233 research outputs found
X-ray absorption spectroscopy on layered cobaltates Na_xCoO_2
Measurements of polarization and temperature dependent soft x-ray absorption
have been performed on Na_xCoO_2 single crystals with x=0.4 and x=0.6. They
show a deviation of the local trigonal symmetry of the CoO_6 octahedra, which
is temperature independent in a temperature range between 25 K and 372 K. This
deviation was found to be different for Co^{3+} and Co^{4+} sites. With the
help of a cluster calculation we are able to interpret the Co L_{23}-edge
absorption spectrum and find a doping dependent energy splitting between the
t_{2g} and the e_g levels (10Dq) in Na_xCoO_2.Comment: 7 pages, 8 figure
Combined 3D characterization of porous zeolites by STEM and FIB tomography
German Research Foundation Priority Program 1570German Research Foundation Cluster of Excellence EXC 315 “Engineering of Advanced Materials
Stabilization of d-Band Ferromagnetism by Hybridization with Uncorrelated Bands
We investigate the influence of s-d or p-d hybridization to d-band
ferromagnetism to estimate the importance of hybridization for the magnetic
properties of transition metals. To focus our attention to the interplay
between hybridization and correlation we investigate a simple model system
consisting of two non-degenerated hybridized bands, one strongly correlated,
the other one quasi-free. To solve this extended Hubbard model, we apply simple
approximations, namely SDA and MAA, that, concerning ferromagnetism in the
single-band model, are known to give qualitatively satisfactory results. This
approach allows us to discuss the underlying mechanism, by which d-band
ferromagnetism is influenced by the hybridization on the basis of analytical
expressions. The latter clearly display the order and the functional
dependencies of the important effects. It is found, that spin-dependent
inter-band particle fluctuations cause a spin-dependent band shift and a
spin-dependent band broadening of the Hubbard bands. The shift stabilizes, the
broadening tends to destabilize ferromagnetism. Stabilization requires
relatively high band distances and small hybridization matrix elements.
Super-exchange and RKKY coupling are of minor importance.Comment: 9 pages, 7 figures, accepted for PR
Synthesis and Characterisation of Hierarchically Structured Titanium Silicalite‐1 Zeolites with Large Intracrystalline Macropores
The successful synthesis of hierarchically structured titanium silicalite‐1 (TS‐1) with large intracrystalline macropores by steam‐assisted crystallisation of mesoporous silica particles is reported. The macropore topology was imaged in 3D by using electron tomography and synchrotron radiation‐based ptychographic X‐ray computed tomography, revealing interconnected macropores within the crystals accounting for about 30 % of the particle volume. The study of the macropore formation mechanism revealed that the mesoporous silica particles act as a sacrificial macropore template during the synthesis. Silicon‐to‐titanium ratio of the macroporous TS‐1 samples was successfully tuned from 100 to 44. The hierarchically structured TS‐1 exhibited high activity in the liquid phase epoxidation of 2‐octene with hydrogen peroxide. The hierarchically structured TS‐1 surpassed a conventional nano‐sized TS‐1 sample in terms of alkene conversion and showed comparable selectivity to the epoxide. The flexible synthesis route described here can be used to prepare hierarchical zeolites with improved mass transport properties for other selective oxidation reactions
Neutrinoless Double Beta Decay in Gauge Theories
Neutrinoless double beta decay is a very important process both from the
particle and nuclear physics point of view. Its observation will severely
constrain the existing models and signal that the neutrinos are massive
Majorana particles. From the elementary particle point of view it pops up in
almost every model. In addition to the traditional mechanisms, like the
neutrino mass, the admixture of right handed currents etc, it may occur due to
the R-parity violating supersymmetric (SUSY) interactions. From the nuclear
physics point of view it is challenging, because: 1) The relevant nuclei have
complicated nuclear structure. 2) The energetically allowed transitions are
exhaust a small part of all the strength. 3) One must cope with the short
distance behavior of the transition operators, especially when the intermediate
particles are heavy (eg in SUSY models). Thus novel effects, like the double
beta decay of pions in flight between nucleons, have to be considered. 4) The
intermediate momenta involved are about 100 MeV. Thus one has to take into
account possible momentum dependent terms in the nucleon current. We find that,
for the mass mechanism, such modifications of the nucleon current for light
neutrinos reduce the nuclear matrix elements by about 25 per cent, almost
regardless of the nuclear model. In the case of heavy neutrinos the effect is
much larger and model dependent.
