Exciton doublet in the Mott-Hubbard insulator LiCuVO4 identified by spectral ellipsometry.

Spectroscopic ellipsometry was used to study the dielectric function of LiCuVO4, a compound comprised of chains of edge-sharing CuO4 plaquettes, in the spectral range 0.75-6.5 eV at temperatures 7-300 K. For photon polarization along the chains, the data reveal a weak but well-resolved two-peak structure centered at 2.15 and 2.95 eV whose spectral weight is strongly enhanced upon cooling near the magnetic ordering temperature. We identify these features as an exciton doublet in the Mott-Hubbard gap that emerges as a consequence of the Coulomb interaction between electrons on nearest and next-nearest-neighbor sites along the chains. Our results and methodology can be used to address the role of the long-range Coulomb repulsion for compounds with doped copper-oxide chains and planes.

Magnetic properties of spin-orbital polarons in lightly doped cobaltates.

We present a numerical treatment of a spin-orbital-polaron model for NaxCoO2 at small hole concentration (0.7 < x 1). We demonstrate how the polarons account for the peculiar magnetic properties of this layered compound: They explain the large susceptibility; their internal degrees of freedom lead both to a negative Curie-Weiss temperature and yet to a ferromagnetic intralayer interaction, thereby resolving a puzzling contradiction between these observations. We make specific predictions on the momentum and energy location of excitations resulting from the internal degrees of freedom of the polaron, and discuss their impact on spin-wave damping.

Wigner crystallization in Na3Cu2O4 and Na8Cu5O10 chain compounds.

We report the synthesis of novel edge-sharing chain systems Na(3)Cu(2)O(4) and Na(8)Cu(5)O(10), which form insulating states with commensurate charge order. We identify these systems as one-dimensional Wigner lattices, where the charge order is determined by long-range Coulomb interaction and the number of holes in the d shell of Cu. Our interpretation is supported by x-ray structure data as well as by an analysis of magnetic susceptibility and specific heat data. Remarkably, due to large second neighbor Cu-Cu hopping, these systems allow for a distinction between the (classical) Wigner lattice and the 4k(F) charge-density wave of quantum mechanical origin.

Effect of an industrial waste water on the nitrification in fixed-bed biofilm reactors--use of fluorescence in-situ hybridization (FISH).

The influence of an industrial waste water containing partly toxic and poorly biodegradable substances on an autotrophic biocenosis was investigated. Nitrifying bacteria were identified using fluorescently-labeled oligonucleotide probes and epifluorescence microscopy. Industrial waste water was used untreated and after ozonation to simulate the effects of indirect discharge to municipal waste water treatment plants. Results were compared to those obtained with acetate as a non-toxic and easily biodegradable substance and a mixture of acetate and pyruvate. The degradation of ammonium and the formation of nitrite and nitrate were measured and compared with the results obtained by fluorescence in-situ hybridization (FISH). The untreated waste water, containing higher amounts of refractory substances, disturbed nitrification, which was restored after reaching higher elimination of the organic substances. However, it showed only minor effects on the bacterial composition. These findings were similar to those reached by the addition of acetate and pyruvate. On the other hand, the ozonated waste water, showing higher toxicity than the untreated waste water, caused a stabilization in nitrification, but the composition of the population of ammonium-oxidizing bacteria changed significantly. In all cases, nitrite-oxidizing bacteria were only little affected both in activity and abundance.

Influence of radiation sterilization on the stability of trifluorothymidine.

The influence of radiation sterilization on the stability of trifluorothymidine (TFT) was investigated. TFT was irradiated under ambient atmosphere with a 60Co-source and with an electron accelerator at 25, 50, and 100 kGy, respectively. The radiation-induced effects were determined by chromatographic and spectroscopic methods as well as potentiometrically with a fluoride selective electrode. TFT was moderately stable to ionizing radiation. The degradation induced by electron-beam irradiation was significantly (P=95%) smaller than by gamma-irradiation. The radiolysis products amounted to about 0.25% after electron-beam irradiation at 25 kGy, and to about 0.50% after gamma-irradiation, respectively. The main irradiation product was 5-trifluoromethyluracil (TFMU). In addition five further impurities were detected with HPLC. Identification of degradation products was performed using HPLC-ESI-MS. A degradation path of TFT after radiation sterilization was shown.

Spin order due to orbital fluctuations: cubic vanadates.

We investigate the highly frustrated spin and orbital superexchange interactions in cubic vanadates. The fluctuations of t(2g) orbitals trigger a novel mechanism of ferromagnetic interactions between spins S = 1 of V3+ ions along one of the cubic directions which operates already in the absence of Hund's rule exchange J(H), and leads to the C-type antiferromagnetic phase in LaVO3. The Jahn-Teller effect can stabilize the orbital ordering and the G-type antiferromagnetic phase at low temperatures, but large entropy due to orbital fluctuations favors again the C phase at higher temperatures, as observed in YVO (3).

Photoemission spectra of LaMnO3 controlled by orbital excitations

We investigate the spectral function of a hole moving in the orbital-ordered ferromagnetic planes of LaMnO3, and show that it depends critically on the type of orbital ordering. While the hole does not couple to the spin excitations, it interacts strongly with the excitations of e(g) orbitals (orbitons), leading to a new type of quasiparticle with a dispersion on the orbiton energy scale and with strongly enhanced mass and reduced weight. Therefore we predict a large redistribution of spectral weight with respect to the bands found in local density approximation (LDA) or in LDA+U.