L. Buisson

Metal-to-metal electron transfer in Co/Fe Prussian Blue molecular analogues: the ultimate miniaturization.

Co/Fe Prussian Blue analogues are known to display both thermally and light induced electron transfer attributed to the switching between diamagnetic {Fe(II)LS(μ-CN)Co(III)LS} and paramagnetic {Fe(III)LS(μ-CN)Co(II)HS} pairs (LS = low spin; HS = high spin). In this work, a dinuclear cyanido-bridged Co/Fe complex, the smallest {Fe(μ-CN)Co} moiety at the origin of the remarkable physical properties of these systems, has been designed by a rational building-block approach. Combined structural, spectroscopic, magnetic and photomagnetic studies reveal that a metal-to-metal electron transfer that can be triggered in solid state by light, temperature and solvent contents, is observed for the first time in a dinuclear complex.

Controlling thermally induced electron transfer in cyano-bridged molecular squares: from solid state to solution.

A new cyano-based {Fe 2 Co 2 } complex has been synthesized, and its optical and magnetic properties described in the frame of an intramolecular electron transfer. Due to methyl functionalization of the ligand, which gives good solubility to this complex, its thermally induced switchable behavior has been transferred to solution and finely tuned over a broad temperature range by adjusting the solvent composition and polarity.

Intermittency in ageing

The fluctuation–dissipation relation is measured on the dielectric properties of a gel (Laponite) and of a polymer glass (polycarbonate). For the gel it is found that during the transition from a fluid-like to a solid-like state the fluctuation–dissipation theorem is strongly violated. The amplitude and the persistence time of this violation are decreasing functions of frequency. Around 1 Hz it may persist for several hours. A very similar behaviour is observed in polycarbonate after a quench below the glass transition temperature. In both cases the origin of this violation is a highly intermittent dynamics characterized by large fluctuations. The relevance of these results for recent models of ageing is discussed.

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 prop- erties of a polymer glass (polycarbonate)in the range 20 mHz-100 Hz. 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 1 Hz it persists for about 3 h. 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 is discussed.

Zero applied stress rheometer

In order to test the fluctuation-dissipation relation on rheological properties of soft materials, we built an experiment to measure thermally excited strain in a sample and compare it to the classical response to an external stress. The rheometer is based on a cylindrical Couette geometry. We use differential interferometry to achieve better than 10−10 rad/Hz sensitivity in angular position above 0.5 Hz. The forcing method, based on electrostatic interaction in a capacitor, generates torques comparable to that of thermal noise. Experiments on a calibrated silicon oil show good agreement between response and fluctuation measurements.

A simple noise subtraction technique

Measuring very low level signals at low frequency is a tedious task, because environmental noise increases in this frequency domain and it is very difficult to filter it efficiently. In order to counteract these major problems, we propose a simple and generic noise subtraction technique, which mixes several features of traditional feedback techniques and those of noise estimators. As an example of application, large band measurements of the thermal fluctuations of a mechanical oscillator are presented. Our results show that the proposed noise subtraction technique is easy to implement and gives good results.

Electrical noise properties in aging materials

The electric thermal noise has been measured in two aging materials, a colloidal suspension (Laponite) and a polymer (polycarbonate), presenting very slow relaxation towards equilibrium. The measurements have been performed during the transition from a fluid-like to a solid-like state for the gel and after a quench for the polymer. 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.

Off equilibrium fluctuations in a polymer glass

Abstract The fluctuation–dissipation relation (FDR) is measured on the dielectric properties of a polymer glass (polycarbonate). It is observed that the fluctuation–dissipation theorem is strongly violated after a quench from above to below the glass transition temperature. The amplitude and the persistence time of this violation are decreasing functions of frequency. Around 1 Hz, it may persist for several hours. The origin of this violation is a highly intermittent dynamics characterized by large fluctuations and strongly non-Gaussian statistics. The intermittent dynamics depends on the quenching rate and it disappears after slow quenches. The relevance of these results for recent models of aging are discussed.

When capacitive transduction meets the thermomechanical limit: Towards femto-newton force sensors at very high frequency

We show that the capacitive transduction of a MEMS device using a setup based on a microwave detection scheme achieves the measurement of the thermomechanical noise spectrum of a high-frequency (>10 MHz) high-stiffness (>105 N/m) resonator, reaching the outstanding displacement resolution of 1 fm/√Hz. This result paves the way for vibrating sensors with exquisite force resolution in the fN/vHz range, enabling large-bandwidth measurements of mechanical interactions at small scale and rheology of fluids at very high frequency. An example of application is given and concerns atomic force microscopy images of biomolecular assemblies.

Importance of thermal gradient in the bitumen bees genesis

Bitumen can be regarded as a complex colloidal suspension. Indeed, the heaviest molecules in bitumen, the asphaltenes, are known to self-associate to form solid particles suspended in a fluid called maltenes. Bitumen is also composed of a crystallizable fraction that partitions between asphaltenes and maltenes. This complex colloidal system exhibits peculiar patterns at their surface called “bees.” By varying the bitumen formulation and also tuning process parameters such as the temperature cooling rate, we demonstrate the role of the various components: asphaltenes, maltenes, and the crystallizable fraction on the bee existence and we evidence that the bee formation results from a complex coupling between different physico-chemical phenomena such as phase separation, crystallization, and buckling. We then propose a mechanism of bee genesis based on the thermal properties of the various fractions and especially on the heat capacity of both asphaltenes and maltenes. We demonstrate that a thermal gradient in the sample is required for the formation of bees and put into evidence the importance of kinetic aspects. We also discuss the bitumen general bulk properties.