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Université de Fribourg

Phase diagram of the Heisenberg spin ladder with ring exchange

Gritsev, Vladimir ; Normand, Bruce ; Baeriswyl, Dionys

In: Physical Review B, 2004, vol. 69, p. 094431-94446

We investigate the phase diagram of a generalized spin-1/2 quantum antiferromagnet on a ladder with rung, leg, diagonal, and ring-exchange interactions. We consider the exactly soluble models associated with the problem, obtain the exact ground states which exist for certain parameter regimes, and apply a variety of perturbative techniques in the regime of strong ring-exchange coupling. By...

Université de Fribourg

Exactly soluble isotropic spin-½ ladder models

Gritsev, Vladimir ; Baeriswyl, Dionys

In: Journal of Physics A: Mathematical and General, 2003, vol. 36, p. 12129-12142

The undeformed limit of the dilute two-colour braid–monoid algebra gives a natural basis for the description of spin-½ ladder models, and allows different Baxterization ansätze. Based on this observation we find the complete classification of exactly soluble generalized isotropic spin-½ ladder models.

Université de Fribourg

Photonic properties of strongly correlated colloidal liquids

Rojas-Ochoa, Luis Fernando ; Mendez-Alcaraz, J. M. ; Sáenz, J. J. ; Schurtenberger, Peter ; Scheffold, Frank

In: Physical Review Letters, 2004, vol. 93, p. 073903-73906

The optical and structural properties of dense colloidal suspensions in the presence of long-range electrostatic repulsion are determined from both light and small-angle neutron scattering experiments. Short-range structural order induces an enhancement of the scattering strength while at the same time the total transmission shows strong wavelength dependence, reminiscent of a photonic crystal....

Université de Fribourg

Scattering functions of semidilute solutions of polymers in a good solvent

Pedersen, Jan Skov ; Schurtenberger, Peter

In: Journal of Polymer Science Part B: Polymer Physics, 2004, vol. 42, p. 3081-3094

Expressions for analyzing small-angle scattering data from semidilute solutions of polymers in a good solvent over a broad range of scattering vectors are examined. Three different scattering function expressions are derived from Monte Carlo simulations. The expressions are similar to those of polymer reference interaction site models, with a scattering-vector-dependent direct correlation...

Université de Fribourg

Optical imaging of the spatiotemporal dynamics of cerebral blood flow and oxidative metabolism in the rat barrel cortex

Weber, B. ; Burger, C. ; Wyss, M. T. ; Schulthess, G. K. von ; Scheffold, Frank ; Buck, Alfred

In: European Journal of Neuroscience, 2004, vol. 20, p. 2664-2670

Oxidative metabolism and cerebral blood flow (CBF) are two of the most important measures in neuroimaging. However, results from concurrent imaging of the two with high spatial and temporal resolution have never been published. We used flavoprotein autofluorescence (AF) and laser speckle imaging (LSI) in the anaesthetized rat to map oxidative metabolism and CBF in response to single vibrissa...

Université de Fribourg

Heterogeneous dynamics of coarsening systems

Mayer, P. ; Bissig, Hugo ; Berthier, L. ; Cipelletti, Luca ; Garrahan, J. P. ; Sollich, P. ; Trappe, Véronique

In: Physical Review Letters, 2004, vol. 93, p. 11570-11573

We show by means of experiments, theory, and simulations that the slow dynamics of coarsening systems displays dynamic heterogeneity similar to that observed in glass-forming systems. We measure dynamic heterogeneity via novel multipoint functions which quantify the emergence of dynamic, as opposed to static, correlations of fluctuations. Experiments are performed on a coarsening foam using...

Université de Fribourg

Burckhardtsche Bestimmung der Raumgruppen II

Strebel, Ralph

In: Elemente der Mathematik, 2004, vol. 59, p. 1-18

Université de Fribourg

Burckhardtsche Bestimmung der Raumgruppen I

Strebel, Ralph

In: Elemente der Mathematik, 2003, vol. 58, p. 141-155

Université de Fribourg

Depolarization of backscattered linear lypolarized ligh

Rojas-Ochoa, Luis Fernando ; Lacoste, David ; Lenke, Ralf ; Schurtenberger, Peter ; Scheffold, Frank

In: Journal of the Optical Society of America: A, 2004, vol. 21, p. 1799-1804

We formulate a quantitative description of backscattered linearly polarized light with an extended photon diffusion formalism taking explicitly into account the scattering anisotropy parameter g of the medium. From diffusing wave spectroscopy measurements, the characteristic depolarization length for linearly polarized light, lp, is deduced. We investigate the dependence of this length on the...

Université de Fribourg

Colloidal gels—low-density disordered solid-like states

Trappe, Veronique ; Sandkühler, Peter

In: Current Opinion in Colloid & Interface Science, 2004, vol. 8, p. 494-500

The analogies between colloidal glasses and gels have stimulated an increased effort in recent years to unify the description of the transitions to these disordered solid-like states within a single conceptual framework. Mode coupling theory, which successfully describes the hard sphere colloidal glass transition, has been extended to describe the effect of weak attractions at large volume...