Exchange and crystal field effects in the ESR spectra of Eu(2+) in LaB(6)

Electron spin resonance of Eu(2+) (4f(7), S=7/2) in a La hexaboride (LaB(6)) single crystal shows a single anisotropic Dysonian resonance. From the observed negative g shift of the resonance, it is inferred that the Eu(2+) ions are covalent exchange coupled to the B 2p-like host conduction electrons. From the anisotropy of the spectra (linewidth and field for resonance), we found that the S ground state of Eu(2+) ions experience a cubic crystal field of a negative fourth order crystal field parameter (CFP), b(4)=-11.5(2.0) Oe, in agreement with the negative fourth order CFP, A(4), found for the non-S ground state R hexaborides. These results support covalency as the dominant contribution to the fourth order CFP for the whole R hexaboride family.

Magnetic susceptibility of an exactly solvable anisotropic spin ladder system

We investigate the thermodynamics of an integrable spin ladder model which possesses a free parameter besides rung and leg couplings. The model is exactly solvable by means of the Bethe ansatz and exhibits a phase transition between a gapped and a gapless spin excitation spectrum. The magnetic susceptibility is obtained numerically and its dependence on the anisotropy parameter is determined. The spin gap obtained from the susceptibility curve and the one obtained from the Bethe ansatz equations are in very good agreement. Our results for the magnetic susceptibility fit well the experimental data for the organometallic compounds (5IAP)(2)CuBr4 . 2H(2)O (Landee C. P. et al., Phys. Rev. B, 63 (2001) 100402(R)) Cu-2(C5H12N2)(2)Cl-4 (Hayward C. A., Poilblanc D. and Levy L. P., Phys. Rev. B, 54 (1996) R12649, Chaboussant G. et al., Phys. Rev. Lett., 19 ( 1997) 925; Phys. Rev. B, 55 ( 1997) 3046.) and (C5H12N)(2)CuBr4 (Watson B. C. et al., Phys. Rev. Lett., 86 ( 2001) 5168) in the strong-coupling regime.

Factors which affect the catalytic activity of iron(III) meso tetrakis(2,6-dichlorophenyl)porphyrin chloride in homogeneous system

An optimization study of the reaction conditions of Fe(TDCPP)Cl when it is used as catalyst in the hydroxylation of cyclohexane by iodosylbenzene (PhIO) has been carried out, It was found that Fe(TDCPP)Cl follows the classical PhIO mechanism described for Fe(TPP)Cl, which involves the monomeric active species Fe-IV(O)P-+. (I). In the optimized condition ([Fe(TDCPP) = 3.0 X 10(-4) mol l(-1) in 1,2-dichloroethane (DCE); ultrasound stirring at 0 degrees C; PhIO/FeP molar ratio = 100), this FeP led to a yield of cyclohexanol (C-ol) of 96% and a turnover number of 96, Therefore, Fe(TDCPP)Cl may be considered a good biomimetic model and a very stable, resistant and selective catalyst, which yields C-ol as the sole product. DCE showed to be a better solvent than dichloromethane (DCM), 1 DCE:1 MeOH mixture or acetonitrile (ACN). Since the Fe-IV(O)P-+. is capable of abstracting hydrogen atom from DCM, MeOH or ACN, the solvent competes with the substrate. Presence of O-2 lowers the yield of C-ol, as it can further oxidize this alcohol to carboxylic acid in the presence of radicals, Presence of H2O also causes a decrease in the yield, since it converts the active species I into Fe-IV(OH)P, which cannot oxidize cyclohexane. Addition of excess imidazole or OH- to the system results in a decrease in the yield of C-ol, due to the formation of the hexacoordinated complexes Fe(TDCPP)Im(2)(+) (low-spin, beta(2) = 2.5 X 10(8) mol(-2) l(2)) and Fe(TDCPP)(OH)(2)(-) (high-spin, beta(2) = 6.3 X 10(7) mol(-2) l(2)), the formation of both Fe(TDCPP)Im(2)(+) and Fe(TDCPP)(OH)(2)(-) complexes were confirmed by EPR studies. The catalytic activities of Fe(TDCPP)C and Fe(TFPP)Cl were compared, the unusually high yields of C-ol with Fe(TFPP)Cl obtained when ultrasound, DCM and O-2 atmosphere were used, suggest that a parallel mechanism involving the mu-oxo dimer form, O-2 and radicals may also be occurring with this FeP, besides the PhIO mechanism.

