246 research outputs found
First-principles study of (BiScO3){1-x}-(PbTiO3){x} piezoelectric alloys
We report a first-principles study of a class of (BiScO3)_{1-x}-(PbTiO3)_x
(BS-PT) alloys recently proposed by Eitel et al. as promising materials for
piezoelectric actuator applications. We show that (i) BS-PT displays very large
structural distortions and polarizations at the morphotropic phase boundary
(MPB) (we obtain a c/a of ~1.05-1.08 and P_tet of ~1.1 C/m^2); (ii) the
ferroelectric and piezoelectric properties of BS-PT are dominated by the onset
of hybridization between Bi/Pb-6p and O-2p orbitals, a mechanism that is
enhanced upon substitution of Pb by Bi; and (iii) the piezoelectric responses
of BS-PT and Pb(Zr_{1-x}Ti_x)O3 (PZT) at the MPB are comparable, at least as
far as the computed values of the piezoelectric coefficient d_15 are concerned.
While our results are generally consistent with experiment, they also suggest
that certain intrinsic properties of BS-PT may be even better than has been
indicated by experiments to date. We also discuss results for PZT that
demonstrate the prominent role played by Pb displacements in its piezoelectric
properties.Comment: 6 pages, with 3 postscript figures embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/ji_bi/index.htm
Almost Maximal Lepton Mixing with Large T Violation in Neutrino Oscillations and Neutrinoless Double Beta Decay
We point out two simple but instructive possibilities to construct the
charged lepton and neutrino mass matrices, from which the nearly bi-maximal
neutrino mixing with large T violation can naturally emerge. The two lepton
mixing scenarios are compatible very well with current experimental data on
solar and atmospheric neutrino oscillations, and one of them may lead to an
observable T-violating asymmetry between \nu_\mu --> \nu_e and \nu_e -->
\nu_\mu transitions in the long-baseline neutrino oscillation experiments.
Their implications on the neutrinoless double beta decay are also discussed.Comment: RevTex 15 pages (2 PS figures
HIV-1 diversity in Cameroon: new insights on the evolution of the virus
Given the central role of HIV-1 diversity in the HIV pandemic and its impact on vaccine development, it is imperative that global molecular epidemiology surveillance continues. Constantly improving phylogenetics-based analytical techniques and rapidly expanding HIV sequence datasets promise to yield important insights into the origin, evolution and spread of HIV-1
Neutrino anomalies and large extra dimensions
Theories with large extra dimensions can generate small neutrino masses when
the standard model neutrinos are coupled to singlet fermions propagating in
higher dimensions. The couplings can also generate mass splittings and mixings
among the flavour neutrinos in the brane. We systematically study the minimal
scenario involving only one singlet bulk fermion coupling weakly to the flavour
neutrinos. We explore the neutrino mass structures in the brane that can
potentially account for the atmospheric, solar and LSND anomalies
simultaneously in a natural way. We demonstrate that in the absence of a priori
mixings among the SM neutrinos, it is not possible to reconcile all these
anomalies. The presence of some structure in the mass matrix of the SM
neutrinos can solve this problem. This is exemplified by the Zee model, which
when embedded in extra dimensions in a minimal way can account for all the
neutrino anomalies.Comment: 23 Revtex pages with 2 eps figure
Connecting bimaximal neutrino mixing to a light sterile neutrino
It is shown that if small neutrino masses owe their origin to the
conventional seesaw mechanism and the MNS mixing matrix is in the exact
bimaximal form, then there exist symmetries in the theory that allow one of the
righthanded neutrinos to become naturally massless, making it a candidate for
the sterile neutrino discussed in the literature. Departures from the exact
bimaximal limit leads to tiny mass for the sterile neutrino as well as its
mixing to the active neutrinos. This provides a minimal theoretical framework
where a simultaneous explanation of the solar, atmospheric and LSND
observations within the so-called 3+1 scenario may be possible.Comment: new references added; paper accepted for publication in Phys. Rev.
D.(rapid communications); note adde
The Increase in the Primordial He-4 Yield in the Two-Doublet Four-Neutrino Mixing Scheme
We assess the effects on Big Bang Nucleosynthesis (BBN) of lepton number
generation in the early universe resulting from the two-doublet four-neutrino
mass/mixing scheme. It has been argued that this neutrino mass/mixing
arrangement gives the most viable fit to the existing data. We study full 4 x 4
mixing matrices and show how possible symmetries in these can affect the BBN
He-4 abundance yields. Though there is as yet no consensus on the reliability
of BBN calculations with neutrino flavor mixing, we show that, in the case
where the sign of the lepton number asymmetry is unpredictable, BBN
considerations may pick out specific relationships between mixing angles. In
particular, reconciling the observed light element abundances with a
\bar\nu_\mu \bar\nu_e oscillation interpretation of LSND would allow unique
new constraints on the neutrino mixing angles in this model.Comment: 12 pages, including 4 figures, to appear in Phys. Rev.
