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Research on semiconductor nanowires underlies the development of the miniaturization of laser devices with low cost and low energy consumption. In general, nanowire lasers are made of direct band gap semiconductors, e.g., GaN, ZnO and CdS, and their band-edge emissions are used to achieve optically pumped laser emission. In addition to the existing class of nanowire lasers, we here show that air-annealed micrometer-sized MgO cubic crystals with well-defined facets exhibit room-temperature stimulated emission at 394 nm under pulsed laser pumping at ~350 nm. Surface midgap states are assumed to be responsible for the excitation and emission processes. The present findings will not only provide opportunities for the development of miniaturized lasers composed of insulating oxides, but will also open up functionality in various families of cubic crystalline materials. [ABSTRACT FROM AUTHOR]

We investigate the interaction between superconductivity and defect-induced d0 ferromagnetism using a composite consisting of MgB2 and MgO nanocrystals. The composite exhibits a ferromagnetic hysteresis behavior in the temperature region from 40 to 300 K. Defective MgO nanocrystals (~20 nm) embedded in the composite are considered to be responsible for the observed ferromagnetism. The zero field cool and field cool magnetization curves show that the superconducting transition occurs at Tc = 38.6K, in agreement with Tc of pure MgB2. In the temperature region from Tc to 0.9Tc (~35 K), the magnetization hysteresis curves show a superposition of ferromagnetic (F) and superconducting (S) signals. When the temperature of the system is decreased below 0.65Tc (~25 K), the S signals dominate over the F signals. The resulting magnetic hysteresis loops are highly asymmetric and the descending filed branch is nearly flat, as predicted in the case of surface pinning. At temperatures below 0.5Tc (~20 K), a sharp peak is developed near zero field in the magnetization hysteresis curves, implying an enhancement of superconducting vortex pinning. The observed pinning enhancement most likely results from magnetic pinning due to randomly distributed magnetic MgO grains, which yield the magnetic inhomogeneity and the related pinning potential in a length scale of ~100nm. Thus, the present ferromagnetic/superconducting composite provides an ideal model system that demonstrates the availability of d0 ferromagnetism as a source of magnetic potential for effective vortex pinning. [ABSTRACT FROM AUTHOR]

Since the seminal observation of room-temperature laser emission from ZnO thin films and nanowires, numerous attempts have been carried out for detailed understanding of the lasing mechanism in ZnO. In spite of the extensive efforts performed over the last decades, the origin of optical gain at room temperature is still a matter of considerable discussion. In this work, we show that a ZnO film consisting of well-packed micrometer-sized ZnO crystals exhibits purely excitonic lasing at room temperature without showing any symptoms of electron-hole plasma emission, even under optical excitation more than 25 times above the excitonic lasing threshold. The lasing mechanism is shifted from the exciton-exciton scattering to the exciton-electron scattering with increasing temperature from 3 to 150 K. The exciton-electron scattering process continues to exist with further increasing temperature from 150 to 300 K. Thus, we present distinct experimental evidence that the room-temperature excitonic lasing is achieved not by exciton-exciton scattering, as has been generally believed, but by exciton-electron scattering. We also argue that the long carrier diffusion length and the low optical loss nature of the micrometer-sized ZnO crystals, as compared to those of ZnO nanostructures, plays a key role in showing room-temperature excitonic lasing. [ABSTRACT FROM AUTHOR]