C. Hermann

Spin-polarized photoemission from AlGaAs/GaAs heterojunction: A convenient highly polarized electron source

We analyze the operation of a spin‐polarized electron source, consisting of a 100 A GaAs cap on top of Al0.3Ga0.7As, excited at 300 or 120 K by a He‐Ne laser. The cap allows easy activation to negative electron affinity while the alloy permits gap matching to the light source, and thus large electron spin polarization (30% at 300 K, 36% at 120 K). We compare yield curves, energy distribution curves, and polarized energy distribution curves obtained on samples with 100 and 1000 A caps and on bulk GaAs. The X conduction minimum position in the alloy is also determined.

Optical pumping and the valence‐band light‐hole effective mass in GaxIn1−xAsyP1−y (y≃2.2x)

We report the use of optical pumping in p‐type GaxIn1−xAsyP1−y nearly lattice‐matched to InP. Analysis of the conduction‐electron spin‐polarized photoluminescence has been used to deduce the valence‐band light‐hole effective mass as a function of alloy composition. Our results are in good agreement with masses calculated using the k⋅p approximation.

Optical detection of electron spin resonance in CdTe

Abstract We report the optical detection of electron spin resonance in p-type CdTe at 1.7 K in optical pumping conditions. The Overhauser shift of the electronic resonance, of the order of 45 G, is related to the sign of the electron g-factor g∗. We measure g ∗ = -1.59±0.02 . Using this g∗ value and the previous results on the Knight shift, we deduce the value of the electron wavefunction on Cd in CdTe, which is consistent with the value in CdS.

Near-field magneto-optics with polarization sensitive STOM

Abstract A scanning tunneling optical microscope (STOM) operating with polarized light has been developed to study thin magnetic films. The magnetic film is deposited on the external face of a prism and illuminated in total reflection conditions with linearly polarized laser light. The evanescent mode close to the magnetic film surface is detected with a tip-ending monomode optical fiber connected at its other end to a light-polarization analyzer mounted at the entrance of a photomultiplier tube. The polarization sensitivity of the whole system, which was found to depend on the tip condition, was characterized on the bare prism with s- and p-polarized excitations. The magneto-optical effect in the evanescent mode is measured through a lock-in amplifier by modulating the magnetic field produced by a coil surrounding the tip. With this set-up we have studied two different systems, both exhibiting perpendicular magnetization. The first one is a dielectric garnet film. The images, obtained on this sample by measuring the magneto-optical effect under very low amplitude of the external magnetic field modulation, show up submicronic details due to magnetic domain wall motion. The second system is a metallic 25 nm Au/1 nm Co/4 nm Au sandwich with a large coercive field (≈ 1 kOe). The magneto-optical effect is here measured by modulating the field with an amplitude larger than the coercive field so that the saturated magnetization is periodically flipped. In this system we have taken advantage of the possibility to excite surface plasmon resonances in noble metal thin films with p-polarized light. Near-field measurements performed with our microscope demonstrate that the intensity of the evanescent mode is strongly enhanced (two orders of magnitude) at resonance. Moreover, the interaction of the light electric field with the gold surface plasmon leads to a related amplification of the magneto-optical effects in the evanescent mode.

Imaging of magnetic domains with scanning tunneling optical microscopy

Near field magneto-optical images of magnetic domains in ferromagnetic soft garnet films have been obtained with a scanning tunneling optical microscope working in total reflection geometry with shear-force control of the tip-to-sample distance. In this geometry a magneto-optical contrast is observed for the first time between domains of opposite magnetization without using modulation techniques. When applying a static or alternating external magnetic field, the magneto-optical images provide the location of domain wall pinning points.

Highly polarized photoluminescence from 2‐μm‐thick strained GaAs grown on CaF2

Intense photoluminescence was measured in a strained 2‐μm‐thick GaAs film grown on a (100)‐oriented CaF2 substrate. Circular polarization of 77%±2% was obtained at 77 K under excitation with circularly polarized photons below 1.575 eV. For excitation above 1.610 eV the polarization is limited to 30%±2%. These results are clear indications of strain induced splitting between the Mj=‖±3/2〉 and the Mj=‖± 1/2〉 hole bands. The deduced splitting was 62.5±2.5 meV, corresponding to a stress of the order of −12 kbar. The polarization is maximum for reception energies very close to the excitation. Strained GaAs/CaF2 is thus a good candidate structure for efficient strongly polarized electron sources.

Photoemission and photoluminescence from GaAs/ GaAlAs superlattices

We report photoemission measurements in a GaAs/GaAlAs superlattice covered by a GaAs surface activated to negative electron affinity by cesium and oxygen coadsorption. The original experimental apparatus allows heating up to 900 K during sample activation, and cooling down to 30 K by helium circulation. The photocurrent versus light excitation energy, i.e., the yield curve, shows definite step-like structures, which appear at the same energy positions as those present in the luminescence excitation spectrum. They are therefore attributed to electrons photoexcited from the valence quantized states into the conduction quantized states of the superlattice, which are transported towards the surface and then into vacuum. The amplitude of the contribution to the photocurrent of the electrons from the wells is interpreted in terms of their tunneling through the superlattice barriers. This photoemission study opens a new field in the direct investigation of the dynamics of electrons in layered structures.

French Women Physicists: Progress and Stability

Here we present a few gender-disaggregated data on women physicists in universities and in public research institutions. We introduce a statistical indicator, the “men’s advantage,” which will help us estimate the glass-ceiling effect in physics as compared with other disciplines. We describe the recently established structures in favor of women scientists in France, which, hopefully, should help empower French women physicists.

Photoelectronic processes in semiconductors activated to negative electron affinity

Note: invited communication Reference LOEQ-CONF-1992-001 Record created on 2007-08-31, modified on 2016-08-08

Photoemission from AlGaAs/GaAs superlattices

Reference LOEQ-CONF-1991-001doi:10.1016/0169-4332(92)90313-MView record in Web of Science Record created on 2007-08-31, modified on 2017-12-13