12,341 research outputs found
Lepton masses and mixings in orbifold models with three Higgs families
We analyse the phenomenological viability of heterotic Z(3) orbifolds with
two Wilson lines, which naturally predict three supersymmetric families of
matter and Higgs fields. Given that these models can accommodate realistic
scenarios for the quark sector avoiding potentially dangerous flavour-changing
neutral currents, we now address the leptonic sector, finding that viable
orbifold configurations can in principle be obtained. In particular,it is
possible to accomodate present data on charged lepton masses, while avoiding
conflict with lepton flavour-violating decays. Concerning the generation of
neutrino masses and mixings, we find that Z(3) orbifolds offer several
interesting possibilities.Comment: 28 pages, 11 figures. References adde
The high-pressure behavior of CaMoO4
We report a high-pressure study of tetragonal scheelite-type CaMoO4 up to 29
GPa. In order to characterize its high-pressure behavior, we have combined
Raman and optical-absorption measurements with density-functional theory
calculations. We have found evidence of a pressure-induced phase transition
near 15 GPa. Experiments and calculations agree in assigning the high-pressure
phase to a monoclinic fergusonite-type structure. The reported results are
consistent with previous powder x-ray-diffraction experiments, but are in
contradiction with the conclusions obtained from earlier Raman measurements,
which support the existence of more than one phase transition in the pressure
range covered by our studies. The observed scheelite-fergusonite transition
induces significant changes in the electronic band gap and phonon spectrum of
CaMoO4. We have determined the pressure evolution of the band gap for the low-
and high-pressure phases as well as the frequencies and pressure dependences of
the Raman-active and infrared-active modes. In addition, based upon
calculations of the phonon dispersion of the scheelite phase, carried out at a
pressure higher than the transition pressure, we propose a possible mechanism
for the reported phase transition. Furthermore, from the calculations we
determined the pressure dependence of the unit-cell parameters and atomic
positions of the different phases and their room-temperature equations of
state. These results are compared with previous experiments showing a very good
agreement. Finally, information on bond compressibility is reported and
correlated with the macroscopic compressibility of CaMoO4. The reported results
are of interest for the many technological applications of this oxide.Comment: 36 pages, 10 figures, 8 table
Photo-desorption of H2O:CO:NH3 circumstellar ice analogs: Gas-phase enrichment
We study the photo-desorption occurring in HO:CO:NH ice mixtures
irradiated with monochromatic (550 and 900 eV) and broad band (250--1250 eV)
soft X-rays generated at the National Synchrotron Radiation Research Center
(Hsinchu, Taiwan). We detect many masses photo-desorbing, from atomic hydrogen
(m/z = 1) to complex species with m/z = 69 (e.g., CHNO, CHO,
CHN), supporting the enrichment of the gas phase.
At low number of absorbed photons, substrate-mediated exciton-promoted
desorption dominates the photo-desorption yield inducing the release of weakly
bound (to the surface of the ice) species; as the number of weakly bound
species declines, the photo-desorption yield decrease about one order of
magnitude, until porosity effects, reducing the surface/volume ratio, produce a
further drop of the yield.
We derive an upper limit to the CO photo-desorption yield, that in our
experiments varies from 1.4 to 0.007 molecule photon in the range ~absorbed photons cm. We apply these findings to a
protoplanetary disk model irradiated by a central T~Tauri star
Spin injection in a single metallic nanoparticle: a step towards nanospintronics
We have fabricated nanometer sized magnetic tunnel junctions using a new
nanoindentation technique in order to study the transport properties of a
single metallic nanoparticle. Coulomb blockade effects show clear evidence for
single electron tunneling through a single 2.5 nm Au cluster. The observed
magnetoresistance is the signature of spin conservation during the transport
process through a non magnetic cluster.Comment: 3 page
Theoretical analysis for critical fluctuations of relaxation trajectory near a saddle-node bifurcation
A Langevin equation whose deterministic part undergoes a saddle-node
bifurcation is investigated theoretically. It is found that statistical
properties of relaxation trajectories in this system exhibit divergent
behaviors near a saddle-node bifurcation point in the weak-noise limit, while
the final value of the deterministic solution changes discontinuously at the
point. A systematic formulation for analyzing a path probability measure is
constructed on the basis of a singular perturbation method. In this
formulation, the critical nature turns out to originate from the neutrality of
exiting time from a saddle-point. The theoretical calculation explains results
of numerical simulations.Comment: 18pages, 17figures.The version 2, in which minor errors have been
fixed, will be published in Phys. Rev.
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