Spin Liquid Phase in the Pyrochlore Antiferromagnet

Correlation functions (CFs) of the classical Heisenberg antiferromagnet on the pyrochlore lattice are studied by solving exactly the infinite-component spin-vector model. % As in many Fully Frustrated Lattices, the constraint due to the minimization of the energy and the particular structure based on corner sharing tetrahedra both contribute to the creation of local degrees of freedom. % The resulting degeneracy destroys any magnetic order at all temperature and we obtain no sign of criticality, even at $T = 0$. % Calculated neutron scattering cross sections have their maxima beyond the first Brillouin Zone and reproduce experimental results obtained on Y(Sc)Mn$_2$ and CsCrNiF$_6$ as well as theoretical predictions previously obtained by classical Monte Carlo simulations. % Evidences for thermal and spatial decoupling of the magnetic modes are found so that the magnetic fluctuations in this system can be approximated by $S(q,T) \approx f(q) h(T)$.Comment: 6 pages (revtex two columns), 7 figures. Submitted to Canadian Journal of Physics for the Proceedings of the Higly Frustrated Magnetism 2000 Conference, Waterloo, Ontario, Canada, June 11-15, 200

Heterogeneous freezing in a geometrically frustrated spin model without disorder: spontaneous generation of two time-scales

By considering the constrained motion of classical spins in a geometrically frustrated magnet, we find a dynamical freezing temperature below which the system gets trapped in metastable states with a "frozen" moment and dynamical heterogeneities. The residual collective degrees of freedom are strongly correlated, and by spontaneously forming aggregates, they are unable to reorganize the system. The phase space is then fragmented in a macroscopic number of disconnected sectors (broken ergodicity), resulting in self-induced disorder and "thermodynamic" anomalies, measured by the loss of a finite configurational entropy. We discuss these results in the view of experimental results on the kagome compounds, SrCr(9p)Ga(12-9p)O19, (H30)Fe3(SO4)2(OH)6, Cu3V2O7(OH)2.2H2O and Cu3BaV2O8(OH)2.Comment: 17 pages, 14 fi

Classical Spin Liquid Properties of the Infinite-Component Spin Vector Model on a Fully Frustrated Two Dimensional Lattice

Thermodynamic quantities and correlation functions (CFs) of the classical antiferromagnet on the checkerboard lattice are studied for the exactly solvable infinite-component spin-vector model, $D \to \infty$. In contrast to conventional two-dimensional magnets with continuous symmetry showing extended short-range order at distances smaller than the correlation length, $r \lesssim \xi_c \propto \exp(T^*/T)$, correlations in the checkerboard-lattice model decay already at the scale of the lattice spacing due to the strong degeneracy of the ground state characterized by a macroscopic number of strongly fluctuating local degrees of freedom. At low temperatures, spin CFs decay as $\propto 1/r^2$ in the range $a_0 \ll r \ll \xi_c \propto T^{-1/2}$, where $a_0$ is the lattice spacing. Analytical results for the principal thermodynamic quantities in our model are very similar with MC simulations, exact and analytical results for the classical Heisenberg model (D=3) on the pyrochlore lattice. This shows that the ground state of the infinite-component spin vector model on the checkerboard lattice is a classical spin liquid.Comment: 9 pages (epj-style, two columns), 7 figures. Version to be published in EPJ

Classical Spin Liquid: Exact Solution for the Infinite-Component Antiferromagnetic Model on the Kagom\'e Lattice

Thermodynamic quantities and correlation functions (CFs) of the classical antiferromagnet on the kagom\'e lattice are studied for the exactly solvable infinite-component spin-vector model, D \to \infty. In this limit, the critical coupling of fluctuations dies out and the critical behavior simplifies, but the effect of would be Goldstone modes preventing ordering at any nonzero temperature is properly accounted for. In contrast to conventional two-dimensional magnets with continuous symmetry showing extended short-range order at distances smaller than the correlation length, r < \xi_c \propto \exp(T^*/T), correlations in the kagom\'e-lattice model decay already at the scale of the lattice spacing due to the strong degeneracy of the ground state characterized by a macroscopic number of strongly fluctuating local degrees of freedom. At low temperatures, spin CFs decay as \propto 1/r^2 in the range a_0 << r << \xi_c \propto T^{-1/2}, where a_0 is the lattice spacing. Analytical results for the principal thermodynamic quantities in our model are in fairly good quantitative agreement with the MC simulations for the classical Heisenberg model, D=3. The neutron scattering cross section has its maxima beyond the first Brillouin zone; at T\to 0 it becomes nonanalytic but does not diverge at any q.Comment: 14 PR pages, 10 figures; Phys. Rev. B; Version 3: final published versio

Evaluation of a pilot program that integrated generic and specific skills on engineering degree: a case study in Catalonia

