2,769 research outputs found

    Internal-strain mediated coupling between polar Bi and magnetic Mn ions in the defect-free quadruple-perovskite BiMn3_3Mn4_4O12_{12}

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    By means of neutron powder diffraction, we investigated the effect of the polar Bi3+^{3+} ion on the magnetic ordering of the Mn3+^{3+} ions in BiMn3_3Mn4_4O12_{12}, the counterpart with \textit{quadruple} perovskite structure of the \textit{simple} perovskite BiMnO3_3. The data are consistent with a \textit{noncentrosymmetric} spacegroup ImIm which contrasts the \textit{centrosymmetric} one I2/mI2/m previously reported for the isovalent and isomorphic compound LaMn3_3Mn4_4O12_{12}, which gives evidence of a Bi3+^{3+}-induced polarization of the lattice. At low temperature, the two Mn3+^{3+} sublattices of the AA' and BB sites order antiferromagnetically (AFM) in an independent manner at 25 and 55 K, similarly to the case of LaMn3_3Mn4_4O12_{12}. However, both magnetic structures of BiMn3_3Mn4_4O12_{12} radically differ from those of LaMn3_3Mn4_4O12_{12}. In BiMn3_3Mn4_4O12_{12} the moments MA\textbf{M}_{A'} of the AA' sites form an anti-body AFM structure, whilst the moments \textbf{M}B_{B} of the BB sites result from a large and \textit{uniform} modulation ±MB,b\pm \textbf{M}_{B,b} along the b-axis of the moments \textbf{M}B,ac_{B,ac} in the acac-plane. The modulation is strikingly correlated with the displacements of the Mn3+^{3+} ions induced by the Bi3+^{3+} ions. Our analysis unveils a strong magnetoelastic coupling between the internal strain created by the Bi3+^{3+} ions and the moment of the Mn3+^{3+} ions in the BB 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 Bi3+^{3+}-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

    Commensurate structural modulation in the charge- and orbitally-ordered phase of the quadruple perovskite (NaMn3_3)Mn4_4O12_{12}

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    By means of synchrotron x-ray and electron diffraction, we studied the structural changes at the charge order transition TCOT_{CO}=176 K in the mixed-valence quadruple perovskite (NaMn3_3)Mn4_4O12_{12}. Below TCOT_{CO} we find satellite peaks indicating a commensurate structural modulation with the same propagation vector q =(1/2,0,-1/2) of the CE magnetic order that appears at low temperature, similarly to the case of simple perovskites like La0.5_{0.5}Ca0.5_{0.5}MnO3_3. In the present case, the modulated structure together with the observation of a large entropy change at TCOT_{CO} gives evidence of a rare case of full Mn3+^{3+}/Mn4+^{4+} charge and orbital order consistent with the Goodenough-Kanamori model.Comment: Accepted for publication in Phys. Rev. B Rapid Communication

    Electron localization and possible phase separation in the absence of a charge density wave in single-phase 1T-VS2_2

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    We report on a systematic study of the structural, magnetic and transport properties of high-purity 1T-VS2_2 powder samples prepared under high pressure. The results differ notably from those previously obtained by de-intercalating Li from LiVS2_2. 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 qCDW{\bf q}_{CDW} \approx (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-VS2_2 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

    Status of KLOE-2

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    In a few months the KLOE-2 detector is expected to start data taking at the upgraded DAΦ\rm{\Phi}NE ϕ\phi-factory of INFN Laboratori Nazionali di Frascati. It aims to collect 25 fb1^{-1} at the ϕ(1020)\phi(1020) peak, and about 5 fb1^{-1} 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

    Study of the time and space distribution of beta+ emitters from 80 MeV/u carbon ion beam irradiation on PMMA

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    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 β+\beta^+ 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 β+\beta^+ rate was parametrized and the dominance of 11C^{11}C emitters over the other species (13N^{13}N, 15O^{15}O, 14O^{14}O) was observed, measuring the fraction of carbon ions activating β+\beta^+ emitters A0=(10.3±0.7)103A_0=(10.3\pm0.7)\cdot10^{-3}. The average depth in the PMMA of the positron annihilation from β+\beta^+ 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

    Measurement of neutron detection efficiency between 22 and 174 MeV using two different kinds of Pb-scintillating fiber sampling calorimeters

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    We exposed a prototype of the lead-scintillating fiber KLOE calorimeter to neutron beam of 21, 46 and 174 MeV at The Svedberg Laboratory, Uppsala, to study its neutron detection efficiency. This has been found larger than what expected considering the scintillator thickness of the prototype. %To check our method, we measured also the neutron %detection efficiency of a 5 cm thick NE110 scintillator. We show preliminary measurement carried out with a different prototype with a larger lead/fiber ratio, which proves the relevance of passive material to neutron detection efficiency in this kind of calorimeters

    Granularity-induced gapless superconductivity in NbN films: evidence of thermal phase fluctuations

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    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.

    Measurement of the neutron detection efficiency of a 80% absorber - 20% scintillating fibers calorimeter

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    The neutron detection efficiency of a sampling calorimeter made of 1 mm diameter scintillating fibers embedded in a lead/bismuth structure has been measured at the neutron beam of the The Svedberg Laboratory at Uppsala. A significant enhancement of the detection efficiency with respect to a bulk organic scintillator detector with the same thickness is observed.Comment: 10 pages, 7 figure
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