301 research outputs found
Ab initio calculations for bromine adlayers on the Ag(100) and Au(100) surfaces: the c(2x2) structure
Ab initio total-energy density-functional methods with supercell models have
been employed to calculate the c(2x2) structure of the Br-adsorbed Ag(100) and
Au(100) surfaces. The atomic geometries of the surfaces and the preferred
bonding sites of the bromine have been determined. The bonding character of
bromine with the substrates has also been studied by analyzing the electronic
density of states and the charge transfer. The calculations show that while the
four-fold hollow-site configuration is more stable than the two-fold
bridge-site topology on the Ag(100) surface, bromine prefers the bridge site on
the Au(100) surface. The one-fold on-top configuration is the least stable
configuration on both surfaces. It is also observed that the second layer of
the Ag substrate undergoes a small buckling as a consequence of the adsorption
of Br. Our results provide a theoretical explanation for the experimental
observations that the adsorption of bromine on the Ag(100) and Au(100) surfaces
results in different bonding configurations.Comment: 10 pages, 4 figure, 5 tables, Phys. Rev. B, in pres
Discontinuous transitions in double exchange materials
It is shown that the double exchange Hamiltonian, with weak antiferromagnetic
interactions, has a rich variety of first order transitions between phases with
different electronic densities and/or magnetizations. For band fillings in the
range , and at finite temperatures, a discontinuous
transition between phases with similar electronic densities but different
magnetizations takes place. This sharp transition, which is not suppressed by
electrostatic effects, and survives in the presence of an applied field, is
consistent with the phenomenology of the doped manganites near the transition
temperature.Comment: three more variational ansatzs considere
Background free search for neutrinoless double beta decay with GERDA Phase II
The Standard Model of particle physics cannot explain the dominance of matter
over anti-matter in our Universe. In many model extensions this is a very
natural consequence of neutrinos being their own anti-particles (Majorana
particles) which implies that a lepton number violating radioactive decay named
neutrinoless double beta () decay should exist. The detection
of this extremely rare hypothetical process requires utmost suppression of any
kind of backgrounds.
The GERDA collaboration searches for decay of Ge
(^{76}\rm{Ge} \rightarrow\,^{76}\rm{Se} + 2e^-) by operating bare detectors
made from germanium with enriched Ge fraction in liquid argon. Here, we
report on first data of GERDA Phase II. A background level of
cts/(keVkgyr) has been achieved which is the world-best if
weighted by the narrow energy-signal region of germanium detectors. Combining
Phase I and II data we find no signal and deduce a new lower limit for the
half-life of yr at 90 % C.L. Our sensitivity of
yr is competitive with the one of experiments with
significantly larger isotope mass.
GERDA is the first experiment that will be background-free
up to its design exposure. This progress relies on a novel active veto system,
the superior germanium detector energy resolution and the improved background
recognition of our new detectors. The unique discovery potential of an
essentially background-free search for decay motivates a
larger germanium experiment with higher sensitivity.Comment: 14 pages, 9 figures, 1 table; ; data, figures and images available at
http://www.mpi-hd.mpg/gerda/publi
Limits on uranium and thorium bulk content in GERDA Phase I detectors
Internal contaminations of U, U and Th in the bulk of
high purity germanium detectors are potential backgrounds for experiments
searching for neutrinoless double beta decay of Ge. The data from GERDA
Phase~I have been analyzed for alpha events from the decay chain of these
contaminations by looking for full decay chains and for time correlations
between successive decays in the same detector. No candidate events for a full
chain have been found. Upper limits on the activities in the range of a few
nBq/kg for Ra, Ac and Th, the long-lived daughter
nuclides of U, U and Th, respectively, have been
derived. With these upper limits a background index in the energy region of
interest from Ra and Th contamination is estimated which
satisfies the prerequisites of a future ton scale germanium double beta decay
experiment.Comment: 2 figures, 7 page
Characterization of 30 Ge enriched Broad Energy Ge detectors for GERDA Phase II
The GERmanium Detector Array (GERDA) is a low background experiment located
at the Laboratori Nazionali del Gran Sasso in Italy, which searches for
neutrinoless double beta decay of Ge into Se+2e. GERDA has
been conceived in two phases. Phase II, which started in December 2015,
features several novelties including 30 new Ge detectors. These were
manufactured according to the Broad Energy Germanium (BEGe) detector design
that has a better background discrimination capability and energy resolution
compared to formerly widely-used types. Prior to their installation, the new
BEGe detectors were mounted in vacuum cryostats and characterized in detail in
the HADES underground laboratory in Belgium. This paper describes the
properties and the overall performance of these detectors during operation in
vacuum. The characterization campaign provided not only direct input for GERDA
Phase II data collection and analyses, but also allowed to study detector
phenomena, detector correlations as well as to test the strength of pulse shape
simulation codes.Comment: 29 pages, 18 figure
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