First-principles calculation of electric field gradients and hyperfine couplings in YBa2Cu3O7

Abstract

Abstract. The local electronic structure of YBa2Cu3O7 has been calculated using first-principles cluster methods. Several clusters embedded in an appropriate background potential have been investigated. The electric field gradients at the copper and oxygen sites are determined and compared to previous theoretical calculations and experiments. Spin polarized calculations with different spin multiplicities have enabled a detailed study of the spin density distribution to be made and a simultaneous determination of magnetic hyperfine coupling parameters. The contributions from on-site and transferred hyperfine fields have been disentangled with the conclusion that the transferred spin densities essentially are due to nearest neighbour copper ions only with marginal influence of ions further away. This implies that the variant temperature dependencies of the planar copper and oxygen NMR spin-lattice relaxation rates are only compatible with commensurate antiferromagnetic correlations. The theoretical hyperfine parameters are compared with those derived from experimental data. PACS

Charge and spin density distributions around Zn impurities in cuprates

“…We have shown previously that with the cluster method all contributions to the EFG can be determined and the resulting values give a direct comparison to NMR and NQR experiments with good agreement 15,17,18 . (For planar copper atoms with vanishing asymmetry parameter η the connection between the z-component of the EFG, V zz , and the quadrupole frequency, ν Q , is given by ν Q = (e/2h)QV zz , where Q is the nuclear quadrupole moment.)…”

“…In cluster calculations, however, the spin state can be chosen and in all cases (irrespective of the functional (LDA or generalized gradient approximation)) the state with the lowest variational energy is found for the antiferromagnetic arrangement (see Refs. 15,16 ). The results of each calculation were examined with the Mulliken population analysis which gives a description of the charge and spin densities in terms of the individual atoms and also the constituent orbitals.…”

“…This observation can be used to split the Fermi contact term into on-site and transferred terms as has been discussed in detail in Refs. 15,17 . We just note here that the onsite term is negative if the Mulliken spin density at the Cu atom under consideration is positive and vice versa and that the transferred term is positive (negative) if the Mulliken spin densities at the neighboring Cu atoms is positive (negative).…”

High sensitivity of O17 NMR to p-d hybridization in transition metal perovskites: First principles calculations of large anisotropic chemical shielding

“…Embedded cluster techniques have been successfully used in crystals and macromolecules to obtain nuclear quadrupole resonance spectrum, ligand to metal charge transfer excitations, photoemission, electric field gradients, and hyperfine coupling in high T c superconductors. 21,22,23,24 Such calculations have generally used standard allelectron quantum chemistry methods, which are widely available in many quantum chemistry computer programs, such as GAUSSIAN. 25,26 These methods are very mature and can calculateσ with a range of approximations, using well tested Gaussian type orbital (GTO) basis sets.…”

Dzyaloshinsky-Moriya antisymmetric exchange coupling in cuprates: Oxygen effects

“…19,20 However, even for a free oxygen atom the electron spin orbital coupling turns out to reach of appreciable magnitude: ξ 2p ∼ = 0.02 eV, 21 while for the oxygen O 2− ion in oxides one expects the visible enhancement of spin-orbital coupling due to a larger compactness of 2p wave function. 22 If to account for ξ nl ∝ r −3 nl and compare these quantities for the copper ( r −3 3d ≈ 6 − 8 a.u. 22 ) and the oxygen ( r −3 2p ≈ 4 a.u.…”