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Local magnetometry measurements of selected superconductors

Abstract : The subject of this work was a detailed study of superconducting CuxTiSe2. Samples with various copper content x, from underdoped to overdoped regions of the phase diagram, were investigated by local Hall probe magnetometry. At first, we constructed the magnetic field profiles in the samples to determine when the first vortices penetrate into the sample. The observed dome-like shape of the profiles indicated that pinning is very small and that the vortex penetration is primarily governed by geometrical barriers, hence enabling us to determine the lower critical field, Hc1. A good agreement between theory and the Hc1(T) data was obtained by using a model with two energy gaps, even though the origin of the smaller energy gap in CuxTiSe2 cannot be related to its electronic structure. Combining those measurements with heat capacity measurements previously performed on these samples, we obtained a non trivial condensation energy. Moreover, measurements performed for various orientations of the magnetic field revealed the presence of a lock-in effect in CuxTiSe2, when superconducting vortices are trapped along a layered structure even for tilted magnetic fields. The orientation of the vortices was experimentally determined and could be well described by a model considering the presence of an additional layered structure in which superconductivity is at least partially suppressed. The lock-in effect was analysed in three different samples and we have shown that the strength of the lock-in is independent of the copper doping, even though this effect can be masked by strong demagnetisation effects in very thin samples. The strength of this lock-in effect indicates that the superconductivity is strongly suppressed on certain layered structure. The nature of this "extrinsic" pinning structure is still unknown but might be related to the recent observation of charge density waves domain walls which may induce strong variations of the superconducting order parameter.
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Submitted on : Tuesday, March 20, 2018 - 12:56:07 PM
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  • HAL Id : tel-01738228, version 1

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Zuzana Medvecka. Local magnetometry measurements of selected superconductors. Superconductivity [cond-mat.supr-con]. Université Pavol Jozef Šafárik (Cassovie, Slovaquie), 2017. English. ⟨NNT : 2017GREAY053⟩. ⟨tel-01738228⟩

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