Directional magnetoelectric effects in MnWO4: magnetic sources of the electric polarization.

The ferroelectric order and magnetic field induced effects observed in the spiral phase of MnWO4 are described theoretically. It is demonstrated explicitly that the Dzyaloshinskii-Moriya antisymmetric interactions contribute to the correlation between spins and electric dipoles in the incommensurate and commensurate ferroelectric phases of magnetic multiferroics. However, other single-site symmetric interactions are shown to be involved in the magnetoelectric process, suggesting the possible existence of an electric polarization originating from purely symmetric effects.

Direct observation of local distortion of a crystal lattice with picometer accuracy using atomic resolution neutron holography.

We demonstrate that neutron holography permits us to extend the determination of atomic positions beyond nearest neighbors at least up to the fourth neighboring shell around cadmium probe atoms alloyed into a lead crystal. The accuracy achieved is sufficient to allow quantitative determination of displacements of atoms due to elastic distortions induced by impurity atoms. The atomic positions derived from the holographic data are in good agreement with those expected theoretically due to Friedel oscillations in this system. In addition, the atomic positions are in qualitative agreement with results obtained in an independent experiment studying the diffuse distortion scattering around Bragg peaks.

Nanocluster formation in electron-irradiated Li2O crystals observed by elastic diffuse neutron scattering.

Previous work performed on electron-irradiated Li2O crystals has demonstrated the simultaneous formation of two populations of colloids of metallic lithium, one is associated with oxygen bubbles and a typical size of >10 microm, while the other one consists of nanoclusters in the range of <10 nm. In the present neutron scattering investigation these small colloids are characterized in detail based on a thorough analysis of distortion scattering around Bragg peaks. It is shown that the small lithium colloids are slightly elongated precipitates with typical dimensions of around approximately 5 nm. For the large lithium colloids a well-defined orientation relation with respect to the Li2O matrix has been determined.

Holographic imaging of atoms using thermal neutrons.

In a recent paper [L. Cser, G. Krexner, and Gy. Török, Europhys. Lett. 54, 747 (2001)]] the use of thermal neutrons with wavelengths close to interatomic distances in condensed matter was proposed to obtain holographic images on an atomic scale. Two experimental methods were considered which either put the radiation source inside and the detector outside the object or vice versa. The second approach, called the inside-detector concept, requires strongly neutron-absorbing isotopes acting as pointlike detectors in the sample. In the present work, we demonstrate the feasibility of this technique by recording a holographic image of a lead nuclei in a Pb(Cd) single crystal.

Suppression of the order parameter correlation length by inhomogeneous strains

Very recently we presented puzzling results of diffuse neutron scattering experiments on KSCN and RbSCN. The data yield an increase of the diffuse intensity with increasing temperature below T(c), whereas the width remains constant. Using molecular dynamics and 3D Monte Carlo simulations, we have shown that below T(c) the width of the correlation functions can be stabilized by strain fields originating from the order parameter strain interactions. Here we construct a novel analytic model which predicts the existence of a second characteristic length scale and explains the suppression of the growth of precursor clusters by the influence of inhomogeneous strain fields.