We report results of a detailed temperature dependence of photoluminescence (PL) decay time and continuous emission properties of the F and F + centers in recently reported lasing MgO microcrystals [T. Uchino and D. Okutsu, Phys. Rev. Lett. 101, 117401 (2008)]. Thermally induced ionization and carrier trapping play a vital role in the entire emission processes of the F-type centers, especially at time scales longer than microseconds. In these time scales the PL decay profiles tend to exhibit a power-law behavior over more than 3−4 decades of time, implying thermally activated hopping and tunneling of the trapped electrons. Such delayed PL signals show two maxima in intensity at temperatures of ∼130 and ∼300 K. This apparently anomalous temperature dependence is also indicative of the thermally stimulated emission processes of the originally photoexcited F and F + centers. The PL spectra above lasing threshold exhibit temperature-dependent broadening, demonstrating that the thermal vibrations of the crystal lattice affect the stimulated emission scheme as well. On the basis of these experimental results, a model of the photoexcitation, trapping, and recombination processes of the F-type centers is presented.

Figure S1. XRD pattern of the as-received Mg 3 N 2 sample. The measurements were carried out under atmospheric conditions.

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.

It has generally been recognized that Mg gives rise to an interstitial donor level in Si although the possibility of the existence of a Mg substitutional acceptor in Si has been suggested. In this work, we explore the synthetic conditions required to obtain substitutional Mg acceptor in Si. We have diffused Mg into Si wafers under the Mg vapor environment created by the thermal decomposition of MgB 2. The Mg vapor pressure is low enough to suppress the formation of the stable silicide phase, Mg 2 Si, but is high enough to induce Mg diffusion into Si. The resultant Mg-diffused Si exhibited degenerate p-type semiconducting behavior. It was proposed that the presence/absence of the Mg 2 Si phase plays a key role in determining the location and solubility of Mg in Si.

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 = 38.6 K, 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 ∼100 nm. 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.