74 research outputs found
Magnetic versus nonmagnetic doping effects on the magnetic ordering in the Haldane chain compound PbNi2V2O8
A study of an impurity driven phase-transition into a magnetically ordered
state in the spin-liquid Haldane chain compound PbNi2V2O8 is presented. Both,
macroscopic magnetization as well as 51V nuclear magnetic resonance (NMR)
measurements reveal that the spin nature of dopants has a crucial role in
determining the stability of the induced long-range magnetic order. In the case
of nonmagnetic (Mg2+) doping on Ni2+ spin sites (S=1) a metamagnetic transition
is observed in relatively low magnetic fields. On the other hand, the magnetic
order in magnetically (Co2+) doped compounds survives at much higher magnetic
fields and temperatures, which is attributed to a significant anisotropic
impurity-host magnetic interaction. The NMR measurements confirm the predicted
staggered nature of impurity-liberated spin degrees of freedom, which are
responsible for the magnetic ordering. In addition, differences in the
broadening of the NMR spectra and the increase of nuclear spin-lattice
relaxation in doped samples, indicate a diverse nature of electron spin
correlations in magnetically and nonmagnetically doped samples, which begin
developing at rather high temperatures with respect to the antiferromagnetic
phase transition.Comment: 10 pages, 7 figure
ESR of MnO embedded in silica nanoporous matrices with different topologies
Electron spin resonance (ESR) experiments were performed with
antiferromagnetic MnO confined within a porous vycor-type glass and within
MCM-type channel matrices. A signal from confined MnO shows two components from
crystallized and amorphous MnO and depends on the pore topology. Crystallized
MnO within a porous glass shows a behavior having many similarities to the
bulk. In contrast with the bulk the strong ESR signal due to disordered
"surface" spins is observed below the magnetic transition. With the decrease of
channel diameter the fraction of amorphous MnO increases while the amount of
crystallized MnO decreases. The mutual influence of amorphous and crystalline
MnO is observed in the matrices with a larger channel diameter. In the matrices
with a smaller channel diameter the ESR signal mainly originates from amorphous
MnO and its behavior is typical for the highly disordered magnetic system.Comment: 7 pages pdf file, 5 figure
Ceramic synthesis of disordered lithium rich oxyfluoride materials
Disordered lithium-rich transition metal oxyfluorides with a general formula LiMOF (M being a transition metal) are gaining more attention due to their high specific capacity which can be delivered from the facecentered cubic (fcc) structure. The most common synthesis procedure involves use of mechanosynthesis. In this work, ceramic synthesis of lithium rich iron oxyfluoride and lithium rich titanium oxyfluoride are reported. Two ceramic synthesis routes are developed each leading to the different level of doping with Li and F and different levels of cationic disorder in the structure. Three different LiMOF samples (x ¼ 0.25, 0.3 and 1) are compared with a sample prepared by mechanochemical synthesis and non-doped LiFeO2 with fcc structure. The obtained lithium rich iron oxyfluoride are characterized by use of M€ossbauer spectroscopy, X-ray absorption spectroscopy, NMR and TEM. Successful incorporation of Li and F have been confirmed and specific capacity that can be obtained from the samples is in the correlation with the level of disorder introduced with doping, nevertheless oxidation state of iron in all samples is very similar. Conclusions obtained from lithium rich iron oxyfluoride are validated by lithium rich titanium oxyfluoride
Ba4Ru3O10.2(OH)1.8 : a new member of the layered hexagonal perovskite family crystallised from water
A new barium ruthenium oxyhydroxide Ba4Ru3O10.2(OH)1.8 crystallises under hydrothermal conditions at 200 °C: powder neutron diffraction data show it adopts an 8H hexagonal perovskite structure with a new stacking sequence, while high resolution electron microscopy reveals regions of ordered layers of vacant Ru sites, and magnetometry shows antiferromagnetism with TN = 200(5) K
Electronic properties of LaOFFeAs in the normal state probed by NMR/NQR
We report 139La, 57Fe and 75As nuclear magnetic resonance (NMR) and nuclear
quadrupole resonance (NQR) measurements on powders of the new LaO1-xFxFeAs
superconductor for x = 0 and x = 0.1 at temperatures up to 480 K, and compare
our measured NQR spectra with local density approximation (LDA) calculations.
For all three nuclei in the x = 0.1 material, it is found that the local Knight
shift increases monotonically with an increase in temperature, and scales with
the macroscopic susceptibility, suggesting a single magnetic degree of freedom.
Surprisingly, the spin lattice relaxation rates for all nuclei also scale with
one another, despite the fact that the form factors for each site sample
different regions of q-space. This result suggests a lack of any q-space
structure in the dynamical spin susceptibility that might be expected in the
presence of antiferromagnetic correlations. Rather, our results are more
compatible with simple quasi-particle scattering. Furthermore, we find that the
increase in the electric field gradient at the As cannot be accounted for by
LDA calculations, suggesting that structural changes, in particular the
position of the As in the unit cell, dominate the NQR response.Comment: 17 pages, 6 figure
Ferromagnetism in graphene nanoribbons: split versus oxidative unzipped ribbons
Two types of graphene nanoribbons: (a) potassium-split graphene nanoribbons
(GNRs), and (b) oxidative unzipped and chemically converted graphene
nanoribbons (CCGNRs) were investigated for their magnetic properties using the
combination of static magnetization and electron spin resonance measurements.
