Steady state fluctuation relations for systems driven by an external random force

We experimentally study the fluctuations of the work done by an external Gaussian random force on two different stochastic systems coupled to a thermal bath: a colloidal particle in an optical trap and an atomic force microscopy cantilever. We determine the corresponding probability density functions for different random forcing amplitudes ranging from a small fraction to several times the amplitude of the thermal noise. In both systems for sufficiently weak forcing amplitudes the work fluctuations satisfy the usual steady state fluctuation theorem. As the forcing amplitude drives the system far from equilibrium, deviations of the fluctuation theorem increase monotonically. The deviations can be recasted to a single master curve which only depends on the kind of stochastic external force.Comment: 6 pages, submitted to EP

Thermal noise properties of two aging materials

In this lecture we review several aspects of the thermal noise properties in two aging materials: a polymer and a colloidal glass. The measurements have been performed after a quench for the polymer and during the transition from a fluid-like to a solid-like state for the gel. Two kind of noise has been measured: the electrical noise and the mechanical noise. For both materials we have observed that the electric noise is characterized by a strong intermittency, which induces a large violation of the Fluctuation Dissipation Theorem (FDT) during the aging time, and may persist for several hours at low frequency. The statistics of these intermittent signals and their dependance on the quench speed for the polymer or on sample concentration for the gel are studied. The results are in a qualitative agreement with recent models of aging, that predict an intermittent dynamics. For the mechanical noise the results are unclear. In the polymer the mechanical thermal noise is still intermittent whereas for the gel the violation of FDT, if it exists, is extremely small.Comment: to be published in the Proceedings of the XIX Sitges Conference on ''Jammming, Yielding and Irreversible Deformation in Condensed Matter'', M.-C.Miguel and M. Rubi eds.,Springer Verlag, Berli

Single beam interferometric angle measurement

We present an application of a quadrature phase interferometer to the measurement of the angular position of a parallel laser beam with interferometric precision. In our experimental realization we reach a resolution of 6.8e-10 rad (1.4e-4 arcsec) for 1 kHz bandwidth in a 2e-2 rad (1 deg) range. This alternative to the optical lever technique features absolute calibration, independence of the sensitivity on the thermal drifts, and wide range of measurement at full accuracy

Out-of-equilibrium dynamics in a gaussian trap model

The violations of the fluctuation-dissipation theorem are analyzed for a trap model with a gausssian density of states. In this model, the system reaches thermal equilibrium for long times after a quench to any finite temperature and therefore all aging effect are of a transient nature. For not too long times after the quench it is found that the so-called fluctuation-dissipation ratio tends to a non-trivial limit, thus inicating the possibility for the definition of a time scale dependent effective temperature. However, different definitions of the effective temperature yield distinct results. In particular plots of the integrated response versus the correlation function strongly depend on the way they are constructed. Also the definition of effective temperatures in the frequency domain is not unique for the model considered. This may have some implications for the interpretation of results from computer simulations and experimental determinations of effective temperatures.Comment: Proceedings of the workshop on non-equilibrium phenomena in supercooled fluids, glasses and amorphous materials (17-22 September, Pisa

Relationships between social forms of organic horticultural production and indicators of environmental quality: a multidimensional approach in Brazil.

Organic farming (OF) is increasingly considered as a possible alternative for designing a "new rural" in Brazil, where OF covers a wide range of production and certification systems. However, the ways small farmers adopt OF in green belts to meet an urban demand in organic vegetables have not been extensively investigated. Likewise, the impact of such practices on environmental quality components has not been sufficiently documented. Our objective was to relate forms of organisation to environmental assessment in a watershed where organic horticulture significantly contributes to landscape and water quality. We showed how small farmers were organised or how they organised themselves to meet urban demands and develop OF. We assumed that associated practices were consistent with environmental impacts, as evaluated by indicators. Based on interviews with stakeholders, we identified four forms of organisation and associated farmers' practices. We related them to environmental assessment in three compartments: landscape ecology, water quality and soil quality. Although organisations share some objectives, namely with regard to visual quality and the "right price" of products, differences appear in their scope and internal operation, their values and relationships with consumers, and their technical and environmental contents. As for technical content, input supply, planning processes and crop diversity vary among organisations, ranging from liberal to hierarchical. Our results also showed similarities and differences among various organisations in terms of environmental impact. Such results are interpreted and discussed in the light of technical and social dimensions that account for the progressive design of new systems in Brazil