Taking the above effects into account, the available nuclear matrix elements
for the experimentally interesting nuclei A = 76, 82, 96, 100, 116, 128, 130,
136 and 150 and the experimental limits on the life times we have extracted new
stringent limits on the average neutrino mass and on the R-parity violating
coupling for various SUSY models.Comment: Latex, 24 pages, 1 postscript figure, uses iopconf.st
Itinerant Ferromagnetism in the Periodic Anderson Model
We introduce a novel mechanism for itinerant ferromagnetism, based on a
simple two-band model. The model includes an uncorrelated and dispersive band
hybridized with a second band which is narrow and correlated. The simplest
Hamiltonian containing these ingredients is the Periodic Anderson Model (PAM).
Using quantum Monte Carlo and analytical methods, we show that the PAM and an
extension of it contain the new mechanism and exhibit a non-saturated
ferromagnetic ground state in the intermediate valence regime. We propose that
the mechanism, which does not assume an intra atomic Hund's coupling, is
present in both the iron group and in some f electron compounds like
Ce(Rh_{1-x} Ru_x)_3 B_2, La_x Ce_{1-x} Rh_3 B_2 and the uranium
monochalcogenides US, USe, and UTe
Grand unified theory constrained supersymmetry and neutrinoless double beta decay
We analyze the contributions to the neutrinoless double decay
(-decay) coming from the Grand Unified Theory (GUT) constrained
Minimal Supersymmetric Standard Model (MSSM) with trilinear R-parity breaking.
We discuss the importance of two-nucleon and pion-exchange realizations of the
quark-level -decay transitions. In this context, the questions
of reliability of the calculated relevant nuclear matrix elements within the
Renormalized Quasiparticle Random Phase Approximation (pn-RQRPA) for several
medium and heavy open-shell nuclei are addressed. The importance of gluino and
neutralino contributions to -decay is also analyzed. We review
the present experiments and deduce limits on the trilinear R-parity breaking
parameter from the non-observability of -decay
for different GUT constrained SUSY scenarios. In addition, a detailed study of
limits on the MSSM parameter space coming from the processes
by using the recent CLEO and OPAL results is performed. Some studies in respect
to the future -decay project GENIUS are also presented.Comment: 29 pages, 8 figure
Recent advances in neutrinoless double beta decay search
Even after the discovery of neutrino flavour oscillations, based on data from
atmospheric, solar, reactor, and accelerator experiments, many characteristics
of the neutrino remain unknown. Only the neutrino square-mass differences and
the mixing angle values have been estimated, while the value of each mass
eigenstate still hasn't. Its nature (massive Majorana or Dirac particle) is
still escaping. Neutrinoless double beta decay (-DBD) experimental
discovery could be the ultimate answer to some delicate questions of elementary
particle and nuclear physics. The Majorana description of neutrinos allows the
-DBD process, and consequently either a mass value could be measured or
the existence of physics beyond the standard should be confirmed without any
doubt. As expected, the -DBD measurement is a very difficult field of
application for experimentalists. In this paper, after a short summary of the
latest results in neutrino physics, the experimental status, the R&D projects,
and perspectives in -DBD sector are reviewed.Comment: 36 pages, 7 figures, To be publish in Czech Journal of Physic
The Majorana neutrino masses, neutrinoless double beta decay and nuclear matrix elements
The effective Majorana neutrino mass is evaluated by using the latest results
of neutrino oscillation experiments. The problems of the neutrino mass
spectrum,absolute mass scale of neutrinos and the effect of CP phases are
addressed. A connection to the next generation of the neutrinoless double beta
decay (0nbb-decay) experiments is discussed. The calculations are performed for
76Ge, 100Mo, 136Xe and 130Te by using the advantage of recently evaluated
nuclear matrix elements with significantly reduced theoretical uncertainty. An
importance of observation of the 0nbb-decay of several nuclei is stressed.Comment: 29 pages, 5 figures, EXO (10 t) experiment considere
The (muon^-,muon^+) conversion in nuclei as a probe of new physics
A detailed study of the muonic analogue of neutrinoless double beta decay,
(muon^-,muon^+) conversion, has been carried out for the A=44 nuclear system.
We studied several lepton number violating (LNV) mechanisms potentially
triggering this process: exchange by light and heavy Majorana neutrinos as well
as exchange by supersymmetric particles participating in R-parity violating
interactions. The nuclear structure has been taken into account within the
renormalized Quasiparticle Random Phase Approximation method. To our knowledge,
this is the first realistic treatment of nuclear structure aspects of the
(muon^-,muon^+) conversion. We estimated the rate of this process utilizing the
existing experimental constraints on the parameters of the underlying LNV
interactions and conclude that the (muon^-,muon^+) conversion is hardly
detectable in the near future experiments.Comment: 23 pages, RevTex, 3 Postscript figure
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