Anisotropic magnetic behavior of GdBa_2Cu_3O_{6+y} single crystals

Magnetic properties of high-quality Al-free nonsuperconducting GdBa_2Cu_3O_{6+y} single crystals grown by flux method have been studied. The magnetic anisotropy below the N\'eel temperature T_N~2.3K corresponds to the direction of Gd^{3+} magnetic moments along the tetragonal c-axis. At T < T_N clear indications of spin-flop transitions for H||c have been observed on magnetization curves at H_{sf}~10kOe. Magnetic phase diagrams have been obtained for H||c as well as for H||ab. A pronounced anisotropy in the magnetic susceptibility (unexpected for Gd-based compounds) has been found above T_N.Comment: 2 pages, 3 figures; LT23 (Aug. 2002; Hiroshima), accepted to Physica

Optical study of 4f excitations in rare earth cuprates

Several recent examples are used to demonstrate that Raman and infrared spectroscopy can be successfully used as a novel experimental tool to study microscopic processes involving 4f electrons in rare earth (RE) cuprates. Raman-active crystal field (CF) excitations in Nd₂CuO₄ were measured under hydrostatic pressure up to ∼7 GPa. The observed pressure-induced shifts of the CF levels were interpreted using density-functional-theory-based ab initio calculations and the superposition model. An infrared transmission study of the ⁴Ij, J=9/2, 11/2, 13/2 multiplets of Nd³⁺ in Nd₂CuO₄ reveals a splitting of the Kramers doublets of the order of a few cm⁻¹ due to the Nd-Cu exchange interaction. This study shows that these splittings can be described by an effective anisotropic exchange Hamiltonian for the Nd³⁺ ion expressed in terms of spherical tensor operators up to the sixth order. The isotropic term in the exchange Hamiltonian vanishes for symmetry reasons in this case. An analysis of the infrared transmission spectra in RE₁₊xBa₂₋xCu3O₆₊δ (RE=Nd,Sm) up to ∼ 10 000 cm⁻¹ indicates that, besides the regular sites, the RE ions also occupy Ba sites, even in the samples with the cation stoichiometry 1–2–3

Electronic and Magnetic Properties of Electron-doped Superconductor, Sm_{1.85}Ce_{0.15}CuO_{4-delta}

Temperature-dependent magnetization (M(T)) and specific heat (C_p(T)) measurements were carried out on single crystal Sm_{1.85}Ce_{0.15}CuO_{4-delta} (T_c = 16.5 K). The magnetic anisotropy in the static susceptibility, chi {equiv} M/H, is apparent not only in its magnitude but also in its temperature dependence, with chi_{perp} for H{perp}c larger than chi_{parallel} for H{parallel}c. For both field orientations, chi does not follow the Curie-Weiss behavior due to the small energy gap of the J = 7/2 multiplet above the J = 5/2 ground-state multiplet. However, with increasing temperature, chi_{parallel}(T) exhibits a broad minimum near 100 K and then a slow increase while chi_{perp}(T) shows a monotonic decrease. A sharp peak in C_p(T) at 4.7 K manifests an antiferromagnetic ordering. The electronic contribution, gamma, to C_p(T) is estimated to be gamma = 103.2 (7) mJ/moleSmK^2. The entropy associated with the magnetic ordering is much smaller than Rln2, where R is the gas constant, which is usually expected for the doublet ground state of Sm^{+3}. The unusual magnetic and electronic properties evident in M(T) and C_p(T) are probably due to a strong anisotropic interaction between conduction electrons and localized electrons at Sm^{+3} sites.Comment: 5 pages, 5 encapsulated postscript figures, late

Raman active phonon and crystal-field studies of Yb3+ doped NdVO4

The F-2(7/2) and F-2(5/2) crystal field energy levels of the Yb3+ ion are measured by infrared absorption in Yb doped NdVO4, with concentrations ranging from 0.25% to 15%, as a function of doping, temperature and magnetic field. Two substitution sites are observed and the presence of Yb3+-Yb3+ interacting pairs is confirmed. Also, the Raman active phonons are detected and analyzed in order to determine the doping effects on the crystalline quality and the phonon contributions to the crystal field vibronics

Crystal-field study of Yb3+ doped LuVO4

Raman active phonons of Yb doped LuVO(4) ranging from 0.25% to 30% are detected and analyzed at low temperature in order to determine the doping effects on the crystalline quality and the phonon contribution to the crystal-field vibronics. Also, Yb(3+) ion (2)F(7/2) and (2)F(5/2) crystal-field energy levels are measured as a function of doping by infrared absorption and luminescence. The appropriate crystal-field Hamiltonian parameters are determined and the presence of Yb(3+)-Yb(3+) magnetically interacting pairs, which play an important role in cooperative emission, is confirme

CRYSTAL-FIELD EFFECTS IN REBa2Cu3O7

Superposition model of the crystal field is used to predict splittings of the lowest multiplets and ground-state magnetic properties of rare earth (RE) ions in the high-Tc superconductor REBa2Cu3O7. The results are compared with experimental data available in the literature