Strain dependence of polarization and piezoelectric response in epitaxial BiFeO3 thin films.

Epitaxial strain has recently emerged as a powerful means to engineer the properties of ferroelectric thin films, for instance to enhance the ferroelectric Curie temperature (T(C)) in BaTiO(3). However, in multiferroic BiFeO(3) thin films an unanticipated strain-driven decrease of T(C) was reported and ascribed to the peculiar competition between polar and antiferrodistortive instabilities. Here, we report a systematic characterization of the room-temperature ferroelectric and piezoelectric properties for strain levels ranging between -2.5% and +1%. We find that polarization and the piezoelectric coefficient increase by about 20% and 250%, respectively, in this strain range. These trends are well reproduced by first-principles-based techniques.

A liquid-He cryostat for structural and thermal disorder studies by X-ray absorption.

A new device operating from 4.2 to 300 K is now installed on the hard X-ray station of the DCI ring in LURE in order to measure absorption coefficients. This liquid-He bath device has three optical windows. One allows the incident beam to impinge on the sample, one located at 180 degrees with respect to the sample allows transmitted beams to be detected, and another located at 90 degrees is used to detect emitted photons. Total electron yield detection mode is also possible thanks to a specific sample holder equipped with an electrode that collects the charges created by the emitted electrons in the He gas brought from the He bath around the sample. The performance of the cryostat is described by measurements of the absorption coefficients versus the temperature for Cu and Co foils. For comparison, the absorption coefficient is also measured for Cu clusters. As expected from dimension effects, the Debye temperature obtained for the clusters is lower than that of bulk Cu.

Sedimentation field-flow fractionation application to Toxoplasma gondii separation and purification.

Toxoplasmosis is a worldwide disease caused by Toxoplasma gondii, an intracellular protozoa of micronic size range (4-10 microm). Its classical purification processes are complex and often associated with low recovery. All investigation procedures concerning this parasite require its isolation and purification from at least the mouse ascitic fluid. For this purpose, a recently developed laboratory technology was used, i.e. sedimentation field-flow fractionation. This chromatographic-like separation technology was demonstrated to be particularly selective for isolation and separation of micron-sized biological particles. Sedimentation field-flow fractionation operated on the steric-hyperlayer mode was used to isolate the parasite from the remanent ascitic contaminants of different origins and from red blood cells. With this technology, 86% recovery with 97% viability was obtained in less than 30 min.

Determination of the optical constants (n, k) of thin dielectric films.

A method for the determination of the optical constants and the thickness of thin dielectric films on transparent substrates is proposed. It requires measurements at normal incidence of the transmission from two films of different thicknesses. The derivation is complicated by the existence of multiple solutions of the relevant equations. A procedure is given for determining the correct solutions for the indices of refraction and absorption and also for accurately fixing the thickness of the films. Advantages of the present method over existing methods are better accuracy and readily available measurement facilities. The method has been applied successfully to films of titanium oxide.