Einfluß von statischen Feldgradientenverteilungen und Interferenzeffekten auf die NAR-Linienform in einkristallinem Niob

Abstract

Nuclear Acoustic Resonance (NAR) in 93 Niobium single Crystals exhibits a hitherto unexplicable resonance line shape.Within the framework of a linear response approach for the dynamically generated magnetic fields and dynamical field gradients generated by the acoustic wave in the bulk crystal the one-particle acoustic susceptibility of the Niobium nucleus is calculated using the quantum mechanical equation of motion.It is demonstrated that because of the simultaneous interactions of the magnetic dipole moment and the electric quadrupole moment of the Niobium nucleus interferences in the NAR-resonance arise depending largely on the sign of the mean static electric field gradient which is unavoidably present due to a static strain of the real cubic crystal lattice.Although especially in Niobium the transition probabilities for both dipole- and quadrupole transitions are nearly equal and therefore the conditions for occurence of a large interference effect on the resonance line shape are fulfilled, the experimentally observed NAR-lineshape in fact does esxhibit only a negligibly small asymmetry in contrast to the theory. Neither are cross relaxation effects due to dynamical fluctuating fields caused by mobile hydrogen atoms in the crystal able to explain the peculiar resonance line shape.As an alternative the observed NAR-lineshape may be explained by appropriate distributions of inhomogeneous fields caused by immobile point defects of built-in lattice defects such as carbon or oxygen atoms.