896 research outputs found
Molecular orbital calculations of two-electron states for P donor solid-state spin qubits
We theoretically study the Hilbert space structure of two neighbouring P
donor electrons in silicon-based quantum computer architectures. To use
electron spins as qubits, a crucial condition is the isolation of the electron
spins from their environment, including the electronic orbital degrees of
freedom. We provide detailed electronic structure calculations of both the
single donor electron wave function and the two-electron pair wave function. We
adopted a molecular orbital method for the two-electron problem, forming a
basis with the calculated single donor electron orbitals. Our two-electron
basis contains many singlet and triplet orbital excited states, in addition to
the two simple ground state singlet and triplet orbitals usually used in the
Heitler-London approximation to describe the two-electron donor pair wave
function. We determined the excitation spectrum of the two-donor system, and
study its dependence on strain, lattice position and inter donor separation.
This allows us to determine how isolated the ground state singlet and triplet
orbitals are from the rest of the excited state Hilbert space. In addition to
calculating the energy spectrum, we are also able to evaluate the exchange
coupling between the two donor electrons, and the double occupancy probability
that both electrons will reside on the same P donor. These two quantities are
very important for logical operations in solid-state quantum computing devices,
as a large exchange coupling achieves faster gating times, whilst the magnitude
of the double occupancy probability can affect the error rate.Comment: 15 pages (2-column
B-meson decay constants: a more complete picture from full lattice QCD
We extend the picture of -meson decay constants obtained in lattice QCD
beyond those of the , and to give the first full lattice QCD
results for the , and . We use improved NonRelativistic QCD
for the valence quark and the Highly Improved Staggered Quark (HISQ) action
for the lighter quarks on gluon field configurations that include the effect of
, and quarks in the sea with quark masses going down to
physical values. For the ratio of vector to pseudoscalar decay constants, we
find = 0.941(26), = 0.953(23) (both
less than 1.0) and = 0.988(27). Taking correlated
uncertainties into account we see clear indications that the ratio increases as
the mass of the lighter quark increases. We compare our results to those using
the HISQ formalism for all quarks and find good agreement both on decay
constant values when the heaviest quark is a and on the dependence on the
mass of the heaviest quark in the region of the . Finally, we give an
overview plot of decay constants for gold-plated mesons, the most complete
picture of these hadronic parameters to date.Comment: 20 pages, 9 figures. Minor updates to the discussion in several
places and some additional reference
Are patterns of fine-scale spatial genetic structure consistent between sites within tropical tree species?
JRS was funded by the Swiss National Science Foundation (SNF) (http://www.snf.ch/en/Pages/default.aspx) grant number PDFMP3_132479 / 1 awarded to JG. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Peer reviewedPublisher PD
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Current experiments in particle physics - particle data group
This report contains summaries of current and recent experiments in Particle Physics. Included are experiments at BEPC (Beijing), BNL, CEBAF, CERN, CESR, DESY, FNAL, Frascati, ITEP (Moscow), JINR (Dubna), KEK, LAMPF, Novosibirsk, PNPI (St. Petersburg), PSI, Saclay, Serpukhov, SLAC, and TRIUMF, and also several proton decay and solar neutrino experiments. Excluded are experiments that finished taking data before 1991. Instructions are given for the World Wide Web (WWW) searching of the computer database (maintained under the SLAC-SPIRES system) that contains the summaries
Research Proposal for an Experiment to Search for the Decay {\mu} -> eee
We propose an experiment (Mu3e) to search for the lepton flavour violating
decay mu+ -> e+e-e+. We aim for an ultimate sensitivity of one in 10^16
mu-decays, four orders of magnitude better than previous searches. This
sensitivity is made possible by exploiting modern silicon pixel detectors
providing high spatial resolution and hodoscopes using scintillating fibres and
tiles providing precise timing information at high particle rates.Comment: Research proposal submitted to the Paul Scherrer Institute Research
Committee for Particle Physics at the Ring Cyclotron, 104 page
Advanced modelling of the Planck-LFI radiometers
The Low Frequency Instrument (LFI) is a radiometer array covering the 30-70
GHz spectral range on-board the ESA Planck satellite, launched on May 14th,
2009 to observe the cosmic microwave background (CMB) with unprecedented
precision. In this paper we describe the development and validation of a
software model of the LFI pseudo-correlation receivers which enables to
reproduce and predict all the main system parameters of interest as measured at
each of the 44 LFI detectors. These include system total gain, noise
temperature, band-pass response, non-linear response. The LFI Advanced RF Model
(LARFM) has been constructed by using commercial software tools and data of
each radiometer component as measured at single unit level. The LARFM has been
successfully used to reproduce the LFI behavior observed during the LFI
ground-test campaign. The model is an essential element in the database of LFI
data processing center and will be available for any detailed study of
radiometer behaviour during the survey.Comment: 21 pages, 15 figures, this paper is part of the Prelaunch status LFI
papers published on JINST:
http://www.iop.org/EJ/journal/-page=extra.proc5/jins
Genetic diversity affects seedling survival but not growth or seed germination in the Bornean endemic dipterocarp Parashorea tomentella
Background: Logging and habitat fragmentation of tropical rain forests may disrupt patterns of gene flow and genetic diversity. Consequently, inbreeding in tree populations may reduce fitness and increase extinction risks, especially among species that are predominantly outcrossing, dependent on biotic pollination and/or display limited seed dispersal such as species of the Dipterocarpaceae.
Aims: To test the hypothesis that heterozygosity of individual progeny affects their likelihood of germination and the growth and survival of seedlings.
Methods: Standardised measure of multilocus heterozygosity (sMLH) was estimated from seven microsatellite loci for individual progeny collected from 18 mother trees of the large dipterocarp Parashorea tomentella. The relationships among sMLH, germination and seedling growth and survival were determined for the progeny.
Results: Seedling survival over 18 months increased with greater sMLH and fresh fruit weight. This result was expressed under all experimentally controlled combinations of light and nutrient availability in the nursery and in the shaded understorey of primary forest where survival overall was much lower than in the nursery. sMLH did not affect the probability of germination or seedling growth rate in any experimental treatment.
Conclusions: These results provide evidence that reduced heterozygosity is associated with reduced seedling survival in a tropical forest tree species
LFI 30 and 44 GHz receivers Back-End Modules
The 30 and 44 GHz Back End Modules (BEM) for the Planck Low Frequency
Instrument are broadband receivers (20% relative bandwidth) working at room
temperature. The signals coming from the Front End Module are amplified, band
pass filtered and finally converted to DC by a detector diode. Each receiver
has two identical branches following the differential scheme of the Planck
radiometers. The BEM design is based on MMIC Low Noise Amplifiers using GaAs
P-HEMT devices, microstrip filters and Schottky diode detectors. Their
manufacturing development has included elegant breadboard prototypes and
finally qualification and flight model units. Electrical, mechanical and
environmental tests were carried out for the characterization and verification
of the manufactured BEMs. A description of the 30 and 44 GHz Back End Modules
of Planck-LFI radiometers is given, with details of the tests done to determine
their electrical and environmental performances. The electrical performances of
the 30 and 44 GHz Back End Modules: frequency response, effective bandwidth,
equivalent noise temperature, 1/f noise and linearity are presented
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