (HPCVD) technique; they have critical temperatures between 29 and 39 K and normal state resistivities between 5 and 250 μohmcm; one of them has been intentionally doped with carbon. The measured critical fields 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 Hc2 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 Hc2 are not trivially linked. The high Hc2 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.