Porous silicon is excited using near-infrared femtosecond pulsed and continuous wave radiation at an average intensity of ∼106 W/cm2 (8×1010 ;W/cm2 peak intensity in pulsed mode). Our results demonstrate the presence of micron-size regions for which the intensity of the photoluminescence has a highly nonlinear threshold, rising by several orders of magnitude near this incident intensity for both the pulsed and continuous wave cases. These results are discussed in terms of stimulated emission from quantum confinement engineered intrinsic Si-Si radiative traps in ultrasmall nanocrystallites, populated following two-photon absorption. © 1999 American Institute of Physics.

Neutron diffraction measurements are presented measuring the responses of both magnetic and structural order parameters of parent and lightly Co-doped Ba(Fe1 xCox)2As2 under the application of uniaxial pressure. We find that the uniaxial pressure induces a thermal shift in the onset of antiferromagnetic order that grows as a percentage of TN as Co doping is increased and the superconducting phase is approached. Additionally, as uniaxial pressure is increased within parent and lightly doped Ba(Fe1 xCox)2As2 on the first-order side of the tricritical point, we observe a decoupling between the onsets of the orthorhombic structural distortion and antiferromagnetism. Our findings place needed constraints on models exploring the nematic susceptibility of the bilayer pnictides in the tetragonal, paramagnetic regime. © 2014 American Physical Society.

We present the results of structural and magnetic phase comparisons of the iron oxychalcogenides La2O2Fe2OM2(M=S,Se). Elastic neutron scattering reveals that M=S and Se have similar nuclear structures at room and low temperatures. Our neutron diffraction data reveals that both materials obtain antiferromagnetic ordering at a Néel temperature TN90.1±0.16 K and 107.2±0.06 K for M=Se and S, respectively. The magnetic arrangements for both M=S, Se compounds are obtained through Rietveld refinement. We find the order parameter exponent β to be 0.129±0.006 for M=Se and 0.133±0.007 for M=S. Each of these values is near the Ising symmetry value of 1/8. This suggests that although lattice and electronic structural modifications result from chalcogen replacement, the nature of the magnetic interactions is similar in these materials.