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The authors have demonstrated photopumped laser action of self-assembled ring-shaped GaAs quantum dots (QDs) grown by droplet epitaxy. Morphological control of the QD shape from conelike dots to ringlike ones results in a narrow spectral band of photoluminescence from the QD ensemble, reflecting their small size distribution. Using ring-shaped QDs as an active laser medium, they observed multimodal stimulated emissions from the ground state at temperatures of up to 300K.

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Thin, c ជ -axis oriented films of the magnetic perovskite Pr 0.5 Sr 0.5 MnO 3 were prepared by dc-magnetron sputtering, structurally characterized by x-ray diffraction and their physical properties investigated by magnetization and electrical transport measurements. Ferromagnetic ordering appears in zero field at 263 K, followed by a second transition into an antiferromagnetic state at 160 K. The zero-field resistivity has a semiconducting behavior above the Curie and below the Néel temperature and is analyzed in the framework of Mott's variable range hopping concept. Quasimetallic behavior is observed within the ferromagnetic state, whose temperature range can be extended by applying an external magnetic field. The negative magnetoresistance effect increases systematically with decreasing temperature and reaches a value of 700% at 1.5 K in a field of 12 T. In the ferro-and antiferromagnetic phases, the field induced resistivity decrease scales with the Brillouin function, suggesting the existence of magnetically polarized regions with a total magnetic moment which is reduced upon entering the antiferromagnetic state.