behavior for an ac magnetic field of amplitude for a generic type II superconductor phase diagram. is strictly equal to zero in the Meissner phase, it assumes finite values with a bell-shape behavior in the mixed state below the irreversibility line and it comes back to zero in the flux flow and normal state regimes. The two vertical arrows refer to the left and right onset, respectively.
(a) Sketch of the experimental setup; (b) details of the coil-sample configuration.
Procedure for the extraction of the first critical field values from the temperature behavior of the third harmonic component (Nb sample): (a) dependence of the left onset on the applied ac field by increasing the amplitude of the current injected in the probing coil. The arrow indicates the increase of ; (b) details of the phase and modulus of the dotted curve shown in (a). The arrow represents the third harmonic onset, whereas the vertical line sets the method to determine the maximum error.
measured values (solid ○) as a function of the reduced temperature for the (a) Nb, (b) NbN, (c) , and (d) YBCO film. In (b), (◻) data represent the experimental results reported in Ref. 41. In (c), (◻) and (○) data represent the experimental results reported in Ref. 43 for thin films and in Refs. 36 and 44 for single crystals, respectively. In (d), (◻) data represent the experimental results reported in Ref. 45. The continuous lines in each graph describe the theoretical predictions.
Summary of sample properties and parameters for the evaluation of the first critical field.
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