235 research outputs found
Optical Spectroscopy of Orbital and Magnetic Excitations in Vanadates and Cuprates
Within the scope of this thesis the low-energy excitations of undoped Mott insulators RVO(3) with R = Y, Ho, and Ce, (Sr,Ca)CuO(2) and La(8)Cu(7)O(19) have been investigated by means of optical spectroscopy. The compounds RVO(3) with R = rare-earth ion recently have attracted a lot of interest because of their unusual structural, orbital, and magnetic properties. The compounds undergo a series of temperature-induced phase transitions accompanied by a change of orbital and magnetic order. Furthermore, it has been proposed that YVO(3) represents the first realization of a one-dimensional orbital liquid and an orbital Peierls phase, with a transition to an orbitally ordered phase at lower temperatures. In this thesis, we present the optical conductivity sig(w) of RVO(3) with R = Y, Ho, and Ce for energies from 0.1 to 1.6 eV as a function of temperature (10-300 K) and polarization of the incident light parallel to the crystallographic axes (sig_a, sig_b, sig_c). Our main experimental result is the observation of two absorption features at 0.55 eV in sig_a and 0.4 eV in sig_c which are assigned to collective orbital excitations, in contrast to conventional local crystal-field transitions. Altogether our results strongly suggest that in RVO(3) with R = Y, Ho, and Ce the orbital exchange interactions play a decisive role. In a second study we have investigated the magnetic excitations of low-dimensional quantum magnets, namely the spin chain (Sr,Ca)CuO(2) and the five-leg ladder La(8)Cu(7)O(19). For (Sr,Ca)CuO(2), two absorption features around 0.4 eV in sig_c (chain direction) and sig_b (inter-chain direction) are identified as magnetic contributions to the optical conductivity. The analysis of sig_c enables the very precise determination of the nearest-neighbor exchange coupling Jc as a function of temperature and Ca substitution. We have found Jc = (227 +- 4) meV for SrCuO(2) at low temperatures and no effect on Jc upon Ca-substitution of 10%. Furthermore, we ascertained that the theoretically predicted two-spinon-plus-phonon contribution describes roughly 74-97% of the observed spectral weight. We suggest that the remaining weight has to be ascribed to multi-spinon excitations. To explain the small magnetic absorption features in sig_b we sketch a new mechanism which gives rise to a dipole moment perpendicular to the chains when holes are virtually exchanged along the chain. In particular, we point out the possible relevance of this contribution to the optical conductivity of many quasi-1D quantum spin systems. Our data of La(8)Cu(7)O(19) are the first experimental result on the magnetic excitations of n-leg ladders (n > 2) at high energies. Amongst others, we compared our data with the well-investigated spectrum of the two-leg ladder and found a surprising resemblance. Although a detailed understanding of the observed line shape of the 5-leg ladder is missing up to now, our data strongly suggest the existence of bound states of magnetic excitations also in this system. The comparison of different low-dimensional S = 1/2 systems suggests that the magnetic excitations in the high-energy continua are similar and develop smoothly from 1D to 2D
Side Effects of Nitrification Inhibitors on Non Target Microbial Processes in Soils
Agricultural chemicals have been used extensively in modern agriculture and toxicological studies suggest a great potential for inducing undesirable effects on non target organisms. A model experiment was conducted in order to determine side effects of three nitrification inhibitors (NIs, 3,4dimethylpyrazolephosphate = DMPP, 4-Chlormethylpyrazole phosphate = ClMPP and dicyandiamide = DCD) on non target microbial processes in soils. Side effects and dose response curve of three NIs were quantified under laboratory conditions using silty clay, loam anda sandy soils. Dehydrogenase, dimethylsulfoxide reductase as well as nitrogenase activity (NA) and potential denitrification capacity were measured as common and specific non target microbial processes. The influence of 5-1000 times the base concentration, dose response curves were examined, and no observable effect level = NOEL, as well as effective dose ED10 and ED50 (10% and 50% inhibition) were calculated. The NOEL for microbial non target processes were about 30–70 times higher than base concentration in all investigated soils. The potential denitrification capacity revealed to be the most sensitive parameter. ClMPP exhibited the strongest influence on the non target microbial processes in the three soils. The NOEL, ED10 and ED50 values were higher in clay than in loamy or sandy soil. The NIs was the most effective in sandy soils.Keywords: microbial non target processes, nitrification inhibitors, soil enzyme
