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Université de Fribourg

Circulating-current states and ring-exchange interactions in cuprates

Normand, Bruce ; Oleś, Andrzej M.

In: Physica C: Superconductivity, 2004, vol. 408-410, p. 252-253

We consider circulating charge- and spin-current states in a generalised Hubbard model for the CuO₂ planes in cuprates. We investigate the parameter regimes for these phases in microscopic calculations on small clusters. We demonstrate that a positive ring-exchange interaction enhances (suppresses) spin- (charge-)flux phases.

Université de Fribourg

Kosmotropes and chaotropes: modelling preferential exclusion, binding and aggregate stability

Moelbert, Susanne ; Normand, Bruce ; De Los Rios, Paolo

In: Biophysical Chemistry, 2004, vol. 112, p. 45-57

Kosmotropic cosolvents added to an aqueous solution promote the aggregation of hydrophobic solute particles, while chaotropic cosolvents act to destabilise such aggregates. We discuss the mechanism for these phenomena within an adapted version of the two-state Muller–Lee–Graziano model for water, which provides a complete description of the ternary water/cosolvent/solute system for small...

Université de Fribourg

Exchange constants and spin dynamics in Mn₁₂-acetate

Honecker, A. ; Fukushima, N. ; Normand, Bruce ; Chaboussant, G. ; Güdel, Hans-Ulrich

In: Journal of Magnetism and Magnetic Materials, 2005, vol. 290-291, p. 966-969

We have obtained new inelastic neutron scattering (INS) data for the molecular magnet Mn₁₂-acetate which exhibit at least six magnetic peaks in the energy range 5–35 meV. These are compared with a microscopic Heisenberg model for the 12 quantum spins localised on the Mn ions, coupled by four inequivalent magnetic exchange constants. A fit to the magnetic susceptibility under the constraint...

Université de Fribourg

Structure and electronic structure of quasicrystal and approximant surfaces: a photoemission study

Naumović, Dusanka

In: Progress in Surface Science, 2004, vol. 75, p. 205-225

The structure and electronic structure of different high-symmetry surfaces of either quasicrystalline or approximant Al–Pd–Mn were studied by means of photoemission-based techniques such as X-ray photoelectron diffraction (XPD) and ultraviolet photoelectron spectroscopy. We find that the twofold (2f), 3f and 5f surfaces of icosahedral Al–Pd–Mn exhibit all the symmetry elements of the...

Université de Fribourg

Kaonic nitrogen X-ray transition yields in a gaseous target

Ishiwatari, T. ; Beer, G. ; Bragadireanu, A. M. ; Cargnelli, M. ; Curceanu-Petrascu, C. ; Egger, J.-P. ; Fuhrmann, H. ; Guaraldo, C. ; Kienle, P. ; Koike, T. ; Iliescu, M. ; Itahashi, K. ; Iwasaki, M. ; Lauss, B. ; Lucherini, V. ; Ludhova, Livia ; Marton, J. ; Mulhauser, Françoise ; Ponta, T. ; Schaller, Lukas A. ; Seki, R. ; Sirghi, D. L. ; Sirghi, F. ; Strasser, P. ; Zmeskal, J.

In: Physics Letters B, 2004, vol. 593, p. 48-54

The first measurement of the yields of three kaonic nitrogen X-ray transitions, using the DEAR (DAΦNE Exotic Atom Research) setup at the DAΦNE collider of Frascati, is reported. The yields are 41.5±8.7(stat.)±4.1(sys.)% for the n=7→6 transition, 55.0±3.9(stat.)±5.5(sys.)% for the n=6→5 transition and 57.4±15.2(stat.)±5.7(sys.)% for the n=5→4 transition at a density ρ=3.4ρNTP. By...

Université de Fribourg

Optical response for a discrete stripe

Benfatto, Larra ; Morais Smith, Cristiane

In: Physica C: Superconductivity, 2004, vol. 408-410, p. 453-454

The optical response of a charge stripe in the presence of pinning impurities is investigated. We address the issue of a discrete description of the stripe, and discuss its quantitative relevance with respect to a continuum one in the light of recent optical-conductivity measurements in cuprate compounds.

Université de Fribourg

Hydrogen density in nanostructured carbon, metals and complex materials

Züttel, Andreas ; Wenger, Pascal ; Sudan, Patrick ; Mauron, Philippe ; Orimo, Shin-ichi

In: Materials Science and Engineering B, 2004, vol. 108, p. 9-18

The challenge in the research on hydrogen storage materials is to pack hydrogen atoms or molecules as close as possible. Hydrogen absorbed in metals can reach a density of more than 150 kg m⁻³ (e.g. Mg₂FeH₆ or Al(BH₄)₃) at atmospheric pressure. For metallic hydrides, however, due to the large atomic mass of the transition metals the gravimetric hydrogen density is limited to less than...

Université de Fribourg

Double-K-shell ionization of Mg and Si induced in collisions with C and Ne ions

Kobal, M. ; Kavčič, M. ; Budnar, M. ; Dousse, Jean-Claude ; Maillard, Yves-Patrick ; Mauron, Olivier ; Raboud, Pierre-Alexandre ; Tökési, K.

In: Physical Review A, 2004, vol. 70, p. 062720-62730

The satellite and hypersatellite K x-ray emission of a thin Mg foil and thick polycrystalline Si target bombarded by 34-MeV C and 50-MeV Ne ions was measured using high-resolution crystal diffractometry. The corresponding projectile reduced velocities v/vK were 1.09 and 0.92 for C ions and 1.02, 0.86 for Ne ions in case of Mg and Si targets, respectively. An energy resolution of approximately 0.5...

Université de Fribourg

High-resolution XES and RIXS studies with a von Hamos Bragg crystal spectrometer

Hoszowska, J. ; Dousse, Jean-Claude

In: Journal of Electron Spectroscopy and Related Phenomena, 2004, vol. 137-140, p. 687-690

The high-resolution von Hamos Bragg crystal spectrometer was constructed for the study of K X-ray emission from low-Z elements and L and M X-ray spectra of medium to high Z elements. Recently, this instrument was applied to high-resolution XES and RIXS studies using X-ray synchrotron radiation at the ID21 and BM5 beamlines at the ESRF. An outline of the spectrometer design and performance...

Université de Fribourg

The cold neutron tomography set-up at SINQ

Materna, T. ; Baechler, Sébastien ; Jolie, J. ; Masschaele, B. ; Dierick, M. ; Kardjilov, N.

In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2004, vol. 525, p. 69-73

The cold neutron tomography station operated at SINQ (Paul Scherrer Institute, Switzerland) is reviewed. The high neutron flux together with a set-up based on a scintillator screen and a CCD camera yielded fast and effective results with resolution down to 250 μm: tomography of small samples (up to 2 cm large) could be performed in less than an hour. The use of a velocity selector improved the...