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Multiferroic CoFe2O4-Ba0.95Ca0.05Ti0.89Sn0.11O3 core-shell nanofibers (CFO@BCTSn NFs) were synthesized by a sol-gel co-axial electrospinning technique. The scanning electron microscope and transmission electron microscope were used to check nanofibers' core-shell structure/configuration. X-ray diffraction and a high-resolution transmission electron microscope were used to confirm the spinel structure of CFO and the perovskite structure of BCTSn. The magnetic character of the resultant CFO@BCTSn NFs was determined by SQUID magnetometry. The piezoelectricity was verified using piezo-response force microscopy, which revealed an entirely covered ferroelectric shell outline, in accordance with SEM and TEM observations. The magnetoelectric (ME) coefficient was measured as a function of the applied external DC magnetic field. The maximum ME coefficient obtained for the CFO@BCTSn NFs was 346 mV cm-1 Oe-1. The high magnetoelectric coupling suggests that CFO@BCTSn NFs could be a promising candidate for magnetic field sensor and magnetoelectric device applications.

We present new relations for calculation of phonon polaritons, plasmons and plasmon polaritons frequencies, which differ from the standard relations. So, we study the frequency of phonon polaritons in a two dimensional lattice with applications to CuO2 lattice. Next, we evaluate plasmon frequencies in graphene and in Cu2D and Cu3D metals. Further, we evaluate the frequency of surface plasmon polaritons at the Cu2D/air and Cu3D/air interfaces. The obtained results are in a good agreement with experimental data. Also, we present a correspondence between the potential vector and the displacement of an oscillator. It is well known that the commutation relations of the operators in second quantization originate from the simple harmonic oscillators that are used to quantize the electromagnetic field. Finally, we present a comparison between photons, phonons and plasmons., 15 pages, 2 Tables

By using a Hamiltonian based on the coupling through flux lines, we have calculated the interaction energy between two fermions via massless bosons as well as via massive particles. In the case of interaction via massless bosons we obtain an equivalent expression for the Coulomb's energy on the form, where is the fine structure constant. In the case of the interaction via massive particles we obtain that the interaction energy contains a term building the potential well. Taking into account the spin-spin interaction of the nucleons, we show that this interaction modulates the interaction potential through a cosine factor. The obtained results are in good agreement with experimental data, for example, of deuteron.

We study the properties of the vortex lattice in unconventional superconductors, using µSQUID microscopy. The most important part is dedicated to the spin triplet superconductor Sr2RuO4. First, we examine the conventional superconductor NbSe2. It is a reference point to the unconventional superconductors imaged afterwards. We observe a hexagonal vortex lattice at low fields, and a rotation of the vortex lattice in respect to the crystallographic axes at higher fields. Next, we present the behavior of vortices in Sr2RuO4. We observe for the first time individual vortices at low fields and vortex coalescence at higher fields.As the pinning is weak and considering the other experiments effectuated till now in this material, we explain the coalescence of vortices by the presence of domains with different chirality. An important role plays the anisotropy, chains of vortices are observed for tilted fields. We find also that for tilting angles close to the layers, a decoration takes place. These results prove that Sr2RuO4 is an unconventional superconductor with strong anisotropy. In the last part, we present images of the magnetic domains taken in the normal phase of the ferromagnetic superconductor URhGe. We also imaged the individual vortices in the unconventional superconductor UPt3.We observe a domain formation, this domains being similar with those seen in Sr2RuO4, a parallel between the two triplet superconductors could be established.; Cette thèse porte sur l'étude, par microscopie à µSQUID, des propriétés du réseau de vortex dans les supraconducteurs non conventionnels. La plus grande partie est dédiée au supraconducteur triplet Sr2RuO4. Dans un premier temps, le supraconducteur conventionnelNbSe2 est étudié. Il sert de référence pour les supraconducteurs non conventionnels images ensuite. Un réseau de vortex hexagonal est observé à bas champ, et une rotation du réseau de30° par rapport aux axes cristallographiques a lieu en augmentant le champ. Dans un second temps, nous présentons l'étude de vortex dans Sr2RuO4. On observe pour la première fois des vortex individuels à bas champ et en augmentant le champ une coalescence a lieu. En tenant compte du fait que le piégeage est faible et des expériences déjà effectuées, on explique cette coalescence par la présence des domaines de différente chiralité. Un rôle important joue aussi l'anisotropie, des chaînes de vortex sont observés pour des champs inclinés. Nous trouvons qu'à des angles d'inclinaison près de couches ab, une décoration des chaînes a lieu.Ces résultats démontrent que Sr2RuO4 est un supraconducteur non conventionnel fortement anisotrope. Dans une dernière partie, nous présentons les images des domaines magnétiques dans la phase normale du supraconducteur ferromagnétique URhGe. Nous observons aussi des vortex individuels et une formation des domaines dans le supraconducteur non conventionnel UPt3. Ces domaines ressemblent à ceux de Sr2RuO4, une parallèle peut être faite entre les deux supraconducteurs triplet.