The ETSEIAT recently executed pilot programs to introduce generic and specific skills into its study plans. As these pilots are now concluding, an evaluation of their efficiency has been conducted. This paper analyses the answers given by professors who were interviewed (via in-person interviews and online tests) to determine how they developed and evaluated their students’ skills. The results of these interviews offer clear data about the progress obtained by the pilot programs, how the professors understood the recently added dynamics and tools, and how the new skills are integrated into the various subjects and coursesPeer ReviewedPostprint (published version

Jean Rouch, un antropòleg de les fronteres

Jean Rouch (1917-2004) és una referència ineludible en la història del cinema etnogràfic. Especialista en els rituals de possessió a l'Àfrica de l'Oest i clarament influenciat pel cinema de Flaherty i de Vertov, Rouch va desenvolpuar un mètode i una teoria cinematogràfica que s'oposaven frontalment als principis del positivisme científic així com a les teories objectivistes del cinema etnogràfic. Més concretament, Rouch va posar en pràctica durant el seu treball de camp una 'antropologia compartida', basada en una concepció no jerarquitzada de les relacions entre l'antropòleg i la comunitat estudiada, i va situar la idea de 'reflexivitat' com a eix principal del coneixement científic i del cinema etnogràfic. Crític amb la clàssica distinció entre art i ciència, el director francès sempre va reivindicar la creativitat, l'experimentació i la llibertat d'estil com a punts essencials de la recerca etnogràfica

Representaciones fílmicas del Caribe: del exotismo a la auto-representación

El objetivo de este artículo es analizar un conjunto de representaciones fílmicas sobre el Caribe, desde el cine clásico norteamericano hasta el documental etnográfico contemporáneo. La idea principal del texto es que existe una evolución en las imágenes del Caribe que van desde el exotismo propio de la primera mitad del siglo xx hasta la situación actual, marcada por un conjunto de producciones heterogéneas que, diversificando la autoría audiovisual, aportan nuevas miradas y discursos sobre la realidad social y cultural de la región

Ising Like Order by Disorder In The Pyrochlore Antiferromagnet with Dzyaloshinskii-Moriya Interactions

It is shown that the mechanism of order out of disorder is at work in the antisymmetric pyrochlore antiferromagnet. Quantum as well as thermal fluctuations break the continuous degeneracy of the classical ground state manifold and reduce its symmetry to $\mathbb{Z}_3 \times \mathbb{Z}_2$. The role of anisotropic symmetric exchange is also investigated and we conclude that this discrete like ordering is robust with respect to these second order like interactions. The antisymmetric pyrochlore antiferromagnet is therefore expected to order at low temperatures, whatever the symmetry type of its interactions, in both the classical and semi classical limits.Comment: 6 pages. 9 figure

Neural Information Processing: between synchrony and chaos

The brain is characterized by performing many different processing tasks ranging from elaborate processes such as pattern recognition, memory or decision-making to more simple functionalities such as linear filtering in image processing. Understanding the mechanisms by which the brain is able to produce such a different range of cortical operations remains a fundamental problem in neuroscience. Some recent empirical and theoretical results support the notion that the brain is naturally poised between ordered and chaotic states. As the largest number of metastable states exists at a point near the transition, the brain therefore has access to a larger repertoire of behaviours. Consequently, it is of high interest to know which type of processing can be associated with both ordered and disordered states. Here we show an explanation of which processes are related to chaotic and synchronized states based on the study of in-silico implementation of biologically plausible neural systems. The measurements obtained reveal that synchronized cells (that can be understood as ordered states of the brain) are related to non-linear computations, while uncorrelated neural ensembles are excellent information transmission systems that are able to implement linear transformations (as the realization of convolution products) and to parallelize neural processes. From these results we propose a plausible meaning for Hebbian and non-Hebbian learning rules as those biophysical mechanisms by which the brain creates ordered or chaotic ensembles depending on the desired functionality. The measurements that we obtain from the hardware implementation of different neural systems endorse the fact that the brain is working with two different states, ordered and chaotic, with complementary functionalities that imply non-linear processing (synchronized states) and information transmission and convolution (chaotic states)

Ground-State Candidate for the Dipolar Kagome Ising Antiferromagnet

We have investigated the low-temperature thermodynamic properties of the dipolar kagome Ising antiferromagnet using at-equilibrium Monte Carlo simulations, in the quest for the ground-state manifold. In spite of the limitations of a single spin-flip approach, we managed to identify certain ordering patterns in the low-temperature regime and we propose a candidate for this unknown state. This novel configuration presents some intriguing features and passes several test-criteria, making it a very likely choice for the dipolar long-range order of this kagome Ising antiferromagnet.Comment: 8 pages, 5 figure