Electric field gradient for muonium and monovacancies in Cu and Al

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The electric field gradient (EFG) and asymmetry parameter (η) at the first nearest neighbour (1NN) site of muonium (μ +) and at the first four NN sites of vacancies in Cu and Al are calculated. The defect induced charge density, used to evaluate the valence effect EFG, is calculated self-consistently in the density functional formalism. The formalism for the size effect EFG for μ + at the octahedral site in the fcc lattice is developed in the elastic continuum model. For both the μ + and the vacancy the components of the strain field tensor are evaluated assuming the lattice of dressed point ions interacting through the screened Coulomb potential. The size effect EFG for μ + is found cylindrically symmetric at the 1NN and 2NN sites. The size effect EFG is found larger than the valence effect EFG at all the NN sites for Cu- and Al-vacancies and the same is obtained at the 1NN site for μ +. The valence effect EFG for an Al-vacancy is found to be relatively larger than for a Cu-vacancy while the contrary is concluded for the μ +. It is noted that η depends upon the relative phases of components of EFG and not on their absolute values independently. The calculated and the experimental values of EFG and η agree reasonably.