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MgB2 : le supraconducteur à 2 gaps

Abstract : “MgB2 : an exotic conventionnal superconductor”..... Behind this apparently paradoxical qualification is hiding the great originality of the system which was the subject of my work. Conventionnal in the electron-phonon coupling mechanism, MgB2 is exotic in the presence of two weakly coupled superconducting gaps. The existence of those two gaps leads to a “pathological” behaviour of the fundamental parameters : the coherence length, xsi and penetration depth, lambda.
Whereas a consensus on the multigap nature of MgB2 was quickly established, the published values of these fundamental parameters and of the associated critical fields expressed a disconcerting dispersion.
My work falls under this period of browsing of the MgB2 properties. It has the originality to use complementary techniques: Hall-probe magnetometry and alternative specific heat. First, we present a thermodynamic study of the H-T phase diagram carried out on single crystals. Specific heat measurements enabled the determination of the upper critical field Hc2. The lower critical field Hc1 has been deduced from magnetic measurements. Its determination required a thorough study of the penetration mechanisms of the vortices. The field distribution deduced from Hall probe array measurements on a serie of samples with various size ratios highlighted the presence of the geometrical barriers that delay the entry of the vortices. It is essential to take into account these barriers in the deduction of Hc1.
The anisotropy of the critical fields is temperature dependent due to the existence of two pairs of slightly coupled superconductive bands whose relative influence varies according to the position in the phase diagram H-T.This leads to a field dependence of these fundamental parameters.
Finally, a study of doped and irradiated samples were performed. The substitution of Mg by Al modifies the electronic doping and the critical fields. On the other hand, the effect of disorder introduced by electron irradiation led to a brutal increase of the peak effect which still remains to be explained.
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Contributor : Laurence Lyard <>
Submitted on : Tuesday, July 31, 2007 - 3:13:06 PM
Last modification on : Friday, December 18, 2020 - 1:34:03 PM
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  • HAL Id : tel-00166057, version 1

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L. Lyard. MgB2 : le supraconducteur à 2 gaps. Supraconductivité [cond-mat.supr-con]. Université Joseph-Fourier - Grenoble I, 2005. Français. ⟨tel-00166057⟩

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