Semi-empirical and ab-initio calculations of the crystal field interaction in rare earth cuprates

摘要

Semi-empirical and ab-initio methods useful for theoretical investigations of the crystal field (CF) interaction in rare earth (RE) cuprates are described. Concrete calculations are performed for Sm_(1+x)Ba_(2-x)Cu_3O_(6+y), a system providing detailed data on intermultiplet infrared-active CF transitions for Sm~(3+) ions on the regular D_(4h) symmetry sites as well as on the substituted C_(4v) symmetry Ba sites. Within this study, the main attention is paid to the second order CF parameters calculated using a parameter free first principles method based on the density functional theory (DFT). A general potential linearized augmented plane wave (LAPW) computational method is used to obtain the ground state charge density. The calculated value of B_(20) = 320 cm~(-1) in D_(4h) symmetry sites of Sm is in a good agreement with the phenomenological value of 282 cm~(-1) obtained from a fit to infrared transmission spectroscopy data. For Sm in the C_(4v) symmetry sites, the DFT value of B_(20) = - 227 cm~(-1) together with the standard superposition model values, B_(40) = 24, B_(44) = -331, B_(60)=-427 and B_(64)=624cm~(-1), allow us to interpret the main features of the available infrared absorption data. It is shown that the sign of the B_(20) in SmBa_2Cu_3O_(6+y) positive in the regular sites and negative in the Ba sites, is governed by the shape of the crystal potential within the LAPW atomic sphere of Sm atom.