ELECTRIC-FIELD GRADIENTS AROUND POINT-DEFECTS IN METALS

The splittings of nuclear energy levels caused by the electric field g radients acting on the quadrupole moments of nuclei in the neighbourho od of atomic defects in cubic metals may serve as ''fingerprints'' pro viding us with a unique characterization of these defects. In favourab le cases the NQDOR technique (nuclear quadrupole double resonance) per mits sensitive measurements of these splittings with good resolution. The present paper outlines the status of the ab-initio calculation of electric field gradients with emphasis on the theoretical basis (densi ty functional theory with local density approximation) and on the tech niques required for handling the specific problems associated with def ects. Recent work by the supercell approach on atomic defects in Al an d Cu, making use either of the full-potential linearized augmented-pla ne-wave method or of the ab-initio pseudopotential method, are reporte d and compared with experiments. The excellent agreement between exper iment and theory for the field gradients acting on the nearest-neighbo ur nuclei of monovacancies in Al demonstrates the reliability and the potential of the theory.