The two types of ribbons possess remarkably different magnetic properties.
While the low temperature ferromagnet-like feature is observed in both types of
ribbons, such room temperature feature persists only in potassium-split
ribbons. The GNRs show negative exchange bias, but the CCGNRs exhibit a
'positive exchange bias'. Electron spin resonance measurements infer that the
carbon related defects may responsible for the observed magnetic behaviour in
both types of ribbons. Furthermore, proton hyperfine coupling strength has been
obtained from hyperfine sublevel correlation experiments performed on the GNRs.
Electron spin resonance provides no indications for the presence of potassium
(cluster) related signals, emphasizing the intrinsic magnetic nature of the
ribbons. Our combined experimental results may infer the coexistence of
ferromagnetic clusters with anti-ferromagnetic regions leading to disordered
magnetic phase. We discuss the origin of the observed contrast in the magnetic
behaviours of these two types of ribbons
Ground state and finite temperature behavior of 1/4-filled band zigzag ladders
We consider the simplest example of lattice frustration in the 1/4-filled
band, a one-dimensional chain with next-nearest neighbor interactions. For this
zigzag ladder with electron-electron as well as electron-phonon interactions we
present numerical results for ground state as well as thermodynamic properties.
In this system the ground state bond distortion pattern is independent of
electron-electron interaction strength. The spin gap in the ground state of the
zigzag ladder increases with the degree of frustration. Unlike in
one-dimension, where the spin-gap and charge ordering transitions can be
distinct, we show that in the ladder they occur simultaneously. We discuss spin
gap and charge ordering transitions in 1/4-filled materials with one, two, or
three dimensional crystal structures. We show empirically that regardless of
dimensionality the occurrence of simultaneous or distinct charge and magnetic
transitions can be correlated with the ground state bond distortion pattern.Comment: 12 pages, 8 eps figure
Genomic traits of Klebsiella oxytoca DSM 29614, an uncommon metal-nanoparticle producer strain isolated from acid mine drainages
Abstract
Background: Klebsiella oxytoca DSM 29614 - isolated from acid mine drainages - grows anaerobically using Fe(III)- citrate as sole carbon and energy source, unlike other enterobacteria and K. oxytoca clinical isolates. The DSM 29614 strain is multi metal resistant and produces metal nanoparticles that are embedded in its very peculiar capsular exopolysaccharide. These metal nanoparticles were effective as antimicrobial and anticancer compounds, chemical catalysts and nano-fertilizers.
Results: The DSM 29614 strain genome was sequenced and analysed by a combination of in silico procedures. Comparative genomics, performed between 85 K. oxytoca representatives and K. oxytoca DSM 29614, revealed that this bacterial group has an open pangenome, characterized by a very small core genome (1009 genes, about 2%), a high fraction of unique (43,808 genes, about 87%) and accessory genes (5559 genes, about 11%). Proteins belonging to COG categories “Carbohydrate transport and metabolism” (G), “Amino acid transport and metabolism” (E), “Coenzyme transport and metabolism” (H), “Inorganic ion transport and metabolism” (P), and “membrane biogenesis-related proteins” (M) are particularly abundant in the predicted proteome of DSM 29614 strain. The results of a protein functional enrichment analysis - based on a previous proteomic analysis – revealed metabolic optimization during Fe(III)- citrate anaerobic utilization. In this growth condition, the observed high levels of Fe(II) may be due to different flavin metal reductases and siderophores as inferred form genome analysis. The presence of genes responsible for the synthesis of exopolysaccharide and for the tolerance to heavy metals was highlighted too. The inferred genomic insights were confirmed by a set of phenotypic tests showing specific metabolic capability in terms of i) Fe2+ and exopolysaccharide production and ii) phosphatase activity involved in precipitation of metal ion-phosphate salts.
Conclusion: The K. oxytoca DSM 29614 unique capabilities of using Fe(III)-citrate as sole carbon and energy source in anaerobiosis and tolerating diverse metals coincides with the presence at the genomic level of specific genes that can support i) energy metabolism optimization, ii) cell protection by the biosynthesis of a peculiar exopolysaccharide armour entrapping metal ions and iii) general and metal-specific detoxifying activities by different proteins and metabolites
Elements of a Model Function for the Atomic Absorption Background in EXAFS Spectra
From the study of multielectron photoexcitations in atoms, a specific ansatz for the EXAFS atomic background is
derived, to improve or replace the conventional spline approximation. The elements of the ansatz describe multielectron
resonances, edges and shake-off features at proper theoretical energies. The approach is tested on the atomic background of Rb
ion and on EXAFS-free absorption spectra of rubidium vapor and krypton
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