22,494 research outputs found
Orientation Determination in Single Particle X-ray Coherent Diffraction Imaging Experiments
Single particle diffraction imaging experiments at free-electron lasers (FEL)
have a great potential for structure determination of reproducible biological
specimens that can not be crystallized. One of the challenges in processing the
data from such an experiment is to determine correct orientation of each
diffraction pattern from samples randomly injected in the FEL beam. We propose
an algorithm (see also O. Yefanov et al., Photon Science - HASYLAB Annual
Report 2010) that can solve this problem and can be applied to samples from
tens of nanometers to microns in size, measured with sub-nanometer resolution
in the presence of noise. This is achieved by the simultaneous analysis of a
large number of diffraction patterns corresponding to different orientations of
the particles. The algorithms efficiency is demonstrated for two biological
samples, an artificial protein structure without any symmetry and a virus with
icosahedral symmetry. Both structures are few tens of nanometers in size and
consist of more than 100 000 non-hydrogen atoms. More than 10 000 diffraction
patterns with Poisson noise were simulated and analyzed for each structure. Our
simulations indicate the possibility to achieve resolution of about 3.3 {\AA}
at 3 {\AA} wavelength and incoming flux of 10^{12} photons per pulse focused to
100\times 100 nm^2.Comment: 23 pages, 10 figures, 40 reference
Spin- and angle-resolved photoemission on topological materials
A historical review of spin- and angle-resolved photoemission on topological
materials is presented, aimed at readers who are new to the field or who wish
to obtain an overview of the activities in the field. The main focus lies on
topological insulators, but also Weyl and other semimetals will be discussed.
Further it will be explained why the measured spin polarisation from a spin
polarised state should always add up to 100% and how spin interference effects
influence the measured spin texture.Comment: Invited review article for special issue "ARPES Studies of
Topological Materials" in Electronic Structur
B decays into light scalar particles and glueball
The recent observations of f_0(980) in charmless B-decays motivate further
studies of scalar particle and glueball production in these processes.
Amplitudes for charmless 2-body B decays involving the members of the scalar
nonet are presented based on the symmetries of the dominant penguin
contribution. Different scenarios for the lightest scalar nonet are
investigated in view of the presently available data. We describe the evidence
from B-decays for f_0(1500) with a flavour octet like mixing and the hints
towards the members of the q qbar nonet of lowest mass. There is further
support for the hypothesis of a broad 0^{++} glueball acting as coherent
background especially in B -> K Kbar K. The estimated B decay rates into
gluonic mesons represent a sizable fraction of the theoretically derived decay
rate for b -> sg.Comment: 34 pages, 5 figures, 8 tables, version to appear in Eur.Phys.J.C.,
with some clarifications in the text, additional refs. and several overall
signs in Tab.2,3 adde
Inelastic neutron scattering due to acoustic vibrations confined in nanoparticles: theory and experiment
The inelastic scattering of neutrons by nanoparticles due to acoustic
vibrational modes (energy below 10 meV) confined in nanoparticles is calculated
using the Zemach-Glauber formalism. Such vibrational modes are commonly
observed by light scattering techniques (Brillouin or low-frequency Raman
scattering). We also report high resolution inelastic neutron scattering
measurements for anatase TiO2 nanoparticles in a loose powder. Factors enabling
the observation of such vibrations are discussed. These include a narrow
nanoparticle size distribution which minimizes inhomogeneous broadening of the
spectrum and the presence of hydrogen atoms oscillating with the nanoparticle
surfaces which enhances the number of scattered neutrons.Comment: 3 figures, 1 tabl
Hyper-Raman scattering analysis of the vibrations in vitreous boron oxide
Hyper-Raman scattering has been measured on vitreous boron oxide,
BO. This spectroscopy, complemented with Raman scattering and
infrared absorption, reveals the full set of vibrations that can be observed
with light. A mode analysis is performed based on the local D symmetry
of BO triangles and BO boroxol rings. The results show that in
BO the main spectral components can be succesfully assigned using
this relatively simple model. In particular, it can be shown that the
hyper-Raman boson peak arises from external modes that correspond mainly to
librational motions of rigid boroxol rings.Comment: 13 pages, 11 figures, 2 table
Looking for a theory of faster-than-light particles
Several principal aspects of a theoretical approach to the theory of
faster-than-light particles (tachyons) are considered in this note. They
concern the resolution of such problems of tachyon theory as the causality
violation by tachyons, the stability of the tachyon vacuum, and the stability
of ordinary particles against the spontaneous emission of tachyons, i.e. the
problems which are generally used as arguments against the possibility of such
particles. It is demonstrated that all these arguments contain nontrivial
loopholes which undermine their validity. A demand for a consistent tachyon
theory is formulated, and several ideas for its construction are suggested.Comment: 41 pages, 5 figure
Tunneling transport in NSN junctions made of Majorana nanowires across the topological quantum phase transition
We theoretically consider transport properties of a normal metal (N)-
superconducting semiconductor nanowire (S)-normal metal (N) structure (NSN) in
the context of the possible existence of Majorana bound states in disordered
semiconductor-superconductor hybrid systems in the presence of spin-orbit
coupling and Zeeman splitting induced by an external magnetic field. We study
in details the transport signatures of the topological quantum phase transition
as well as the existence of the Majorana bound states in the electrical
transport properties of the NSN structure. Our theory includes the realistic
nonperturbative effects of disorder, which is detrimental to the topological
phase (eventually suppressing the superconducting gap completely), and the
effects of the tunneling barriers (or the transparency at the tunneling NS
contacts), which affect (and suppress) the zero bias conductance peak
associated with the zero energy Majorana bound states. We show that in the
presence of generic disorder and barrier transparency the interpretation of the
zero bias peak as being associated with the Majorana bound state is problematic
since the nonlocal correlations between the two NS contacts at two ends may not
manifest themselves in the tunneling conductance through the whole NSN
structure. We establish that a simple modification of the standard transport
measurements using conductance differences (rather than the conductance itself
as in a single NS junction) as the measured quantity can allow direct
observation of the nonlocal correlations inherent in the Majorana bound states
and enables the mapping out of the topological phase diagram (even in the
presence of considerable disorder) by precisely detecting the topological
quantum phase transition point.Comment: 34 pages, 7 figures, 1 table. New version with minor modifications
and more physical discussion
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