FERROMAGNETIC RELAXATION IN POROUS POLYCRYSTALLINE FERRITES

On a mesuré la largeur de raie effective W d'échantillon de YIG polycristallin de porosité comprise entre 0,006 et 0,04. Les mesures ont été effectuées à la fréquence de 9 GHz pour des champs appliqués He compris entre 1,5 et 8 kOe. Les valeurs de W en dehors du domaine d'existence des ondes de spin sont plus élevées que le prévoit la théorie. Une modification de la théorie est proposée pour tenir compte de la perturbation des ondes de spin elles-mêmes par la porosité. L'accord entre la théorie et l'expérience est bon.The effective linewidth W has been measured in polycrystalline YIG with porosities between 0.006 and 0.04, at a frequency of 9 GHz and for external magnetic fields He between 1.5 kOe and 8 kOe. The values of W outside the spinwave manifold are much larger than predicted by existing spin wave theories. A new treatment is proposed, in which secondary scattering of the spin waves is taken into account, by considering the spin waves to be broadened over a field range of 4 πM. Fair agreement is obtained between theory and experiment

Superfluorescence from photoexcited semiconductor quantum wells: Magnetic field, temperature, and excitation power dependence

Superfluorescence (SF) is a many-body process in which a macroscopic polarization spontaneously builds up from an initially incoherent ensemble of excited dipoles and then cooperatively decays, producing a delayed pulse of coherent radiation. SF arising from electron-hole recombination has recently been observed in In0.2Ga0.8As/GaAs quantum wells [G. T. Noe et al., Nature Phys. 8, 219 (2012) and J.-H. Kim et al., Sci. Rep. 3, 3283 (2013)], but its observability conditions have not been fully established. Here, by performing magnetic field (B), temperature (T), and pump power (P) dependent studies of SF intensity, linewidth, and delay time through time-integrated and time-resolved magnetophotoluminescence spectroscopy, we have mapped out the B−T−P region in which SF is observable. In general, SF can be observed only at sufficiently low temperatures, sufficiently high magnetic fields, and sufficiently high laser powers with characteristic threshold behavior. We provide theoretical insights into these behaviors based primarily on considerations on how the growth rate of macroscopic coherence depends on these parameters. These results provide fundamental new insight into electron-hole SF, highlighting the importance of Coulomb interactions among photogenerated carriers as well as various scattering processes that are absent in SF phenomena in atomic and molecular systems