Min-Quan Kuang

Theoretical Investigations of the Knight Shifts and the Hyperfine Structure Constants for the Tetragonal Cu2+ Sites in the Bismuth- and Thallium-Based High-T-c Superconductors

The Knight shifts and hyperfine structure constants for the tetragonal Cu2+ sites in bismuth- and thallium-based high-Tc uperconductors ( Bi1:6Pb0:4Sr2Ca2Cu3O10, TlSr2CaCu2O7-y, and Tl2Ba2CuOy) are theoretically investigated from the high-order perturbation formulas of these parameters for a 3d9 ion under tetragonally elongated octahedra in a unified way. The calculation results show good agreement with the observed values. The significant anisotropies of the Knight shifts are attributed to the local tetragonal elongation distortions of the five-(or six-)coordinated Cu2+ sites in these systems. The present studies would be beneficial to establish a complete physical scheme for unified understandings of electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectral behaviours of Cu2+ (or other similar 3d9 ions) in the high-Tc superconductors.

Investigations on the EPR parameters and tetragonal distortions for Cu2+ in alkali lead tetraborate glasses

The electron paramagnetic resonance parameters (g factors and hyperfine structure constants) and local structures are theoretically investigated for Cu2+ in alkali lead tetraborate 90R2B4O7·9PbO·CuO (R = Li, Na and K) glasses based on the high-order perturbation calculations for a tetragonally elongated octahedral 3d9 complex. The [CuO6]10− complexes are found to experience the relative tetragonal elongation ratios 18%, 23% and 30% for R = Li, Na and K, respectively, due to the Jahn–Teller effect, much larger than those for similar ARbB4O7 (A = Li, Na and K) glasses. This point is attributed to the lattice expansion (longer A–O bond lengths) with doped PbO, yielding lower force constants and more intense Jahn–Teller elongations in the 90R2B4O7·9PbO·CuO glasses. The increasing tendency (Li > Na > K) of the relative elongation ratio λ, covalency and the ratio Δg///Δg⊥ for g-shifts are systematically analysed in a uniform way.

Anisotropic g factors of the tetragonal Cu2+ monomer in Tl-2223 superconductor

The gyromagnetic factors of the Cu2+ monomer in Tl-2223 superconductor are quantitatively investigated from the perturbation formulas of these factors for a 3d9 ion in a tetragonally elongated octahedron. The local tetragonal distortion of the system is attributable to the axial elongation along c axis, corresponding to the five-fold coordinated Cu2+(2) site with almost 30% longer Cu–O bond length for the apical oxygen as compared to the four planar ones. The significant anisotropic behaviors of the EPR spectra perpendicular to and parallel with the ab (CuO2) layers are analyzed on the basis of the local tetragonal elongation.

Studies of the Local Distortions and the EPR Parameters for Cu2+ in xLi2O-(30–x)Na2O-69·5B2O Glasses

Abstract The local distortions and electron paramagnetic resonance (EPR) parameters for Cu2+ in lithium sodium borate (LNB) glasses xLi2O·(30–x)·Na2O·69.5B2O3 (5≤x≤25 mol%) are theoretically studied at various concentrations x in a consistent way. Owing to the Jahn–Teller effect, the [CuO6]10− clusters are found to experience the significant tetragonal elongations of 16% along C4 axis. Despite the nearly unchanging observed g factors, measured d–d transition band (or cubic field parameter Dq) shows remarkable linear increases with concentration x, whose influences on g‖ and g⊥ are actually cancelled by the linearly increasing covalency factor N and relative elongation ratio η with x. The almost unvarying hyperfine structure constants are attributed to the fact that the influences of the linearly increasing N and the linearly decreasing core polarisation constant κ largely cancel one another. The microscopic mechanisms of the above concentration dependences for these quantities are illustrated from mixed alkali effect (modification of B2O3 network by transforming some BO3 units into BO4 ones with variations in modifier Li2O concentration).

Theoretical investigations of 63Cu2+ orbital Knight shifts for YBa2Cu4O8

The temperature-independent orbital Knight shifts for the orthorhombic 63Cu2+(1) site in YBa2Cu4O8 (Y124) are investigated by utilizing the high order perturbation formulae of these parameters for a 3d9 ion situated into orthorhombically elongated octahedra. The calculation results are in good agreement with the experimental data. The moderate quasi-axial anisotropies of the Knight shifts are ascribed to the elongation distortion of the four-fold coordinated Cu2+(1) site. The g factors are also theoretically calculated in a uniform way for further experimental verification.