Computer Vision and Graphics for Heritage Preservation and Digital Archaeology

The goal of this work is to provide attendees with a survey of topics related to Heritage Preservation and Digital Archeology, which are challenging and motivating subjects to both computer vision and graphics community. These issues have been gaining increasing attention and priority within the scientific scenario and among funding agencies and development organizations over the last years. Motivations to this work are the recent efforts in the digital preservation of cultural heritage objects and sites before degradation or damage caused by environmental factors or human development. One of the main focuses of these researches is the development of new techniques for realistic 3D model building from images, preserving as much information as possible. We intend to introduce and discuss several emerging topics in computer vision and graphics related to the proposed theme while highlighting the major contributions and advances in these fields

On the Symmetries of Integrability

We show that the Yang-Baxter equations for two dimensional models admit as a group of symmetry the infinite discrete group $A_2^{(1)}$. The existence of this symmetry explains the presence of a spectral parameter in the solutions of the equations. We show that similarly, for three-dimensional vertex models and the associated tetrahedron equations, there also exists an infinite discrete group of symmetry. Although generalizing naturally the previous one, it is a much bigger hyperbolic Coxeter group. We indicate how this symmetry can help to resolve the Yang-Baxter equations and their higher-dimensional generalizations and initiate the study of three-dimensional vertex models. These symmetries are naturally represented as birational projective transformations. They may preserve non trivial algebraic varieties, and lead to proper parametrizations of the models, be they integrable or not. We mention the relation existing between spin models and the Bose-Messner algebras of algebraic combinatorics. Our results also yield the generalization of the condition $q^n=1$ so often mentioned in the theory of quantum groups, when no $q$ parameter is available.Comment: 23 page

Symmetrical Temperature-Chaos Effect with Positive and Negative Temperature Shifts in a Spin Glass

The aging in a Heisenberg-like spin glass Ag(11 at% Mn) is investigated by measurements of the zero field cooled magnetic relaxation at a constant temperature after small temperature shifts $|\Delta T/T_g| < 0.012$. A crossover from fully accumulative to non-accumulative aging is observed, and by converting time scales to length scales using the logarithmic growth law of the droplet model, we find a quantitative evidence that positive and negative temperature shifts cause an equivalent restart of aging (rejuvenation) in terms of dynamical length scales. This result supports the existence of a unique overlap length between a pair of equilibrium states in the spin glass system.Comment: 4 page

Surfing on a critical line: Rejuvenation without chaos, Memory without a hierarchical phase space

The dynamic behaviour of glassy materials displays strong nonequilibrium effects, such as ageing in simple protocols, memory, rejuvenation and Kovacs effects in more elaborated experiments. We show that this phenomenology may be easily understood in the context of the nonequilibrium critical dynamics of non-disordered systems, the main ingredient being the existence of an infinite equilibrium correlation length. As an example, we analytically investigate the behaviour of the 2D XY model submitted to temperature protocols similar to experiments. This shows that typical glassy effects may be obtained by `surfing on a critical line' without invoking the concept of temperature chaos nor the existence of a hierarchical phase space, as opposed to previous theoretical approaches. The relevance of this phenomenological approach to glassy dynamics is finally discussed.Comment: Version to be published in Europhysics Letters. Slight modifs + ref to "surfing" adde

Intermittent origin of the large violations of the fluctuation dissipation relations in an aging polymer glass

The fluctuation-dissipation relation (FDR) is measured on the dielectric properties of a polymer glass (polycarbonate)in the range $20mHz - 100Hz$. It is found that after a quench below the glass transition temperature the fluctuation dissipation theorem is strongly violated. The amplitude and the persistence time of this violation are decreasing functions of frequency. At frequencies larger than 1Hz it persists for about $3h$. The origin of this violation is a highly intermittent dynamics characterized by large fluctuations. The relevance of these results for recent models of aging dynamics are discussed.Comment: to be published in Europhysics Letter