First critical field measurements of superconducting films by third harmonic analysis

The temperature behavior of the first critical field (BC1) of superconducting thin film samples can be determined with high accuracy using an inductive and contactless method. Driving a sinusoidal current in a single coil placed in front of the sample, a nonzero third harmonic voltage V3 is induced in it when Abrikosov vortices enter the sample. Conditions to be satisfied for the quantitative evaluation of BC1 using this technique are detailed. As a validation test, different type II superconductors (Nb, NbN, MgB2, and Y1Ba2Cu3O7−d under the form of thin films) have been measured. The comparison between experimental results, data presented in literature, and theoretical predictions is presented and discussed.