The magnetic field homogeneity of coils by means of the space harmonics suppression of the current density distribution

Electromagnetic coils are ubiquitously used in the modern world in motors, antennas, etc. In many applications (magnetic field coil calibration and nuclear magnetic resonance spectroscopy and imaging) there is a strong need for a homogeneous magnetic field. In this paper, we propose a simple modelling based on serial Fourier decomposition allowing the determination of the electrical conductor distribution to make the magnetic field homogeneous. The method is valid both for plane and axisymmetric geometries. The method allows the retrieval of the classical configurations of saddle coil for the plane geometry and Helmholtz coil for the axisymmetric one. The method is generalized for any number of electrical conductors and brings the perspective of new homogeneous magnetic resonance imaging (MRI) coil configuration

Orientational order and dynamics of a nematic liquid crystal in porous media

We report on orientational order and dynamics of a nematic liquid crystal confined in porous silica. 13C NMR lineshape is used as a fingerprint for this study. A procedure based on exchange of non-coupled sites is implemented in order to obtain the corresponding local order parameter S and the characteristic time for molecular reorientation τ. Our results evidence that in the nematic phase, S decreases when confinement increases. By means of Langevin dynamics simulations of confined liquid crystals, we show that this behavior is due to topological defects, originated by the absorbed-layers phenomenon as a consequence of the strong anchoring of these molecules on silica surfaces

Properties of K,Rb-intercalated C60 encapsulated inside carbon nanotubes called peapods derived from nuclear magnetic resonance

We present a detailed experimental study on how magnetic and electronic properties of Rb, K-intercalated C-60 encapsulated inside carbon nanotubes called peapods can be derived from C-13 nuclear magnetic resonance investigations. Ring currents do play a basic role in those systems; in particular, the inner cavities of nanotubes offer an ideal environment to investigate the magnetism at the nanoscale. We report the largest diamagnetic shifts down to -68.3 ppm ever observed in carbon allotropes, which is connected to the enhancement of the aromaticity of the nanotube envelope upon intercalation. The metallization of intercalated peapods is evidenced from the chemical shift anisotropy and spin-lattice relaxation (T-1) measurements. The observed relaxation curves signal a three-component model with two slow and one fast relaxing components. We assigned the fast component to the unpaired electrons charged C-60 that show a phase transition near 100 K. The two slow components can be rationalized by the two types of charged C-60 at two different positions with a linear regime following Korringa behavior, which is typical for metallic system and allow us to estimate the density of sate at Fermi level n(E-F)