engleski

Microwave study of the coherence lengths in superconductors

The temperature dependence of the coherence lengths in anisotropic and doped superconductors has been studied by the microwave method. We show that one can experimentally make distinction between the coherence lengths in the mixed state, and at the transition to the normal state. In the former case, the vortex involves many Landau levels, whereas in the latter, only the lowest Landau level is left. In MgB2 superconductor, we find that the anisotropy ratio, as determined from the coherence lengths, is not the same deeply in the mixed state and close to the transition to the normal state. These results can be interpreted in terms of the two gap structure of MgB2 which yields superposition of vortices with different radii. In YBCO superconductor, we observed variation of the coherence length with doping. In contrast, to some previous reports in the literature, our results show systematically that lower doping increases the coherence length, i.e. the pairing strength is diminished.

superconductivity; microwave impedance; upper critical field

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