4,344 research outputs found
The planar-to-tubular structural transition in boron clusters from optical absorption
The optical response of the lowest energy isomers of the B_20 family is
calculated using time-dependent density functional theory within a real-space,
real-time scheme. Significant differences are found among the absorption
spectra of the clusters studied. We show that these differences can be easily
related to changes in the overall geometry. Optical spectroscopy is thus an
efficient tool to characterize the planar to tubular structural transition,
known to be present in these boron based systems
Poly(hydroxyalkanoate) production by Cupriavidus necator from fatty waste can be enhanced by phaZ1 inactivation
PHA production from waste oils or fats requires microorganisms that should be both excellent PHA producers and equipped with enzymatic activities allowing hydrolysation of triglycerides. Unfortunately, microbes with the combination of substrate-utilization and PHA production are not currently available, and the strategies to be adopted are the use of costly commercial enzymes, or genetic modification of microorganisms exhibiting high PHA product yields. In the present work, after a general investigation on the ability of Cupriavidus necator to grow on a number of fatty substrates, the possibility to enhance PHA production by limiting intracellular depolymerisation, was investigated. By knocking out the related phaZ1 gene, the construction of C. necator recombinant strains impaired in depolymerase (PhaZ1) activity was achieved. The polymer yield of the recombinant strain was finally compared to that of the parental C. necator DSM 545
Prediction of a novel monoclinic carbon allotrope
A novel allotrope of carbon with symmetry was identified during an
\emph{ab-initio} minima-hopping structural search which we call -carbon.
This structure is predicted to be more stable than graphite at pressures above
14.4 GPa and consists purely of bonds. It has a high bulk modulus and is
almost as hard as diamond. A comparison of the simulated X-ray diffraction
pattern shows a good agreement with experimental results from cold compressed
graphite.Comment: 3 pages, 3 figure
Speeding up the solution of the Bethe-Salpeter equation by a double-grid method and Wannier interpolation
The Bethe-Salpeter equation is a widely used approach to describe optical
excitations in bulk semiconductors. It leads to spectra that are in very good
agreement with experiment, but the price to pay for such accuracy is a very
high computational burden. One of the main bottlenecks is the large number of
k-points required to obtain converged spectra. In order to circumvent this
problem we propose a strategy to solve the Bethe-Salpeter equation based on a
double-grid technique coupled to a Wannier interpolation of the Kohn-Sham band
structure. This strategy is then benchmarked for a particularly difficult case,
the calculation of the absorption spectrum of GaAs, and for the well studied
case of Si. The considerable gains observed in these cases fully validate our
approach, and open the way for the application of the Bethe-Salpeter equation
to large and complex systems.Comment: 5 pages, 3 figures. Accepted for Phys. Rev.
Implementing clinical guidelines in an organizational setup
Outcomes research in healthcare has been a topic much addressed in recent years. Efforts in this direction have been supplemented by work in the areas of guidelines for clinical practice and computer-interpretable workflow and careflow models.In what follows we present the outlines of a framework for understanding the relations between organizations, guidelines, individual patients and patient-related functions. The derived framework provides a means to extract the knowledge contained in the guideline text at different granularities, in ways that can help us to assign tasks within the healthcare organization and to assess clinical performance in realizing the guideline. It does this in a way that preserves the flexibility of the organization in the adoption of the guidelines
Identification of fullerene-like CdSe nanoparticles from optical spectroscopy calculations
Semiconducting nanoparticles are the building blocks of optical nanodevices
as their electronic states, and therefore light absorption and emission, can be
controlled by modifying their size and shape. CdSe is perhaps the most studied
of these nanoparticles, due to the efficiency of its synthesis, the high
quality of the resulting samples, and the fact that the optical gap is in the
visible range. In this article, we study light absorption of CdSe
nanostructures with sizes up to 1.5 nm within density functional theory. We
study both bulk fragments with wurtzite symmetry and novel fullerene-like
core-cage structures. The comparison with recent experimental optical spectra
allows us to confirm the synthesis of these fullerene-like CdSe clusters
Low-density silicon allotropes for photovoltaic applications
Silicon materials play a key role in many technologically relevant fields,
ranging from the electronic to the photovoltaic industry. A systematic search
for silicon allotropes was performed by employing a modified ab initio minima
hopping crystal structure prediction method. The algorithm was optimized to
specifically investigate the hitherto barely explored low-density regime of the
silicon phase diagram by imitating the guest-host concept of clathrate
compounds. In total 44 metastable phases are presented, of which 11 exhibit
direct or quasi-direct band-gaps in the range of 1.0-1.8 eV, close to
the optimal Shockley-Queisser limit of 1.4 eV, with a stronger overlap
of the absorption spectra with the solar spectrum compared to conventional
diamond silicon. Due to the structural resemblance to known clathrate compounds
it is expected that the predicted phases can be synthesized
Low-Energy Polymeric Phases of Alanates
Low-energy structures of alanates are currently known to be described by
patterns of isolated, nearly ideal tetrahedral [AlH] anions and metal
cations. We discover that the novel polymeric motif recently proposed for
LiAlH plays a dominant role in a series of alanates, including LiAlH,
NaAlH, KAlH, Mg(AlH), Ca(AlH) and Sr(AlH). In
particular, most of the low-energy structures discovered for the whole series
are characterized by networks of corner-sharing [AlH] octahedra, forming
wires and/or planes throughout the materials. Finally, for Mg(AlH) and
Sr(AlH), we identify two polymeric phases to be lowest in energy at low
temperatures.Comment: 9 pages, 8 figures, 2 tables, including supplemental materia
Exact Effective action for (1+1)-dimensional fermions in an Abelian background at finite temperature and chemical potential
In this paper we study the effects of a nonzero chemical potential in the
effective action for massless fermions in (1+1) dimensions in an abelian gauge
field background at finite temperature. We calculate the n-point function and
show that the structure of the amplitudes corresponds to a generalization of
the structure noted earlier in a calculation without a chemical potential (the
associated integrals carry the dependence on the chemical potential). Our
calculation shows that the chiral anomaly is unaffected by the presence of a
chemical potential at finite temperature. However, unlike the earlier
calculation (in the absence of a chemical potential) odd point functions do not
vanish. We trace this to the fact that in the presence of a chemical potential
the generalized charge conjugation symmetry of the theory allows for such
amplitudes. In fact, we find that all the even point functions are even
functions of the chemical potential while the odd point functions are odd
functions of it which is consistent with this generalized charge conjugation
symmetry. We show that the origin of the structure of the amplitudes is best
seen from a formulation of the theory in terms of left and right handed
spinors. The calculations are also much simpler in this formulation and it
clarifies many other aspects of the theory
First-principles predicted low-energy structures of NaSc(BH4)4
According to previous interpretations of experimental data, sodium-scandium
double-cation borohydride NaSc(BH) crystallizes in the crystallographic
space group where each sodium (scandium) atom is surrounded by six
scandium (sodium) atoms. A careful investigation of this phase based on
\textit{ab initio} calculations indicates that the structure is dynamically
unstable and gives rise to an energetically and dynamically more favorable
phase with symmetry and nearly identical x-ray diffraction pattern. By
additionally performing extensive structural searches with the minima-hopping
method we discover a class of new low-energy structures exhibiting a novel
structural motif in which each sodium (scandium) atom is surrounded by four
scandium (sodium) atoms arranged at the corners of either a rectangle with
nearly equal sides or a tetrahedron. These new phases are all predicted to be
insulators with band gaps of eV. Finally, we estimate the influence
of these structures on the hydrogen-storage performance of NaSc(BH).Comment: Version publishe
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