Studies on the Local Angular Distortion and Spin Hamiltonian Parameters for the Trigonal Co2+ Center in MgCl2

Abstract The local angular distortion and spin Hamiltonian parameters (g factors g||, g⊥ and the hyperfine structure constants) for the trigonal Co2+ center in MgCl2 are theoretically studied by diagonalizing the 6×6 energy matrix of ground 4T1 state for a trigonally distorted octahedral 3d7 cluster. Based on the cluster approach, the contributions from the admixtures of various J (= 1/2;3/2;5/2) states and the ligand orbital and spin-orbit coupling interactions are taken into account in a uniform way. The local impurity-ligand bond angle in the Co2+ center is found to be about 3.44° larger than the host metal-ligand bond angle in the pure crystal due to substitution of smaller Mg2+ by bigger Co2+, inducing a further compressed ligand octahedron. The calculated spin Hamiltonian parameters using the above local angular distortion are in good agreement with the experimental data. The present studies on the local structure and the spin Hamiltonian parameters for Co2+ in MgCl2 are tentatively extended to a more general case by comparing the relevant impurity behaviours for Co2+ in various trigonal environments.

Theoretical investigations of the spin Hamiltonian parameters for the CoCl(PPh3)3 molecules

The perturbation formulas of the spin Hamiltonian parameters (zero-field splitting and g factor g// and g⊥) are established for a 3d8 ion in trigonally distorted tetrahedra for the first time. In the theoretical treatments, the contributions from the Jahn–Teller effect, the ligand orbital and spin–orbit coupling interactions and configuration interactions are taken into account from the cluster approach in a uniform way. The above formulas are applied to the studies of the spin Hamiltonian parameters for the three CoCl(PPh3)3 molecules, and the experimental electron paramagnetic resonance spectra of all the molecules are satisfactorily explained. The significant compressions of the ligand tetrahedra with mixed chlorine and PPh3 groups around Co+ are analysed for three distinct CoCl(PPh3)3 compounds, characterised by the Cl–Co–P bond angles θ larger than the tetrahedral angle of 109.47°. The local trigonal distortions are discussed in view of the Jahn–Teller effect.

Investigations on the spin Hamiltonian parameters of Cu(2) site in (Tl0.5Pb0.5)Sr2CaCu2O7 superconductors

The spin Hamiltonian parameters (g factors and hyperfine structure constants) of the five-fold coordinated Cu(2) site in (Tl0.5Pb0.5)Sr2CaCu2O7 superconductors are investigated from the high order perturbation formulas of these parameters for a 3d9 ion in tetragonally elongated octahedron. The observed anisotropies (g∥ > g⊥ and |A∥| > |A⊥|) are ascribed to the 2B 1g ground state for Cu2+ under octahedral tetragonal elongation distortion. The comparable magnitudes of the parallel and perpendicular hyperfine structure constants may be illustrated by the small core polarization constant and the reduction factor H. The ligand orbital and spin–orbit coupling contributions are found to be important due to significant covalency and should be considered in the calculations.

Theoretical studies on the Knight shifts for the tetragonal Cu2+ site in HgBa2CuO4+δ

The Knight shifts for the tetragonal Cu2+ site in HgBa2CuO4+δ are theoretically studied from the high order perturbation formulas of the Knight shifts for a tetragonally elongated octahedral 3d9 cluster. The significant anisotropy of the Knight shifts is attributable to the obvious tetragonal elongation distortion of the Cu2+ site. The anisotropic g factors of this system are uniformly analyzed, and the calculation results and the local structure of the copper site are also discussed.

Theoretical investigations of the spin Hamiltonian parameters and local angular variations for the trigonal V3+ centers in alum compounds

The spin Hamiltonian parameters (zero-field splittings D, g factors g// and g⊥) for V3+ in the alum compounds are theoretically studied using the perturbation formulas for these parameters for a 3d2 ion in the trigonally distorted octahedra. The contributions from the dynamical Jahn–Teller effect, the configuration interactions and the ligand orbital and spin–orbit coupling interactions are considered from the cluster approach in a uniform way. The angles in the deformed octahedra around V3+ ions are increased by 1.1°–2.7°, as compared with those ones for the host. The theoretical spin Hamiltonian parameters based on the above angular variations and the related effects show good agreement with the experimental data, which reveal that these effects (especially the Jahn–Teller effect) can bring forward significant influences on the spin Hamiltonian parameters, and should be taken into account in electron paramagnetic resonance (EPR) analysis. The contributions from the configuration interactions and the ligand orbital and spin–orbit coupling interactions are quantitatively involved from the cluster approach in a uniform way. The results are discussed.