Characterization and catalytic activity of 2,6-dichlorophenyl substituted iron(III) porphyrin supported on silica gel and imidazole propyl gel

In this work we have made use of the study of the interaction between Fe(TDCPP)(+) and the axial ligands OH- and imidazole in order to help characterize the heterogenized catalysts Fe(TDCPP)SG and Fe(TDCPP)IPG through UV-VIS and EPR spectroscopies and thus, better understand their different catalytic activity in the oxidation of cyclohexane by PhIO. We have found out that in Fe(TDCPP)SG (containing 1.2 X 10(-6) mol Fe(TDCPP)(+)/g of support), the FeP bis-coordinates to silica gel through Fe-O coordination and it is high-spin (FeP)-P-III species. In Fe(TDCPP)IPG 1 (containing 1.1 X 10(-6) mol Fe(TDCPP)(+) and 2.2 X 10(-4) mol imidazole/g of support), the FeP is bis-ligated to imidazole propyl gel through Fe-imidazole coordination and using NO as a paramagnetic probe, we present evidence that Fe(TDCPP)(+) is present as a mixture of low-spin (FeP)-P-III and (FeP)-P-II species. This catalyst led to a relative low yield of cyclohexanol (25%) because the bis-coordination of the (FeP)-P-III to the support partially blocks the reaction between Fe(TDCPP)(+) and PhIO, thus leading to the formation of only a small amount of the active species Fe-IV(OP+, while the (FeP)-P-II species do not react with the oxygen donor. Increasing the amount of Fe(TDCPP)(+) and decreasing the amount of imidazole in the support led to the obtention of high-spin (FeP)-P-III EPR signals in the spectra of Fe(TDCPP)IPG 5 (containing 4.4 X 10(-6) mol Fe(TDCPP)(+) and 2.2 X 10(-5) mol imidazole/g of IPG), together with low-spin (FeP)-P-III species. This latter catalyst led to better cyclohexanol yields (67%) than Fe(TDCPP)IPG 1. Fe(TDCPP)IPG 5 was further used in a study of the optimization of its catalytic activity and in recycling experiments in the optimized conditions. Recycling oxidation reactions of Fe(TDCPP)IPG 5 led to a total turnover number of 201 and total cyclohexanol yield of 201%, which could not be attained with Fe(TDCPP)Cl in homogeneous solution (turnover = 96) due to the difficulty in recovering and reusing it.

Role of oxygen vacancies in the magnetic and dielectric properties of the high-dielectric-constant system CaCu3Ti4O12: An electron-spin resonance study

We report experiments of electron spin resonance (ESR) of Cu2+ in polycrystalline samples of CaCu3Ti4O12 post-annealed in different atmospheres. After being synthesized by solid state reaction, pellets of CaCu3Ti4O12 were annealed for 24 h at 1000 degrees C under air, Ar or O-2. Our temperature dependent ESR data revealed for all samples nearly temperature independent g value (2.15(1)) and linewidth for T > T-N approximate to 25 K. However, the values of ESR linewidth are strongly affected by the oxygen content in the sample. For instance, argon post-annealed samples show a much larger linewidth than the O-2 or air post-annealed samples. We attribute this broadening to an increase of the dipolar homogeneous broadening of the Cu2+ ESR lines due to the presence of oxygen vacancies which induce an S=1/2 spin inside the TiO6 octahedra. Correlation between a systematic dependence of the ESR linewidth on the oxygen content and the high dielectric constant of these materials is addressed. Also, ESR, magnetic susceptibility, and specific heat data for a single crystal of CaCu3Ti4O12 and for polycrystals of CdCu3Ti4O12 are reported.

