Evidence of a quantum entangled state between the muon spin and 51V nuclear spin (I=7/2) in A15 compounds

The purpose of this thesis is the study of the interaction at the atomic level between the positive muon μ+, with spin S = 1/2, and nuclei having spin I >1/2. Normally the μ+, a particle with a charge equal (in absolute value) to that of the electron and which interacts with the magnetic fields through its magnetic moment, can form a quantum entangled state when it is implanted between two nuclei with spin I=1/2, such as fluorine; this phenomenon causes a peculiar trend of the muon polarization. This phenomenon has never been observed for materials with nuclei having large spin. Very recently a similar phenomenon has been observed in a system containing Na (I= 3/2). Here we study a system with V which has larger nuclear spin I= 7/2. Nuclei with spin I >1/2 have a quadrupolar moment Q and can be subject to an electric field gradient (EFG) based on the symmetry of the crystal lattice. Furthermore, the muon itself induces an EFG within the crystalline structure, which combines with that, if present, generated by the material itself. This phenomenon has recently been observed in compounds with A15 crystal structure by using the muon resonance spectroscopy. These materials, with the chemical formula A3B (where A is a transition metal and B any other element), have type-II superconductivity but they do not exhibit magnetism. In this thesis we performed a study of V3B samples with B = Si, Sn, Ge by a) a structural characterization by a Rietveld analysis of high resolution X-rays Diffraction patterns, b) the data analysis of the muon polarization and a subsequent simulation of the latter.