301 research outputs found
How are soil use and management reflected by soil organic matter characteristics: a spectroscopic approach
We studied the quantitative and qualitative changes of soil organic matter (SOM) due to different land
uses (arable versus grassland) and treatments (organic manure and mineral fertilizer) within an agricultural crop rotation in a long-term field experiment, conducted since 1956 at Ultuna, Sweden, on a Eutric
Cambisol. The organic carbon (OC) content of the grassland plot was 1.8 times greater than that of the
similarly fertilized Ca(NO3)
2 treated cropped plots. The comparison of two dispersion techniques (a lowenergy sonication and a chemical dispersion which yield inherent soil aggregates) showed that increasing
OC contents of the silt-sized fractions were not matched by a linear increase of silt-sized aggregates. This
indicated saturation of the aggregates with OC and a limited capacity of particles to protect OC
physically. Thermogravimetric analyses suggested an increase of free organic matter with increasing
OC contents. Transmission FT-IR spectroscopy showed relative enrichment of carboxylic, aromatic,
CH and NH groups in plots with increasing OC contents. The silt-sized fractions contained the largest
SOM pool and, as revealed by 13C NMR spectroscopy, were qualitatively more influenced by the plant
residue versus manure input than the clay fractions. Alkyl and O-alkyl C in the silt-sized fractions
amounted to 57.4% of organic carbon in the animal manure treated plots and 50–53% in the other
treatments.We thank the Austrian Science Fund (Fonds zur FÖrderung
der wissenschaftlichen Forschung) for funding this bilateral
project.Peer reviewe
Ultrafast Optical Excitation of a Persistent Surface-State Population in the Topological Insulator Bi2Se3
Using femtosecond time- and angle- resolved photoemission spectroscopy, we
investigated the nonequilibrium dynamics of the topological insulator Bi2Se3.
We studied p-type Bi2Se3, in which the metallic Dirac surface state and bulk
conduction bands are unoccupied. Optical excitation leads to a meta-stable
population at the bulk conduction band edge, which feeds a nonequilibrium
population of the surface state persisting for >10ps. This unusually long-lived
population of a metallic Dirac surface state with spin texture may present a
channel in which to drive transient spin-polarized currents
La differenciation plastidale chez l'acetabulaire etudiee par l'emission de fluorescence a 77° K
Plastidal differentiation in Acetabularia studied by fluorescence emission at 77° K. Examination of the 77° K fluorescence emission spectra provides a good criterion for distinguishing chloroplasts of the apical from those of the basal region of the stalk of Acetabularia cells at stage 4. Extraction increases the differences between the emission spectra of apical and basal chloroplasts. By using the 77° K fluorescence emission spectra technique, it is possible to show the differentiation of the plastids and to follow it during the regeneration of anucleate and nucleate fragments. © 1975
Persistent order due to transiently enhanced nesting in an electronically excited charge density wave
Non-equilibrium conditions may lead to novel properties of materials with
broken symmetry ground states not accessible in equilibrium as vividly
demonstrated by non-linearly driven mid-infrared active phonon excitation.
Potential energy surfaces of electronically excited states also allow to
direct nuclear motion, but relaxation of the excess energy typically excites
fluctuations leading to a reduced or even vanishing order parameter as
characterized by an electronic energy gap. Here, using femtosecond time- and
angle-resolved photoemission spectroscopy, we demonstrate a tendency towards
transient stabilization of a charge density wave after near-infrared
excitation, counteracting the suppression of order in the non-equilibrium
state. Analysis of the dynamic electronic structure reveals a remaining energy
gap in a highly excited transient state. Our observation can be explained by a
competition between fluctuations in the electronically excited state, which
tend to reduce order, and transiently enhanced Fermi surface nesting
stabilizing the order
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