STRESS MODULATION EFFECT ON TRANSMISSION AND RESISTIVITY NEAR THE CURIE TEMPERATURE

La piezorésistance et la piezotransmission de EuO montrent un comportement critique près de Tc. Quand T décroît, dans le domaine paramagnétique les signaux de piezorésistance et de piezotransmission augmentent, passent par un maximum, puis diminuent très rapidement. Ils inversent leur signe et présentent un minimum dans le domaine ferromagnétique. De plus, la piezorésistance présente une structure à une température voisine de la transition isolant-métal. La variation dans la région paramagnétique est maintenant bien comprise, et est due à la variation de l'énergie d'échange avec la contrainte. Les effets de piezorésistance et de piezotransmission sont d'origine différente dans la région ferromagnétique. Le premier effet est dû au déplacement de Tc avec la contrainte. La structure au voisinage de la température de la TIM étant liée à l'effet de contrainte sur le piegeage des électrons des polarons magnétiques. Le changement de signe de la piezotransmission provient de la présence de domaines magnétiques. L'application d'un faible champ magnétique inverse le signe et le spectre de piezotransmission correspond alors à la dérivée des fonctions de correlation de spin.Piezoresistance and piezotransmission of EuO show a critical behaviour near the Curie temperature. When T decreases in the paramagnetic range towards Tc, the piezoresistance as well as the piezotransmission increase, pass through a maximum, then decrease very fast, invert their sign at Tc and in the ferromagnetic range exhibit a minimum and increase. The piezoresistance in addition shows another peak near the temperature of the isolant metal transition. In the two cases, the paramagnetic part is well understood, and due to the change of the second order exchange energy Ɗεk with the strain. The ferromagnetic part of the piezoresistance and piezotransmission temperature dependence are due to different phenomena. The change of sign of the piezoresistance is explained only by the shift of Tc under strain, and the second peak below Tc by the pressure effect on the self trapping of electrons on magnetic polarons. The change of sign of piezotransmission effect is due to the presence of magnetic domains very sensible to the strain. The application of a small magnetic field gives a piezotransmission spectra which corresponds to the derivative of the spin correlation functions

PIEZOTRANSMISSION BEHAVIOR OF FERROMAGNETIC EuO UNDER MAGNETIC FIELD

Dans un précédent article, nous avons décrit le comportement du signal de piézotransmission de l'Oxyde d'Europium lorsque la température varie. Ce signal, [MATH] Tr/Tr, qui est directement relié au coefficient de pression de la transition optique observée change son signe à la temperature de Curie. Dans la région ferromagnétique la comparaison des mesures d'absorption et de piézotransmission met en évidence deux contributions du spectre "d'absorption apparente". Un faible champ magnétique supprime l'une d'elles. Les spectres de piézotransmission ont alors la même allure dans la région ferromagnétique H [MATH] Hs et dans la région paramagnétique. Cette forme est caractéristique d'une transition bande à bande. Les spectres d'absorption et de piézotransmission sont fortement influencés dans la région ferromagnétique, par la diffusion de la lumière due aux domaines magnétiques et doivent donc être analysés avec attention.It was reported in a previous paper that piezotransmission signal (ATr/Tr) changes its sign at the Curie temperature Tc. It can be shown that the piezotransmission signal is directly related to the pressure coefficient of the optical transition observed (4f 5d). Transmission and piezotransmission measurements have been carried out and in the ferromagnetic state on other contribution to the "apparent optical absorption" appears. Under a small magnetic field this other contribution disappears. Indeed the piezotransmission spectra have the same shape in the ferromagnetic state under a magnetic field than in the paramagnetic state. The shape is characteristic of a band to band transition. The piezotransmission and optical absorption in the ferromagnetic range are thus strongly influenced by light diffusion by domains and must be carefully analysed

BIAXIAL-STRAIN EFFECT ON EXCITONIC TRANSITIONS E(0) AND E(0)+DELTA(0) IN THE TEMPERATURE-RANGE 4.5-200 K AND ZEEMAN SPLITTING IN ZNSE/GAAS EPILAYERS