Toxicity of Nitrification Inhibitors on Dehydrogenase Activity in Soils
The objective of this research was to determine the effects of nitrification inhibitors (NIs) such as 3,4-dimethylpyrazolephosphate=DMPP, 4-Chlor-methylpyrazole phosphate=ClMPP and dicyandiamide,DCD) which might be expected to inhibit microbial activity, on dehydrogenase activity (DRA),in three different soils in laboratory conditions. Dehydrogenase activity were assessed via reduction of 2-p-Iodophenyl-3-p-nitrophenyl-5-phenyltetrazoliumchloride (INT). The toxicity and dose response curve of three NIs were quantified under laboratory conditions using a loamy clay, a sandy loam and a sandy soil. The quantitative determination of DHA was carried out spectrophotometrically. In all experiments, the influence of 5-1000 times the base concentration were examined. To evaluate the rate of inhibition with the increasing NI concentrations, dose reponse curves were presented and no observable effect level =NOEL, as well as effective dose ED10 and ED 50(10% and 50% inhibition) were calculated. The NOEL for common microbial activity such as DHA was about 30–70 times higher than base concentration in all investigated soils. ClMPP exhibited the strongest influence on the non target microbial processes in the three soils if it compare to DMPP and DCD. The NOEL,ED10 and ED50 values higher in clay than in loamy or sandy soil. The NIs were generally most effective in sandy soils. The three NIs considered at the present state of knowledge as environmentally safe in use
Dimensionality Control of Electronic Phase Transitions in Nickel-Oxide Superlattices
The competition between collective quantum phases in materials with strongly
correlated electrons depends sensitively on the dimensionality of the electron
system, which is difficult to control by standard solid-state chemistry. We
have fabricated superlattices of the paramagnetic metal LaNiO3 and the wide-gap
insulator LaAlO3 with atomically precise layer sequences. Using optical
ellipsometry and low-energy muon spin rotation, superlattices with LaNiO3 as
thin as two unit cells are shown to undergo a sequence of collective
metalinsulator and antiferromagnetic transitions as a function of decreasing
temperature, whereas samples with thicker LaNiO3 layers remain metallic and
paramagnetic at all temperatures. Metal-oxide superlattices thus allow control
of the dimensionality and collective phase behavior of correlated-electron
systems
Digital modulation of the nickel valence state in a cuprate-nickelate heterostructure
Layer-by-layer oxide molecular beam epitaxy has been used to synthesize
cuprate-nickelate multilayer structures of composition
(LaCuO)/LaO/(LaNiO). In a combined experimental and
theoretical study, we show that these structures allow a clean separation of
dopant and doped layers. Specifically, the LaO layer separating cuprate and
nickelate blocks provides an additional charge that, according to density
functional theory calculations, is predominantly accommodated in the
interfacial nickelate layers. This is reflected in an elongation of bond
distances and changes in valence state, as observed by scanning transmission
electron microscopy and x-ray absorption spectroscopy. Moreover, the predicted
charge disproportionation in the nickelate interface layers leads to a
thickness-dependent metal-to-insulator transition for , as observed in
electrical transport measurements. The results exemplify the perspectives of
charge transfer in metal-oxide multilayers to induce doping without introducing
chemical and structural disorder
Tunable Charge and Spin Order in PrNiO Thin Films and Superlattices
We have used polarized Raman scattering to probe lattice vibrations and
charge ordering in 12 nm thick, epitaxially strained PrNiO films, and in
superlattices of PrNiO with the band-insulator PrAlO. A carefully
adjusted confocal geometry was used to eliminate the substrate contribution to
the Raman spectra. In films and superlattices under tensile strain, which
undergo a metal-insulator transition upon cooling, the Raman spectra reveal
phonon modes characteristic of charge ordering. These anomalous phonons do not
appear in compressively strained films, which remain metallic at all
temperatures. For superlattices under compressive strain, the Raman spectra
show no evidence of anomalous phonons indicative of charge ordering, while
complementary resonant x-ray scattering experiments reveal antiferromagnetic
order associated with a modest increase in resistivity upon cooling. This
confirms theoretical predictions of a spin density wave phase driven by spatial
confinement of the conduction electrons.Comment: PRL, in pres
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