Jacob Larsen

CAMEA ESS: The Continuous Angle Multi-Energy Analysis Indirect Geometry Spectrometer for the European Spallation Source

The CAMEA ESS neutron spectrometer is designed to achieve a high detection efficiency in the horizontal scattering plane, and to maximize the use of the long pulse European Spallation Source. It is an indirect geometry time-of-flight spectrometer that uses crystal analysers to determine the final energy of neutrons scattered from the sample. Unlike other indirect gemeotry spectrometers CAMEA will use ten concentric arcs of analysers to analyse scattered neutrons at ten different final energies, which can be increased to 30 final energies by use of prismatic analysis. In this report we will outline the CAMEA instrument concept, the large performance gain, and the potential scientific advancements that can be made with this instrument.

Measurement of unique magnetic and superconducting phases in oxygen-doped high-temperature superconductors La(2-x)Sr(x)CuO(4+y).

L. Udby, J. Larsen, N. B. Christensen, M. Boehm, Ch. Niedermayer, H. E. Mohottala, T. B. S. Jensen, R. Toft-Petersen, 6 F. C. Chou, N. H. Andersen, K. Lefmann, and B. O. Wells Nanoscience Center, Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark Department of Physics, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark Institut Laue-Langevin, 38042 Grenoble Cedex 9, France Laboratory for Neutron Scattering, Paul Scherrer Institut, CH-5232 VIlligen PSI, Switzerland University of Hartford, West Hartford, Connecticut 06117, USA Helmholtz Zentrum Berlin für Materialien und Energie, D-14109 Berlin, Germany Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan Department of Physics, University of Connecticut, Storrs, Connecticut 06269-3046, USA

Magnetic ground state and magnon-phonon interaction in multiferroic h−YMnO3

Inelastic neutron scattering has been used to study the magnetoelastic excitations in the multiferroic manganite hexagonal YMnO3. An avoided crossing is found between magnon and phonon modes close to the Brillouin zone boundary in the (a,b) plane. Neutron polarization analysis reveals that this mode has mixed magnon-phonon character. An external magnetic field along the c axis is observed to cause a linear field-induced splitting of one of the spin-wave branches. A theoretical description is performed, using a Heisenberg model of localized spins, acoustic phonon modes, and a magnetoelastic coupling via the single-ion magnetostriction. The model quantitatively reproduces the dispersion and intensities of all modes in the full Brillouin zone, describes the observed magnon-phonon hybridized modes, and quantifies the magnetoelastic coupling. The combined information, including the field-induced magnon splitting, allows us to exclude several of the earlier proposed models and point to the correct magnetic ground state symmetry, and provides an effective dynamic model relevant for the multiferroic hexagonal manganites.

Non-negative Matrix Factorization for binary data

We propose the Logistic Non-negative Matrix Factorization for decomposition of binary data. Binary data are frequently generated in e.g. text analysis, sensory data, market basket data etc. A common method for analysing non-negative data is the Non-negative Matrix Factorization, though this is in theory not appropriate for binary data, and thus we propose a novel Non-negative Matrix Factorization based on the logistic link function. Furthermore we generalize the method to handle missing data. The formulation of the method is compared to a previously proposed logistic matrix factorization without non-negativity constraint on the features. We compare the performance of the Logistic Non-negative Matrix Factorization to Least Squares Non-negative Matrix Factorization and Kullback-Leibler (KL) Non-negative Matrix Factorization on sets of binary data: a synthetic dataset, a set of student comments on their professors collected in a binary term-document matrix and a sensory dataset. We find that choosing the number of components is an essential part in the modelling and interpretation, that is still unresolved.

Shoaling of sixth-order Stokes waves on a current

Jonsson and Arneborg (Jonsson, I.G., Arneborg, L., 1995. Energy properties and shoaling of higher-order Stokes waves on a current. Ocean Engng 22, 819-857.) combined energy flux and set-down to make shoaling predictions for fourth-order Stokes waves with and without a net volume flux. With basis in their expressions, sixth-order expressions are derived and combined to make shoaling predictions correct to sixth order with an arbitrary net volume flux. The new sixth-order results are compared with the fourth-order results and the practically exact results obtained by Sobey and Bando (Sobey R.J., Bando K., 1991. Variations on higher-order shoaling. J. Waterway, Port, Coastal Ocean Engng ASCE 117, 348-368) with a Fourier 18 model. The effects of introducing sixth-order theory rather than the fourth-order theory in shoaling calculations are in general found to be small. As expected the deviations increase with increasing wave-steepness, decreasing depth and opposing currents. Also as expected, the results obtained with the sixth-order expressions improve the results obtained with the fourth-order expressions when compared to the results of Sobey and Bando. As novelties, some considerations regarding the consistency of odd- and even-order shoaling calculations, and the magnitude of the bottom slope, are presented. Furthermore a comparison between the wave-induced current and the total current is given.