International audienceIdentification of heavy-hole and light-hole excitonic transitions is made by reflectivity and magnetocircular dichroism in strained metal-organic vapor-phase-epitaxy-grown ZnSe/GaAs epilayers with a thickness range between 0.10 and 0.78 mum at 4.5 K. The observed splitting of the exciton transitions due to the lattice mismatch is constant up to 200 K. It is explained by the very small variation (< 2%) of the lattice mismatch strain in this temperature range. Zeeman splitting of \+/- 3/2, +/- 1/2] and \+/- 1/2, -/+ 1/2] transitions are measured at 5.5 T and effective g values g3/2 = -0. 24 and g1/2 = 0. 5 7 are determined

BIAXIAL-STRAIN EFFECT ON EXCITONIC TRANSITIONS E(0) AND E(0)+DELTA(0) IN THE TEMPERATURE-RANGE 4.5-200 K AND ZEEMAN SPLITTING IN ZNSE/GAAS EPILAYERS

International audienceIdentification of heavy-hole and light-hole excitonic transitions is made by reflectivity and magnetocircular dichroism in strained metal-organic vapor-phase-epitaxy-grown ZnSe/GaAs epilayers with a thickness range between 0.10 and 0.78 mum at 4.5 K. The observed splitting of the exciton transitions due to the lattice mismatch is constant up to 200 K. It is explained by the very small variation (< 2%) of the lattice mismatch strain in this temperature range. Zeeman splitting of \+/- 3/2, +/- 1/2] and \+/- 1/2, -/+ 1/2] transitions are measured at 5.5 T and effective g values g3/2 = -0. 24 and g1/2 = 0. 5 7 are determined

Magnetoexcitons in a narrow single GaAs-Ga0.5Al0.5As quantum well grown by Molecular Beam Epitaxy.

International audienceWe present a series of magnetoreflectance investigations performed at liquid-helium temperature and up to 55 kG in the Faraday configuration for a narrow single GaAs-Ga0.5Al0.5As quantum well (width around 79 Å). Both right-hand and left-hand circularly polarized magnetoexciton polaritons have been selected for the heavy-hole- and light-hole-related transitions. From the experimental data, taking into account the electron-hole exchange interaction, we could obtain the diamagnetic shifts of both the light-hole and the heavy-hole excitons. We measured an enhancement of the electron Landé factor (gc=-6.8±0.2) and a weak influence of the two-dimensional confinement on the valence-band Luttinger parameter (κ=1.37±0.13)

Magnetoreflectivity study of type I - type II transition in CdTe/(Cd, Mn)Te quantum wells

The magnetic field induced type I - type II transition in a 20-Å-thick CdTe/Cd0.875Mn0.125Te single quantum well has been documented by the resonance light reflection measurements. The pronounced splitting of the lower Zeeman-component exciton resonance has been observed to appear at 1.5T and to increase at higher fields, which manifests the formation of direct and indirect exciton states in the type II quantum well

Heterostructure of binaries II-VI semiconductors ZnTe and ZnSe / III-V (GaAs, InP, GaSb)

Epitaxial ZnSe, ZnTe and multilayers have been grown on III-V (InP, GaAs and GaSb) substrates by organometallic vapor phase epitaxy. First, the OMVPE growth technique used is described. The microstructural quality of the epilayers was determined by SEM (EDX) and X-Ray diffraction. Mechanichal strain, due firstly to a mismatch between II-VI layers and III-V substrates and secondly to different thermal expansion coefficients is studied. Reflectivity and photoluminescence spectra are presented for one type of material. Growth parameters are identified and problems associated to experimental conditions are discussed

Giant second-harmonic generation in a one-dimensional GaN photonic crystal

International audienc

Giant second-harmonic generation in a one-dimensional GaN photonic crystal

International audienc

Equifrequency surfaces in GaN/sapphire photonic crystals

International audiencePhotonic crystals are a new class of materials where photonic band gaps, large dispersion and anisotropy occur. By exploiting these properties GaN photonic crystals should have important potential for optoelectronic applications, principally in the areas of high-efficiency light emitters and second-harmonic generators. We present measurements of the equifrequency surfaces of the radiative Bloch modes for a photonic crystal etched in a GaN/sapphire film. The photonic band structure is calculated by using a scattering matrix method that reproduces well the anisotropy of the equifrequency surfaces exhibited by the photonic crystal. (C) 2002 Elsevier Science B.V. All rights reserved