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Université de Fribourg

Adaptivity and ‘Per learning’

Wakeling, Joseph Rushton

One of the key points addressed by Per Bak in his models of brain function was that biological neural systems must be able not just to learn, but also to adapt—to quickly change their behaviour in response to a changing environment. I discuss this in the context of various simple learning rules and adaptive problems, centred around the Chialvo-Bak ‘minibrain’ model (Neurosci. 90 (1999)...

Université de Fribourg

Laser-based precision magnetometry in fundamental and applied research

Weis, Antoine ; Wynands, Robert

In: Optics and Lasers in Engineering, 2005, vol. 43, p. 387-401

In this paper we review research done by the recently founded Fribourg group for Atomic Physics (FRAP) using laser-assisted spin coherence spectroscopy of atomic samples in applied and fundamental physics. Spin coherence in optically pumped ensembles of paramagnetic atoms may be extremely long-lived when the samples are well shielded from external perturbations, like static or oscillating...

Université de Fribourg

Muon capture by ³He nuclei followed by proton and deuteron production

Bystritsky, V. M. ; Boreiko, V. F. ; Filipowicz, M. ; Gerasimov, V. V. ; Huot, Olivier ; Knowles, Paul E. ; Mulhauser, Françoise ; Pavlov, V. N. ; Schaller, Lukas A. ; Schneuwly, Hubert ; Sandukovsky, V. G. ; Stolupin, V. A. ; Volnykh, V. P. ; Woźniak, J.

In: Physical Review A, 2004, vol. 69, p. 012712-12727

The paper describes an experiment aimed at studying muon capture by He nuclei in pure ³He and D₂+³He mixtures at various densities. Energy distributions of protons and deuterons produced via µ₋+³He-->p + n + n + νµ and µ₋+³He-->d + n + νµ are measured for the energy intervals 10–49 MeV and 13–31 MeV, respectively. Muon capture rates lambdacapp(ΔEp) and...

Université de Fribourg

CD81-induced behavioural changes during chronic cocaine administration: in vivo gene delivery with regulatable lentivirus

Bahi, Amine ; Boyer, Frederic ; Kafri, Tal ; Dreyer, Jean-Luc

In: European Journal of Neuroscience, 2004, vol. 19, p. 1621-1633

CD81, a tetraspanin transmembrane protein involved in cell adhesion, is up-regulated in the mesolimbic dopaminergic pathway 24 h following acute administration of high doses of cocaine [Brenz-Verca et al., (2001) Mol. Cell. Neurosci., 17, 303-316]. Further evidence consecutive with this observation and based on microarray analysis are presented here. In addition, a regulatable lentivirus was...

Université de Fribourg

Planar LAAPDs: temperature dependence, performance, and application in low-energy X-ray spectroscopy

Ludhova, Livia ; Amaro, F. D. ; Antognini, A. ; Biraben, F. ; Cardoso, J. M. R. ; Conde, C. A. N. ; Covita, D. S. ; Dax, A. ; Dhawan, S. ; Fernandes, L. M. P. ; Hänsch, T. W. ; Hughes, V.-W. ; Huot, Olivier ; Indelicato, P. ; Julien, L. ; Knowles, Paul E. ; Kottmann, F. ; Lopes, J. A. M. ; Liu, Y.-W. ; Monteiro, C. M. B. ; Mulhauser, Françoise ; Nez, F. ; Pohl, R. ; Rabinowitz, P. ; Santos, J. M. F. dos ; Schaller, Lukas A. ; Taqqu, D. ; Veloso, J. F. C. A.

In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2005, vol. 540, p. 169-179

An experiment measuring the 2S Lamb shift in muonic hydrogen (μ-p) was performed at the Paul Scherrer Institute, Switzerland. It required small and compact detectors for 1.9 keV X-rays (2P–1S transition) with an energy resolution around 25% at 2 keV, a time resolution better than 100 ns, a large solid angle coverage, and insensitivity to a 5 T magnetic field. We chose Large Area Avalanche...

Université de Fribourg

Shedding light on El Farol

Challet, Damien ; Marsili, M. ; Ottino, Gabriele

In: Physica A: Statistical Mechanics and its Applications, 2004, vol. 332, p. 469-480

We mathematize El Farol bar problem and transform it into a workable model. We find general conditions on the predictor space under which the convergence of the average attendance to the resource level does not require any intelligence on the side of the agents. Secondly, specializing to a particular ensemble of continuous strategies yields a model similar to the Minority Game. Statistical...

Université de Fribourg

Magnon dispersion in field-induced magnetically ordered phases

Matsumoto, Masashige ; Normand, Bruce ; Rice, T. M. ; Sigrist, Manfred

In: Journal of Magnetism and Magnetic Materials, 2004, vol. 272-276, p. 267-268

The magnon dispersion in the field-induced and pressure-induced antiferromagnetically (AF) ordered phase in TlCuCl₃ are studied. In the pressure-induced AF phase, there are two transverse phase (Goldstone) modes and one gapped longitudinal amplitude mode. A magnetic field lifts the degeneracy, and it gives rise to three separated magnon branches in the field-induced AF phase, where the lowest...

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...