The Interplay Between the "Low" and "High" Energy CP-Violation in Leptogenesis
We analyse within the "flavoured" leptogenesis scenario of baryon asymmetry
generation, the interplay of the "low energy" CP-violation, originating from
the PMNS neutrino mixing matrix , and the "high energy" CP-violation which
can be present in the matrix of neutrino Yukawa couplings, , and can
manifest itself only in "high" energy scale processes. The type I see-saw model
with three heavy right-handed Majorana neutrinos having hierarchical spectrum
is considered. The "orthogonal" parametrisation of the matrix of neutrino
Yukawa couplings, which involves a complex orthogonal matrix , is employed.
In this approach the matrix is the source of "high energy" CP-violation.
Results for normal hierarchical (NH) and inverted hierarchical (IH) light
neutrino mass spectrum are derived in the case of decoupling of the heaviest RH
Majorana neutrino. It is shown that taking into account the contribution to
due to the CP-violating phases in the neutrino mixing matrix can
change drastically the predictions for , obtained assuming only "high
energy" CP-violation from the -matrix is operative in leptogenesis. In the
case of IH spectrum, in particular, there exist significant regions in the
corresponding parameter space where the purely "high energy" contribution in
plays a subdominant role in the production of baryon asymmetry compatible
with the observations.Comment: Results unchanged; comments and references added; version to be
puplished in Eur.Phys.J.
Renormalization Group Running of the Neutrino Mass Operator in Extra Dimensions
We study the renormalization group (RG) running of the neutrino masses and
the leptonic mixing parameters in two different extra-dimensional models,
namely, the Universal Extra Dimensions (UED) model and a model, where the
Standard Model (SM) bosons probe an extra dimension and the SM fermions are
confined to a four-dimensional brane. In particular, we derive the beta
function for the neutrino mass operator in the UED model. We also rederive the
beta function for the charged-lepton Yukawa coupling, and confirm some of the
existing results in the literature. The generic features of the RG running of
the neutrino parameters within the two models are analyzed and, in particular,
we observe a power-law behavior for the running. We note that the running of
the leptonic mixing angle \theta_{12} can be sizable, while the running of
\theta_{23} and \theta_{13} is always negligible. In addition, we show that the
tri-bimaximal and the bimaximal mixing patterns at a high-energy scale are
compatible with low-energy experimental data, while a tri-small mixing pattern
is not. Finally, we perform a numerical scan over the low-energy parameter
space to infer the high-energy distribution of the parameters. Using this scan,
we also demonstrate how the high-energy \theta_{12} is correlated with the
smallest neutrino mass and the Majorana phases.Comment: 20 pages, 5 figures, REVTeX4-1. (v2) Final version published in J.
High Energy Phys. (v3) A short clarification at the end of the appendix has
been adde
Long Baseline Neutrino Physics with a Muon Storage Ring Neutrino Source
We examine the physics capabilities of known flavor neutrino beams from
intense muon sources. We find that long-baseline neutrino experiments based on
such beams can provide precise measurements of neutrino oscillation mass and
mixing parameters. Furthermore, they can test whether the dominant atmospheric
neutrino oscillations are \nu_\mu --> \nu_\tau and/or \nu_\mu --> \nu_s,
determine the \nu_\mu --> \nu_e content of atmospheric neutrino oscillations,
and measure \nu_e --> \nu_\tau appearance. Depending on the oscillation
parameters, they may be able to detect Earth matter and CP violation effects
and to determine the ordering of some of the mass eigenstates.Comment: 38 pages, Revtex with epsf.sty, 21 postscript figures. Minor text
revisions, some new numbers in Tables II and II
Bimaximal mixing from the leptonic new texture for triangular mass matrices
An analysis of the leptonic texture for the new triangular mass matrices has
been carried out. In particular, it is shown that both bimaximal and nearly
bimaximal solutions for solar and atmospheric neutrino anomalies can be
generated within this pattern. We have also derived exact and compact
parametrization of the leptonic mixing matrix in terms of the lepton masses and
the parameters and . A consistency with the CHOOZ
reactor result for and a smallness of the Jarlskog's invariant
parameter are obtained.Comment: 16 pages, late
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