2,350 research outputs found
Anisotropic glass-like properties in tetragonal disordered crystals
The low temperature acoustic and thermal properties of amorphous, glassy
materials are remarkably similar. All these properties are described
theoretically with reasonable quantitative accuracy by assuming that the
amorphous solid contains dynamical defects that can be described at low
temperatures as an ensemble of two-level systems (TLS), but the deep nature of
these TLSs is not clarified yet. Moreover, glassy properties were found also in
disordered crystals, quasicrystals, and even perfect crystals with a large
number of atoms in the unit cell. In crystals, the glassy properties are not
universal, like in amorphous materials, and also exhibit anisotropy. Recently
it was proposed a model for the interaction of two-level systems with arbitrary
strain fields (Phys. Rev. B 75, 64202, 2007), which was used to calculate the
thermal properties of nanoscopic membranes at low temperatures. The model is
also suitable for the description of anisotropic crystals. We describe here the
results of the calculation of anisotropic glass-like properties in crystals of
various lattice symmetries, emphasizing the tetragonal symmetry.Comment: 5 pages, no figure
Elastic response of [111]-tunneling impurities
We study the dynamic response of a [111] quantum impurity, such as lithium or
cyanide in alkali halides, with respect to an external field coupling to the
elastic quadrupole moment. Because of the particular level structure of a
eight-state system on a cubic site, the elastic response function shows a
biexponential relaxation feature and a van Vleck type contribution with a
resonance frequency that is twice the tunnel frequency . This
basically differs from the dielectric response that does not show relaxation.
Moreover, we show that the elastic response of a [111] impurity cannot be
reduced to that of a two-level system. In the experimental part, we report on
recent sound velocity and internal friction measurements on KCl doped with
cyanide at various concentrations. At low doping (45 ppm) we find the dynamics
of a single [111] impurity, whereas at higher concentrations (4700 ppm) the
elastic response rather indicates strongly correlated defects. Our theoretical
model provides a good description of the temperature dependence of
and at low doping, in particular the relaxation peaks, the absolute
values of the amplitude, and the resonant contributions. From our fits we
obtain the value of the elastic deformation potential eV.Comment: 19 pages, 5 figure
Scattering of phonons on two-level systems in disordered crystals
We calculate the scattering rates of phonons on two-level systems in
disordered trigonal and hexagonal crystals. We apply a model in which the
two-level system, characterized by a direction in space, is coupled to the
strain field of the phonon via a tensor of coupling constants. The structure of
the tensor of coupling constants is similar to the structure of the tensor of
elastic stiffness constants, in the sense that they are determined by the same
symmetry transformations. In this way, we emphasize the anisotropy of the
interaction of elastic waves with the ensemble of two-level systems in
disordered crystals. We also point to the fact that the ratio
has a much broader range of allowed values in disordered
crystals than in isotropic solids.Comment: 5 pages, no figure
The challenge of modelling nitrogen management at the field scale : simulation and sensitivity analysis of N2O fluxes across nine experimental sites using DailyDayCent
Peer reviewedPublisher PD
Modelling spatial and inter-annual variations of nitrous oxide emissions from UK cropland and grasslands using DailyDayCent
This work contributes to the Defra funded projects AC0116: ‘Improving the nitrous oxide inventory’, and AC0114: ‘Data Synthesis, Management and Modelling’. Funding for this work was provided by the UK Department for Environment, Food and Rural Affairs (Defra) AC0116 and AC0114, the Department of Agriculture, Environment and Rural Affairs for Northern Ireland, the Scottish Government and the Welsh Government. Rothamsted Research receives strategic funding from the Biotechnology and Biological Sciences Research Council. This study also contributes to the projects: N-Circle (BB/N013484/1), U-GRASS (NE/M016900/1) and GREENHOUSE (NE/K002589/1).Peer reviewedPublisher PD
Internal Friction of Amorphous Silicon in a Magnetic Field
The internal friction of e-beam amorphous silicon was measured in a magnetic
field between 0 and 6 T, from 1.5-20 K, and was found to be independent of the
field to better than 8%. It is concluded that the low energy excitations
observed in this experiment are predominantly atomic in nature.Comment: 4 pages, 4 figures, REVTe
Spectral Shape of Relaxations in Silica Glass
Precise low-frequency light scattering experiments on silica glass are
presented, covering a broad temperature and frequency range (9 GHz < \nu < 2
THz). For the first time the spectral shape of relaxations is observed over
more than one decade in frequency. The spectra show a power-law low-frequency
wing of the relaxational part of the spectrum with an exponent
proportional to temperature in the range 30 K < T < 200 K. A comparison of our
results with those from acoustic attenuation experiments performed at different
frequencies shows that this power-law behaviour rather well describes
relaxations in silica over 9 orders of magnitude in frequency. These findings
can be explained by a model of thermally activated transitions in double well
potentials.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Using legume-based mixtures to enhance the nitrogen use efficiency and economic viability of cropping systems - Final report (LK09106/HGCA3447)
As costs for mineral fertilisers rise, legume-based leys are recognised as a potential alternative nitrogen source for crops. Here we demonstrate that including species-rich legume-based leys in rotations helps to maximise synergies between agricultural productivity and other ecosystem services. By using functionally diverse plant species mixtures, these services can be optimised and fine-tuned to regional and farm-specific needs. Replicated field experiments were conducted over three years at multiple locations, testing the performance of 12 legume species and 4 grass species sown in monocultures, as well as in a mixture of 10 of the legumes and all 4 grasses (called the All Species Mix, ASM). In addition, we compared this complex mixture to farmer-chosen ley mixtures on 34 sites across the UK.
The trials showed that there is a large degree of functional complementarity among the legume species. No single species scored high on all evaluation criteria. In particular, the currently most frequently used species, white clover, is outscored by other legume species on a number of parameters such as early development and resistance to decomposition. Further complementarity emerged from the different responses of legume species to environmental variables, with soil pH and grazing or cutting regime being among the more important factors. For example, while large birdsfoot trefoil showed better performance on more acidic soils, the opposite was true for sainfoin, lucerne and black medic. In comparison with the monocultures, the ASM showed increased ground cover, increased above-ground biomass and reduced weed biomass. Benefits of mixing species with regard to productivity increased over time. In addition, the stability of biomass production across sites was greater in the ASM than in the legume monocultures. Within the on-farm trials, we further found that on soils low in organic matter the biomass advantage of the ASM over the Control ley was more marked than on the soils with higher organic matter content. Ecological modelling revealed that the three best multifunctional mixtures all contained black medic, lucerne and red clover.
Within the long term New Farming Systems (NFS) rotational study, the use of a clover bi-crop showed improvement to soil characteristics compared to current practice (e.g. bulk density and water infiltration rate). Improvements in wheat yield were also noted with respect to the inclusion of a clover bi-crop in 2010, but there was evidence of a decline in response as the N dose was increased. Cumulatively, over both the wheat crop and the spring oilseed rape crop, the clover bi-crop improved margin over N. The highest average yield response (~9%) was associated with the ASM legume species mix cover cropping approach
Mechanical Relaxation in Glasses and at the Glass Transition
The Gilroy-Phillips model of relaxational jumps in asymmetric double-well
potentials, developed for the Arrhenius-type secondary relaxations of the glass
phase, is extended to a formal description of the breakdown of the shear
modulus at the glass transition, the flow process.Comment: 13 pages, 11 figures, 49 ref
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