5,998 research outputs found
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
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
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
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
Scintillation efficiency of liquid argon in low energy neutron-argon scattering
Experiments searching for weak interacting massive particles with noble gases
such as liquid argon require very low detection thresholds for nuclear recoils.
A determination of the scintillation efficiency is crucial to quantify the
response of the detector at low energy. We report the results obtained with a
small liquid argon cell using a monoenergetic neutron beam produced by a
deuterium-deuterium fusion source. The light yield relative to electrons was
measured for six argon recoil energies between 11 and 120 keV at zero electric
drift field.Comment: 21 pages, 19 figures, 4 table
Identification of novel Cu, Ag, and Au ternary oxides from global structural prediction
We use ab initio global structural prediction, and specifically the minima
hopping method, to explore the periodic table in search of novel oxide phases.
In total, we study 183 different compositions of the form MXO2, where M=(Cu,
Ag, Au) and X is an element of the periodic table. This set includes the
well-known Cu delafossite compounds that are, up to now, the best p-type
transparent conductive oxides known to mankind. Our calculations discover 81
stable compositions, out of which only 36 are included in available databases.
Some of these new phases are potentially good candidates for transparent
electrodes. These results demonstrate, on one hand, how incomplete is still our
knowledge of the phase-space of stable ternary materials. On the other hand, we
show that structural prediction combined with high-throughput approaches is a
powerful tool to extend that knowledge, paving the way for the experimental
discovery of new materials on a large scale
Three channel model of meson-meson scattering and scalar meson spectroscopy
New solutions on the scalar -- isoscalar phase shifts are analysed
together with previous results using a separable potential model of
three coupled channels (, and an effective
system). Model parameters are fitted to two sets of solutions obtained in a
recent analysis of the CERN-Cracow-Munich measurements of the reaction on a polarized target. A relatively
narrow (90 -- 180 MeV) scalar resonance is found, in contrast
to a much broader ( MeV) state emerging from the analysis
of previous unpolarized target data.Comment: 10 Latex pages + 6 postscript figure
Properties of scalar--isoscalar mesons from multichannel interaction analysis below 1800 MeV
Scalar-isoscalar mesons are studied using an unitary model in three channels:
pi-pi, K-anti K and an effective 2pi-2pi.
All the solutions, fitted to the pi-pi and K-anti K data, exhibit a wide
f0(500), a narrow f0(980) and two relatively narrow resonances, lying on
different sheets between 1300 MeV and 1500 MeV.
These latter states are similar to the f0(1370) and f0(1500) seen in
experiments at CERN. Branching ratios are compared with available data. We have
started investigations of some crossing symmetry and chiral constraints imposed
near the pi-pi threshold on the scalar-isoscalar, scalar-isotensor and P-wave
pi-pi amplitudes.Comment: Talk given at XVth Particles and Nuclei Int. Conf. (PANIC99),
Uppsala, Sweden, June 10-16, 1999; 4 pages, 3 figures, file espcrc1.sty
include
Tests of silicon sensors for the CMS pixel detector
The tracking system of the CMS experiment, currently under construction at
the Large Hadron Collider (LHC) at CERN (Geneva, Switzerland), will include a
silicon pixel detector providing three spacial measurements in its final
configuration for tracks produced in high energy pp collisions. In this paper
we present the results of test beam measurements performed at CERN on
irradiated silicon pixel sensors. Lorentz angle and charge collection
efficiency were measured for two sensor designs and at various bias voltages.Comment: Talk presented at 6th International Conference on Large Scale
Applications and Radiation Hardness of Semiconductor Detectors, September
29-October 1, 2003, Firenze, Italy. Proceedings will be published in Nuclear
Instr. & Methods in Phys. Research, Section
Why the high lying glueball does not mix with the neighbouring
Chiral symmetry restoration in high-lying hadron spectra implies that hadrons
which belong to different irreducible representations of the parity-chiral
group cannot mix. This explains why the , which was suggested
to be a glueball, and hence must belong to the scalar (0,0) representation of
the chiral group, cannot mix with the neighbouring , which
was interpreted as a state, and that belongs to the
representation of the chiral group. If confirmed, then we have an access to a
"true" glueball of QCD.Comment: 4 pages, LaTeX, final version, Eur. Phys. J. A 19 (2004) 15
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