MAGNETOOPTICAL PROPERTIES OF F-2(+) CENTER IN KCL-SH-

The magnetic circular dichroism (MCD) of F2+ centers in KCl:SH- has been measured in absorption in the 1ssigma(g) --> 2p(y)pi(u) transitions at 493 and 509 nm, with fields up to 5 T and in the temperature range 1.5 K < T < 77 K. Within the limit of detection, no MCD is observed in the near infrared transition 1ssigma(g) --> 2psigma(u) as well as in both emissions 2ppi(u) --> 1ssigma(g) and 2psigma(u) --> 1ssigma(g). The optical detection of EPR in the F2+ ground state presents an isotropic single band with g = 1.965 +/- 0.007. The spin-lattice relaxation measured at H = 0.32 T is typical of a direct process T-1 = 4.3 x 10(-2_ coth (gmu(B)H/2k(B)T). The spectral variation of the MCD is calculated using perturbation theory to first order. The Hamiltonian includes the spin-orbit interaction in the 2ppi(u) excited state and the orbital molecular wave functions are obtained by a linear combination of 1s and 2p atomic orbitals. The calculated MCD is in good agreement with the observed one, for the spin-orbit interaction strength Pound(z) = 3.6 meV.

Rare Earth Doped SnO(2) Nanoscaled Powders and Coatings: Enhanced Photoluminescence in Water and Waveguiding Properties

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Spin-orbit coupling for tidally evolving super-Earths

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Higher spin symmetries and w∞ algebra in the conformal affine Toda model

As recently shown the conformal affine Toda models can be obtained via hamiltonian reduction from a two-loop Kac-Moody algebra. In this paper we propose a systematic procedure to analyze the higher spin symmetries of the conformal affine Toda models. The method is based on an explicit construction of infinite towers of extended conformal symmetry generators. Two fundamental building blocks of this construction are special spin-one and -two primary fields characterizing the conformal structure of these models. The connection to the algebra of area preserving diffeomorphisms on a two-manifold (w∞ algebra) is established.

A free relativistic anyon with canonical spin algebra

We discuss a relativistic free particle with fractional spin in 2+1 dimensions, where the dual spin components satisfy the canonical angular momentum algebra {Sμ, Sν} = εμνγSγ. It is shown that it is a general consequence of these features that the Poincaré invariance is broken down to the Lorentz one, so indicating that it is not possible to keep simultaneously the free nature of the anyon and the translational invariance.

Translating conduction-electron spin-resonance lines into lorentzian lines

The Dysonian line in the limit d < or ∼ δ, where d is the thickness and 6 the skin depth, was fitted to a combination of absorption and dispersion Lorentzian lines. This procedure allows one to determine not only microwave conductivity from the Dysonian line but also the true g value, linewidth, and paramagnetic susceptibility by the measurement of five parameters of the ESR absorption-derivative Dysonian line. ©1990 Academic Press, inc.

Competition between spin, charge, and bond waves in a Peierls-Hubbard model

We study a one-dimensional extended Peierls-Hubbard model coupled to intracell and intercell phonons for a half-filled band. The calculations are made using the Hartree-Fock and adiabatic approximations for arbitrary temperature. In addition to static spin, charge, and bond density waves, we predict intermediate phases that lack inversion symmetry, and phase transitions that reduce symmetry on increasing temperature.

A simple action for a free fractional spin particle

By studying classical realizations of the sl(2, R-fraktur sign) algebra in a two dimensional phase space (q,π), we have derived a continuous family of new actions for free fractional spin particles in 2 + 1 dimensions. For the case of light-like spin vector (SμSμ = 0), the action is remarkably simple. We show the appearence of the Zitterbewegung in the solutions of the equations of motion, and relate the actions to others in the literature at classical level. © 1997 Elsevier Science B.V.