450 research outputs found
Modelling understorey dynamics in temperate forests under global change : challenges and perspectives
The understorey harbours a substantial part of vascular plant diversity in temperate forests and plays an important functional role, affecting ecosystem processes such as nutrient cycling and overstorey regeneration. Global change, however, is putting these understorey communities on trajectories of change, potentially altering and reducing their functioning in the future. Developing mitigation strategies to safeguard the diversity and functioning of temperate forests in the future is challenging and requires improved predictive capacity. Process-based models that predict understorey community composition over time, based on first principles of ecology, have the potential to guide mitigation endeavours but such approaches are rare. Here, we review fourteen understorey modelling approaches that have been proposed during the last three decades. We evaluate their inclusion of mechanisms that are required to predict the impact of global change on understorey communities. We conclude that none of the currently existing models fully accounts for all processes that we deem important based on empirical and experimental evidence. Based on this review, we contend new models are needed to project the complex impacts of global change on forest understoreys. Plant functional traits should be central to such future model developments, as they drive community assembly processes and provide valuable information on the functioning of the understorey. Given the important role of the overstorey, a coupling of understorey models to overstorey models will be essential to predict the impact of global change on understorey composition and structure, and how it will affect the functioning of temperate forests in the future
Determination of the crystal structure of CuSnTi by full profile Rietveld analysis
The crystal structure of the new ternary phase CuSnTi is determined by full profile Rietveld analysis of the powder diffractogram. 104 reflections were refined to a final RBragg value of 5.60%. CuSnTi crystallizes with the spacegroup P63/mmc and is isostructural to InNi2. The lattice parameters are a=0.439 555(5) nm and c=0.601 505(9) n
Spin wave spectrum of the quantum ferromagnet on the pyrochlore lattice Lu2V2O7
Neutron inelastic scattering has been used to probe the spin dynamics of the
quantum (S=1/2) ferromagnet on the pyrochlore lattice Lu2V2O7. Well-defined
spin waves are observed at all energies and wavevectors, allowing us to
determine the parameters of the Hamiltonian of the system. The data are found
to be in excellent overall agreement with a minimal model that includes a
nearest- neighbour Heisenberg exchange J = 8:22(2) meV and a
Dzyaloshinskii-Moriya interaction (DMI) D =1:5(1) meV. The large DMI term
revealed by our study is broadly consistent with the model developed by Onose
et al. to explain the magnon Hall effect they observed in Lu2V2O7 [1], although
our ratio of D=J = 0:18(1) is roughly half of their value and three times
larger than calculated by ab initio methods [2].Comment: 5 pages, 4 figure
Spin wave dispersion softening in the ferromagnetic Kondo lattice model for manganites
Spin dynamics is calculated in the ferromagnetic (FM) state of the
generalized Kondo lattice model taking into account strong on-site correlations
between e_g electrons and antiferromagnetic (AFM) exchange among t_{2g} spins.
Our study suggests that competing FM double-exchange and AFM super-exchange
interaction lead to a rather nontrivial spin-wave spectrum. While spin
excitations have a conventional Dq^2 spectrum in the long-wavelength limit,
there is a strong deviation from the spin-wave spectrum of the isotropic
Heisenberg model close to the zone boundary. The relevance of our results to
the experimental data are discussed.Comment: 6 RevTex pages, 3 embedded PostScript figure
The Force Between Giant Magnons
We compute the force and torque between well-separated, slowly-moving Giant
Magnons with arbitrary orientations on S^5. We propose an effective Hamiltonian
for Giant Magnons in this regime
Magneto-transport characteristics of La1.4Ca1.6Mn2O7 thin film deposited by spray pyrolysis
Polycrystalline thin films of double layer manganite La_1.4Ca_1.6Mn_2O_7
(DLCMO) have been deposited by nebulized spray pyrolysis on single crystal
LaAlO_3 substrates. These single phase films having grain size in the range
70-100 nm exhibit ferromagnetic transition at T_C ~ 107K. The short range
ferromagnetic ordering due to in plane spin coherence is evidenced to occur at
a higher temperature around 225 K. Insulator/semiconductor to metal transition
occurs at a lower temperature T_P ~ 55K. The transport mechanism above T_C is
of Mott`s variable range hopping type. Below T_C the current-voltage
characteristics show non-linear behaviour that becomes stronger with decreasing
temperature. At low temperatures below T_CA ~ 30K a magnetically frustrated
spin canted state is observed. The DLCMO films exhibit resonable low field
magnetoresistance and at 77K the magnetoresistance ratio is ~ 5% at 0.6 kOe and
\~ 13% at 3 kOe.Comment: 14 pages, 7 figure
Relation between crystal and magnetic structures of the layered manganites La2-2xSr1+2xMn2O7 (0.30 =< x =< 0.50)
Comprehensive neutron-powder diffraction and Rietveld analyses were carried
out to clarify the relation between the crystal and magnetic structures of
La2-2xSr1+2xMn2O7 (0.30 =< x =< 0.50). The Jahn-Teller (JT) distortion of Mn-O6
octahedra, i.e., the ratio of the averaged apical Mn-O bond length to the
equatorial Mn-O bond length, is Delta_JT=1.042(5) at x=0.30, where the magnetic
easy-axis at low temperature is parallel to the c axis. As the JT distortion
becomes suppressed with increasing x, a planar ferromagnetic structure appears
at x =< 0.32, which is followed by a canted antiferromagnetic (AFM) structure
at x =< 0.39. The canting angle between neighboring planes continuously
increases from 0 deg (planar ferromagnet: 0.32 =< x < 0.39) to 180 deg (A-type
AFM: x=0.48 where Delta_JT=1.013(5)). Dominance of the A-type AF structure with
decrease of JT distortion can be ascribed to the change in the eg orbital state
from d3z^2-r^2 to dx^2-y^2
The Skyrme model predictions for the mass spectrum and the - mass splittings
The -plet mass spectrum and the - mass splittings are computed in the framework of the minimal SU(3)
extended Skyrme model. As functions of the Skyrme charge and the SU(3)
symmetry breaking parameters the predictions are presented in tabular form. The
predicted mass splitting - is the smallest among
all SU(3) baryonic multiplets.Comment: 4 pages, 2 tables, version to appear in Phys. Rev.
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