Pulsed-laser-induced quenching of ferromagnetic order has intrigued researchers since pioneering works in the 1990s. It was reported that demagnetization in gadolinium proceeds within 100 ps, but three orders of magnitude faster in ferromagnetic transition metals such as nickel. Here we show that a model based on electron-phonon-mediated spin-flip scattering explains both timescales on equal footing. Our interpretation is supported by ab initio estimates of the spin-flip scattering probability, and experimental fluence dependencies are shown to agree perfectly with predictions. A phase diagram is constructed in which two classes of laser-induced magnetization dynamics can be distinguished, where the ratio of the Curie temperature to the atomic magnetic moment turns out to have a crucial role. We conclude that the ultrafast magnetization dynamics can be well described disregarding highly excited electronic states, merely considering the thermalized electron system.

We present a theoretical study of broadening of defect luminescence bands due to vibronic coupling. Numerical proof is provided for the commonly used assumption that a multi-dimensional vibrational problem can be mapped onto an effective one-dimensional configuration coordinate diagram. Our approach is implemented based on density functional theory with a hybrid functional, resulting in luminescence lineshapes for important defects in GaN and ZnO that show unprecedented agreement with experiment. We find clear trends concerning effective parameters that characterize luminescence bands of donor-and acceptor-type defects, thus facilitating their identification.

Based on the Fermi surface breathing model of Kamberský, a phenomenological extension of the ab initio density-functional electron theory is used to derive an equation of motion for the spin dynamics in magnets. It is shown that even in the simple case of a homogeneous magnetization M the damping term ͑1/ M͒M ϫ ͓␣dM / dt͔ of the commonly used Gilbert equation with the damping scalar ␣ has to be replaced by a term of the form ͑1/ M͒M ϫ ͓␣ = ͑M͒ · dM / dt͔ with a damping matrix ␣ = which depends on the orientation of M. Explicit calculations are performed for bulk, monolayers, and monatomic wires of Fe, Co, and Ni. The variation of ␣ = with an orientation of M is quite substantial already for the bulk materials ͑up to a factor of 4 in hcp Co͒ but most dramatic in the monolayers and monatomic wires in which for some orientations the damping is even zero. This represents an additional option for optimizing the magnetization reversal process in a magnetic nanostructure. It is shown that there is no simple relation between damping and magnetic anisotropy energy.

We investigate the electrical and optical properties of oxygen vacancies (VO), zinc vacancies (VZn), hydrogenated VZn, and isolated dangling bonds in ZnO using hybrid functional calculations. While the formation energy of VO is high in n-type ZnO, indicating that this center is unlikely to form, our results for optical absorption signals associated with VO are consistent with those observed in irradiated samples, and give rise to emission with a peak at less than 1 eV. Under realistic growth conditions, we find that VZn is the lowest-energy native defect in n-type ZnO, acting as an acceptor that is likely to compensate donor doping. Turning to optical transitions, we first examine NO as a case study, since N-related transitions have been identified in experiments on ZnO. We also examine how hydrogen, often unintentionally present in ZnO, forms stable complexes with VZn and modifies its optical properties. Compared with isolated VZn, VZn-H complexes have charge-state transition levels lower in the band gap as well as have lower formation energies. These complexes also lead to characteristic vibrational frequencies which compare favorably with experiment. Oxygen dangling bonds show behavior mostly consistent with VZn, while zinc dangling bonds give rise to transition levels near the ZnO conduction-band minimum and emission peaking near 2.4 eV. We discuss our results in view of the available experimental literature.

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