Spin-Exchange Interaction in ZnO-based Quantum Wells

Wurtzitic ZnO/(Zn,Mg)O quantum wells grown along the (0001) direction permit unprecedented tunability of the short-range spin exchange interaction. In the context of large exciton binding energies and electron-hole exchange interaction in ZnO, this tunability results from the competition between quantum confinement and giant quantum confined Stark effect. By using time-resolved photoluminescence we identify, for well widths under 3 nm, the redistribution of oscillator strengths between the A and B excitonic transitions, due to the enhancement of the exchange interaction. Conversely, for wider wells, the redistribution is cancelled by the dominant effect of internal electric fields, which dramatically reduce the exchange energy.Comment: 14 pages, 3 figure

Magnetic Anisotropy of Co2+ as Signature of Intrinsic Ferromagnetism in ZnO:Co

We report on the magnetic properties of thoroughly characterized Zn1-xCoxO epitaxial thin films, with low Co concentration, x=0.003-0.005. Magnetic and EPR measurements, combined with crystal field theory, reveal that isolated Co2+ ions in ZnO possess a strong single ion anisotropy which leads to an "easy plane" ferromagnetic state when the ferromagnetic Co-Co interaction is considered. We suggest that the peculiarities of the magnetization process of this state can be viewed as a signature of intrinsic ferromagnetism in ZnO:Co materials.Comment: 4 pages, 4 figure

Influence of s,p-d and s-p exchange couplings on exciton splitting in (Zn,Mn)O

This work presents results of near-band gap magnetooptical studies on (Zn,Mn)O epitaxial layers. We observe excitonic transitions in reflectivity and photoluminescence, that shift towards higher energies when the Mn concentration increases and split nonlinearly under the magnetic field. Excitonic shifts are determined by the s,p-d exchange coupling to magnetic ions, by the electron-hole s-p exchange, and the spin-orbit interactions. A quantitative description of the magnetoreflectivity findings indicates that the free excitons A and B are associated with the Gamma_7 and Gamma_9 valence bands, respectively, the order reversed as compared to wurtzite GaN. Furthermore, our results show that the magnitude of the giant exciton splittings, specific to dilute magnetic semiconductors, is unusual: the magnetoreflectivity data is described by an effective exchange energy N_0(beta-alpha)=+0.2+/-0.1 eV, what points to small and positive N_0 beta. It is shown that both the increase of the gap with x and the small positive value of the exchange energy N_0 beta corroborate recent theory describing the exchange splitting of the valence band in a non-perturbative way, suitable for the case of a strong p-d hybridization.Comment: 8 pages, 8 figure

Transport of indirect excitons in ZnO quantum wells

We report on spatially- and time-resolved emission measurements and observation of transport of indirect excitons in ZnO/MgZnO wide single quantum wells

Territories and grassland exploitation by horses in France

International audienceThis papers aims to present a review of french knowledge about the exploitation of territories and grassland by horses

Fractura de elastómeros EPDM cargados con microesferas de vidrio mediante el trabajo esencial de fractura

In this work, the essential work of fracture concept (EWF) has been applied to untreated and silane- treated glass bead-filled EPDM. It has been proved that this theory is not applicable to pure EPDM due to its high elastic character, but being possible in glass bead-filled EPDM, thus the presence of these particles induces certain plasticity on the EPDM. The results show that the higher adhesion between matrix and particles, the higher value of plastic work of fracture, and the higher final instability of crack propagation. Moreover the influence of processing temperature on the mechanical characteristics of these materials has been studied, noticing a marked drop of stiffness and strength from a temperature processing of 200 ºC. The higher adhesion obtained between EPDM matrix and glass beads through surface treatment has been confirmed by scanning electron microscopy.<br><br>En el presente trabajo se ha aplicado el concepto de trabajo esencial de fractura (EWF) a una serie de compuestos de EPDM y microesferas de vidrio, sin tratamiento superficial y tratadas con silanos. Se ha comprobado que esta teoría no es aplicable al EPDM puro debido a su elevado carácter elástico, pudiéndose en cambio aplicar a sus compuestos cargados con microesferas de vidrio, pues la presencia de estas partículas induce cierta plasticidad en el EPDM. Los resultados indican que una mayor adhesión entre partícula y matriz se traduce en un mayor valor de trabajo plástico de fractura, así como en una mayor inestabilidad final en la propagación de la grieta. Asimismo se ha estudiado la influencia de la temperatura de procesado sobre las características mecánicas de estos materiales, observándose una caída acusada de la rigidez y de la resistencia a la tracción en todos los compuestos a partir de una temperatura de procesado de 200 ºC. La mayor adhesión que se consigue entre la matriz de EPDM y las microesferas de vidrio mediante el tratamiento superficial con silanos se ha confirmado a través de la microscopía electrónica de barrido