2,263 research outputs found

    Pressure-induced enhancement of superconductivity and superconducting-superconducting transition in CaC_6\_6

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    We measured the electrical resistivity, ŌĪ(T)\varrho(T), of superconducting CaC_6\_6 at ambient and high pressure up to 16 GPa. For P‚ȧP \leq8 GPa, we found a large increase of T_cT\_c with pressure from 11.5 up to 15.1 K. At 8 GPa, T_cT\_c drops and levels off at 5 K above 10 GPa. Correspondingly, the residual ŌĪ\varrho increases by ‚Čą\approx 200 times and the ŌĪ(T)\varrho(T) behavior becomes flat. The recovery of the pristine behavior after depressurization is suggestive of a phase transition at 8 GPa between two superconducting phases with good and bad metallic properties, the latter with a lower T_cT\_c and more static disorder

    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 DAő¶\rm{\Phi}NE Ōē\phi-factory of INFN Laboratori Nazionali di Frascati. It aims to collect 25 fb‚ąí1^{-1} at the Ōē(1020)\phi(1020) peak, and about 5 fb‚ąí1^{-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

    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 A‚Ä≤A' 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‚Ä≤\textbf{M}_{A'} of the A‚Ä≤A' 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 ¬ĪMB,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

    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‚Čą{\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

    The Low Energy Tagger for the KLOE-2 experiment

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    The KLOE experiment at the upgraded DAFNE e+e- collider in Frascati (KLOE-2) is going to start a new data taking at the beginning of 2010 with its detector upgraded with a tagging system for the identification of gamma-gamma interactions. The tagging stations for low-energy e+e- will consist in two calorimeters The calorimeter used to detect low-energy e+e- will be placed between the beam-pipe outer support structure and the inner wall of the KLOE drift chamber. This calorimeter will be made of LYSO crystals readout by Silicon Photomultipliers, to achieve an energy resolution better than 8% at 200 MeV.Comment: 4 pages, 5 figures, in the proceedings of "Frontier detectors for frontier physics", isola d'Elba, Italy, May 200

    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.

    Hall helps Ohm: some corrections to negative-U centers approach to transport properties of YBa2_2Cu3_3Ox_x and La2‚ąíx_{2-x}Srx_xCuO4_4

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    For broad oxygen and strontium doping ranges, temperature dependences (T-dependences) of the normal state resistivity \rho(T) of YBa_2Cu_3O_x (YBCO) and La_(2-x)Sr_xCuO_4 (LSCO) are calculated and compared to experiments. Holes transport was taken in the \tau-approximation, where \tau(T,\epsilon) is due to acoustic phonons. Besides, T-dependence of the chemical potential \mu(T) and effective carrier mass m* ~10-100 free electron masses, obtained by negative-U centers modelling the T-dependence of the Hall coefficient, were used to calculate \rho(T). In addition, it is demonstrated that anisotropy of the cuprates does not affect the calculated T-variation of neither Hall coefficient nor \rho, but only rescale their magnitudes by factors depending on combinations of m_ab and m_c.Comment: 4th International Conference Fundamental Problems of High-Temperature Superconductivity, Moscow-Zvenigorod (October 3-7, 2011) Submitted to J. Supercond. Nov. Magn.: after revision. Extension for Supercond. Sci. Technol. 24 075026 (2011), DOI: 10.1088/0953-2048/24/7/075026 Contains: 2 pages, 3 figure
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