443 research outputs found
Soft phonon modes in rutile TiO
The lattice dynamics of TiO in the rutile crystal structure was studied
by a combination of thermal diffuse scattering, inelastic x-ray scattering and
density functional perturbation theory. We experimentally confirm the existence
of an anomalous soft transverse acoustic mode with energy minimum at q = (1/2
1/2 1/4). The phonon energy landscape of this particular branch is reported and
compared to the calculation. The harmonic calculation underestimates the phonon
energies but despite this the shape of both the energy landscape and the
scattering intensities are well reproduced. We find a significant temperature
dependence in energy of this transverse acoustic mode over an extended region
in reciprocal space which is in line with a substantially anharmonic mode
potential-energy surface. The reported low energy branch is quite different
from the ferroelectric mode that softens at the Brillouin zone center and may
help explain anomalous convergence behavior in calculating TiO surface
properties
A likely role for anoxygenic photosynthetic microbes in the formation of ancient stromatolites
Although cyanobacteria are the dominant primary producers in modern stromatolites and other microbialites, the oldest stromatolites pre-date geochemical evidence for oxygenic photosynthesis and cyanobacteria in the rock record. As a step towards the development of laboratory models of stromatolite growth, we tested the potential of a metabolically ancient anoxygenic photosynthetic bacterium to build stromatolites. This organism, Rhodopseudomonas palustris, stimulates the precipitation of calcite in solutions already highly saturated with respect to calcium carbonate, and greatly facilitates the incorporation of carbonate grains into proto-lamina (i.e. crusts). The appreciable stimulation of the growth of proto-lamina by a nonfilamentous anoxygenic microbe suggests that similar microbes may have played a greater role in the formation of Archean stromatolites than previously assumed
Multiscale modelling and simulation, 13th international workshop
Multiscale Modelling and Simulation (MMS) is a cornerstone in the today's research in computational science. Simulations containing multiple models, with each model operating at a different temporal or spatial scale, are a challenging setting that frequently require innovative approaches in areas such as scale bridging, code deployment, error quantification, and scientific analysis. The aim of the MMS workshop is to encourage and consolidate the progress in this multidisciplinary research field, both in the areas of the scientific applications and the underlying infrastructures that enable these applications. Here we briefly introduce the scope of the workshop and highlight some of the key aspects of this year's submissions
Phonon surface mapping of graphite: disentangling quasi--degenerate phonon dispersions
The two-dimensional mapping of the phonon dispersions around the point of
graphite by inelastic x-ray scattering is provided. The present work resolves
the longstanding issue related to the correct assignment of transverse and
longitudinal phonon branches at . We observe an almost degeneracy of the
three TO, LA and LO derived phonon branches and a strong phonon trigonal
warping. Correlation effects renormalize the Kohn anomaly of the TO mode, which
exhibits a trigonal warping effect opposite to that of the electronic band
structure. We determined the electron--phonon coupling constant to be
166 in excellent agreement to calculations. These results
are fundamental for understanding angle-resolved photoemission,
double--resonance Raman and transport measurements of graphene based systems
The phonon dispersion of graphite by inelastic x-ray scattering
We present the full in-plane phonon dispersion of graphite obtained from
inelastic x-ray scattering, including the optical and acoustic branches, as
well as the mid-frequency range between the and points in the Brillouin
zone, where experimental data have been unavailable so far. The existence of a
Kohn anomaly at the point is further supported. We fit a fifth-nearest
neighbour force-constants model to the experimental data, making improved
force-constants calculations of the phonon dispersion in both graphite and
carbon nanotubes available.Comment: 7 pages; submitted to Phys. Rev.
Conformity of spin fluctuations in alkali-metal iron selenide superconductors inferred from the observation of a magnetic resonant mode in K(x)Fe(2-y)Se(2)
Spin excitations stemming from the metallic phase of the ferrochalcogenide
superconductor K(0.77)Fe(1.85)Se(2) (T_c=32 K) were mapped out in the ab plane
by means of the time-of-flight neutron spectroscopy. We observed a magnetic
resonant mode at Q_res=(1/2 1/4), whose energy and in-plane shape are almost
identical to those in the related compound Rb(0.8)Fe(1.6)Se(2). This lets us
infer that there is a unique underlying electronic structure of the bulk
superconducting phase K(x)Fe(2)Se(2), which is universal for all alkali-metal
iron selenide superconductors and stands in contrast to the doping-tunable
phase diagrams of the related iron pnictides. Furthermore, the spectral weight
of the resonance on the absolute scale, normalized to the volume fraction of
the superconducting phase, is several times larger than in optimally doped
BaFe(2-x)Co(x)As(2). We also found no evidence for any additional low-energy
branches of spin excitations away from Q_res. Our results provide new input for
theoretical models of the spin dynamics in iron based superconductors
- …