9,421 research outputs found

    Large Magneto-Dielectric Effects in Orthorhombic HoMnO3 and YMnO3

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    We have found a remarkable increase (up to 60 %) of the dielectric constant with the onset of magnetic order at 42 K in the metastable orthorhombic structures of YMnO3 and HoMnO3 that proves the existence of a strong magneto-dielectric coupling in the compounds. Magnetic, dielectric, and thermodynamic properties show distinct anomalies at the onset of the incommensurate magnetic order and thermal hysteresis effects are observed around the lock-in transition temperature at which the incommensurate magnetic order locks into a temperature independent wave vector. The orders of Mn3+ spins and Ho3+ moments both contribute to the magneto-dielectric coupling. A large magneto-dielectric effect was observed in HoMnO3 at low temperature where the dielectric constant can be tuned by an external magnetic field resulting in a decrease of up to 8 % at 7 Tesla. By comparing data for YMnO3 and HoMnO3 the contributions to the coupling between the dielectric response and Mn and Ho magnetic orders are separated.Comment: revised manuscrip

    Ferroelectricity in perovskite HoMnO3HoMnO_3 and YMnO3YMnO_3

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    Ferroelectricity is observed in orthorhombic HoMnO3HoMnO_3 and YMnO3YMnO_3 at the magnetic lock-in transitions into an E-type structure or an incommensurate phase with a temperature independent wave vector, respectively. In HoMnO3HoMnO_3 the ferroelectric polarization strongly depends on the external magnetic field indicating the involvement of the rare earth moment order in this compound. The results are discussed within the framework of recent theoretical models, in particular the double exchange driven polar displacements predicted for E-type magnetic structures.Comment: 5 pages, 3 figure

    Superconductivity in pure and electron doped MgB2: Transport properties and pressure effects

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    The normal state and superconducting properties of MgB2 and Mg1-xAlxB2 are discussed based on structural, transport, and high pressure experiments. The positive Seebeck coefficient and its linear temperature dependence for Tc<T<160 K provide evidence that the low-temperature transport in MgB2 is due to hole-like metallic carriers. Structural and transport data show the important role of defects as indicated by the correlation of Tc, the residual resistance ratio, and the microstrain extracted from x-ray spectra. The decrease of Tc with hydrostatic pressure is well explained by the strong-coupling BCS theory. The large scatter of the pressure coefficients of Tc for different MgB2 samples, however, cannot be explained within this theory. We speculate that pressure may increase the defect density, particularly in samples with large initial defect concentration.Comment: Presented at NATO Advanced Research Workshop "New Trends in Superconductivity", Yalta (Ukraine) 16-20 September, 200

    Modified null broadening adaptive beamforming: constrained optimisation approach

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    A constrained optimisation approach for null broadening adaptive beamforming is proposed. This approach improves the robustness of the traditional MVDR beamformer by broadening nulls for interference direction and the mainlobe for the desired direction. This optimisation is efficiently solved by semidefinite programming. The proposed approach, when applied to high altitude platform communications using a vertical linear antenna array, provides significantly better coverage performance than a previously reported null broadening technique

    Theoretical Constraints and Systematic Effects in the Determination of the Proton Form Factors

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    We calculate the two-photon exchange corrections to electron-proton scattering with nucleon and Δ\Delta intermediate states. The results show a dependence on the elastic nucleon and nucleon-Δ\Delta-transition form factors used as input which leads to significant changes compared to previous calculations. We discuss the relevance of these corrections and apply them to the most recent and precise data set and world data from electron-proton scattering. Using this, we show how the form factor extraction from these data is influenced by the subsequent inclusion of physical constraints. The determination of the proton charge radius from scattering data is shown to be dominated by the enforcement of a realistic spectral function. Additionally, the third Zemach moment from the resulting form factors is calculated. The obtained radius and Zemach moment are shown to be consistent with Lamb shift measurements in muonic hydrogen.Comment: minor changes, added references, version to appear in PR

    The Suppression and Recovery of the Ferroelectric Phase in Multiferroic MnWO4MnWO_4

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    We report the discovery of a complete suppression of ferroelectricity in MnWO4MnWO_4 by 10 % iron substitution and its restoration in external magnetic fields. The spontaneous polarization in Mn0.9Fe0.1WO4Mn_{0.9}Fe_{0.1}WO_4 arises below 12 K in external fields above 4 T. The magnetic/ferroelectric phase diagram is constructed from the anomalies of the dielectric constant, polarization, magnetization, and heat capacity. The observations are qualitatively described by a mean field model with competing interactions and strong anisotropy. We propose that the magnetic field induces a non-collinear inversion symmetry breaking magnetic structure in Mn0.9Fe0.1WO4Mn_{0.9}Fe_{0.1}WO_4
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