2,406 research outputs found
Internal-strain mediated coupling between polar Bi and magnetic Mn ions in the defect-free quadruple-perovskite BiMnMnO
By means of neutron powder diffraction, we investigated the effect of the
polar Bi ion on the magnetic ordering of the Mn ions in
BiMnMnO, the counterpart with \textit{quadruple} perovskite
structure of the \textit{simple} perovskite BiMnO. The data are consistent
with a \textit{noncentrosymmetric} spacegroup which contrasts the
\textit{centrosymmetric} one previously reported for the isovalent and
isomorphic compound LaMnMnO, which gives evidence of a
Bi-induced polarization of the lattice. At low temperature, the two
Mn sublattices of the and sites order antiferromagnetically
(AFM) in an independent manner at 25 and 55 K, similarly to the case of
LaMnMnO. However, both magnetic structures of
BiMnMnO radically differ from those of LaMnMnO.
In BiMnMnO the moments of the sites form
an anti-body AFM structure, whilst the moments \textbf{M} of the
sites result from a large and \textit{uniform} modulation along the b-axis of the moments \textbf{M} in the
-plane. The modulation is strikingly correlated with the displacements of
the Mn ions induced by the Bi ions. Our analysis unveils a strong
magnetoelastic coupling between the internal strain created by the Bi
ions and the moment of the Mn ions in the sites. This is ascribed to
the high symmetry of the oxygen sites and to the absence of oxygen defects, two
characteristics of quadruple perovskites not found in simple ones, which
prevent the release of the Bi-induced strain through distortions or
disorder. This demonstrates the possibility of a large magnetoelectric coupling
in proper ferroelectrics and suggests a novel concept of internal strain
engineering for multiferroics design.Comment: 9 pages, 7 figures, 5 table
Status of KLOE-2
In a few months the KLOE-2 detector is expected to start data taking at the
upgraded DANE -factory of INFN Laboratori Nazionali di
Frascati. It aims to collect 25 fb at the peak, and about 5
fb in the energy region between 1 and 2.5 GeV. We review the status and
physics program of the projectComment: 6 pages, 5 figures, to appear in the Proceedings of the PHIPSI09
Workshop, Oct 13-16, 2009, Beijing, Chin
Electron localization and possible phase separation in the absence of a charge density wave in single-phase 1T-VS
We report on a systematic study of the structural, magnetic and transport
properties of high-purity 1T-VS powder samples prepared under high
pressure. The results differ notably from those previously obtained by
de-intercalating Li from LiVS. First, no Charge Density Wave (CDW) is found
by transmission electron microscopy down to 94 K. Though, \textit{ab initio}
phonon calculations unveil a latent CDW instability driven by an acoustic
phonon softening at the wave vector (0.21,0.21,0)
previously reported in de-intercalated samples. A further indication of latent
lattice instability is given by an anomalous expansion of the V-S bond distance
at low temperature. Second, infrared optical absorption and electrical
resistivity measurements give evidence of non metallic properties, consistent
with the observation of no CDW phase. On the other hand, magnetic
susceptibility and NMR data suggest the coexistence of localized moments with
metallic carriers, in agreement with \textit{ab initio} band structure
calculations. This discrepancy is reconciled by a picture of electron
localization induced by disorder or electronic correlations leading to a phase
separation of metallic and non-metallic domains in the nm scale. We conclude
that 1T-VS is at the verge of a CDW transition and suggest that residual
electronic doping in Li de-intercalated samples stabilizes a uniform CDW phase
with metallic properties.Comment: 22 pages, 10 Figures. Full resolution pictures available at
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.23512
Granularity-induced gapless superconductivity in NbN films: evidence of thermal phase fluctuations
Using a single coil mutual inductance technique, we measure the low
temperature dependence of the magnetic penetration depth in superconducting NbN
films prepared with similar critical temperatures around 16 K but with
different microstructures. Only (100) epitaxial and weakly granular (100)
textured films display the characteristic exponential dependence of
conventional BCS s-wave superconductors. More granular (111) textured films
exhibit a linear dependence, indicating a gapless state in spite of the s-wave
gap. This result is quantitatively explained by a model of thermal phase
fluctuations favored by the granular structure.Comment: 10 pages, 4 figures, to appear in Phys. Rev.
A description of the ratio between electric and magnetic proton form factors by using space-like, time-like data and dispersion relations
We use the available information on the ratio between the electric and
magnetic proton form factors coming from recently published space-like data and
from the few available time-like data. We apply a dispersive procedure on these
data to evaluate the behaviour of this ratio, as a complex function, for all
values of q^2.Comment: 12 pages, 7 Encapsulated Postscript figures, uses epsfig, rotating,
exscale, amsmath, cite, latexsym, graphics, color packages, added reference
Suppression of flavor violation in an A4 warped extra dimensional model
In an attempt to simultaneously explain the observed masses and mixing
patterns of both quarks and leptons, we recently proposed a model
(JHEP08(2010)115) based on the non abelian discrete flavor group A4,
implemented in a custodial RS setup with a bulk Higgs. We showed that the
standard model flavor structure can be realized within the zero mode
approximation (ZMA), with nearly tribimaximal (TBM) neutrino mixing and a
realistic CKM matrix with rather mild assumptions. An important advantage of
this framework with respect to flavor anarchic models is the vanishing of the
dangerous tree level KK gluon contribution to \epsilon_K and the suppression of
the new physics one loop contributions to the neutron EDM, \epsilon'/\epsilon,
b->s\gamma and Higgs mediated flavor changing neutral current (FCNC) processes.
These results are obtained beyond the ZMA, in order to account for the the full
flavor structure and mixing of the zero modes and first Kaluza-Klein (KK) modes
of all generations. The resulting constraints on the KK mass scale are shown to
be significantly relaxed compared to the flavor anarchic case, showing
explicitly the role of non abelian discrete flavor symmetries in relaxing
flavor violation bounds within the RS setup. As a byproduct of our analysis we
also obtain the same contributions for the custodial anarchic case with two
SU(2)_R doublets for each fermion generation.Comment: 8 pages, 4 figures; contribution prepared for DISCRETE'10 - Symposium
on Prospects in the Physics of Discrete Symmetrie
Study of the time and space distribution of beta+ emitters from 80 MeV/u carbon ion beam irradiation on PMMA
Proton and carbon ion therapy is an emerging technique used for the treatment
of solid cancers. The monitoring of the dose delivered during such treatments
and the on-line knowledge of the Bragg peak position is still a matter of
research. A possible technique exploits the collinear 511\ \kilo\electronvolt
photons produced by positrons annihilation from emitters created by
the beam. This paper reports rate measurements of the 511\ \kilo\electronvolt
photons emitted after the interactions of a 80\ \mega\electronvolt / u fully
stripped carbon ion beam at the Laboratori Nazionali del Sud (LNS) of INFN,
with a Poly-methyl methacrylate target. The time evolution of the
rate was parametrized and the dominance of emitters over the other
species (, , ) was observed, measuring the fraction of
carbon ions activating emitters . The
average depth in the PMMA of the positron annihilation from emitters
was also measured, D_{\beta^+}=5.3\pm1.1\ \milli\meter, to be compared to the
expected Bragg peak depth D_{Bragg}=11.0\pm 0.5\ \milli\meter obtained from
simulations
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