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- ItemTemperature-Dependent Charge Carrier Diffusion in [0001¯] Direction of GaN Determined by Luminescence Evaluation of Buried InGaN Quantum Wells(Weinheim : Wiley-VCH, 2020) Netzel, Carsten; Hoffmann, Veit; Tomm, Jens W.; Mahler, Felix; Einfeldt, Sven; Weyers, MarkusTemperature-dependent transport of photoexcited charge carriers through a nominally undoped, c-plane GaN layer toward buried InGaN quantum wells is investigated by continuous-wave and time-resolved photoluminescence spectroscopy. The excitation of the buried InGaN quantum wells is dominated by charge carrier diffusion through the GaN layer; photon recycling contributes only slightly. With temperature decreasing from 310 to 10 K, the diffusion length in [0001⎯⎯] direction increases from 250 to 600 nm in the GaN layer. The diffusion length at 300 K also increases from 100 to 300 nm when increasing the excitation power density from 20 to 500 W cm−2. The diffusion constant decreases from the low-temperature value of ∼7 to 1.5 cm2 s−1 at 310 K. The temperature dependence of the diffusion constant indicates that the diffusivity at room temperature is limited by optical phonon scattering. Consequently, higher diffusion constants in GaN-based devices require a reduced operation temperature. To increase diffusion lengths at a fixed temperature, the effective recombination time has to be prolonged by reducing the number of nonradiative recombination centers.
- ItemAb-Initio Real-Time Magnon Dynamics in Ferromagnetic and Ferrimagnetic Systems(Weinheim : Wiley-VCH, 2020) Singh, Nisha; Elliott, Peter; Dewhurst, J. Kay; Gross, E.K.U.Magnonics—an emerging field of physics—is based on the collective excitations of ordered spins called spin waves. These low-energy excitations carry pure spin currents, paving the way for future technological devices working at low energies and on ultrafast timescales. The traditional ab-initio approach to predict these spin-wave energies is based on linear-response time-dependent density functional theory (LR-TDDFT) in the momentum and frequency regime. Herein, the simulation of magnon dynamics using real-time time-dependent density functional theory is demonstrated, thus extending the domain of ab-initio magnonic studies. Unlike LR-TDDFT, this enables us to observe atom-resolved dynamics of individual magnon modes and, using a supercell approach, the dynamics of several magnon modes can be observed simultaneously. The energies of these magnon modes are concurrent with those found using LR-TDDFT. Next, the complex dynamics of the superposition of magnon modes is studied, before finally studying the element-resolved modes in multisublattice magnetic systems.
- ItemTailored Disorder in Photonics: Learning from Nature(Weinheim : Wiley-VCH, 2021) Rothammer, Maximilian; Zollfrank, Cordt; Busch, Kurt; Freymann, Georg vonDisorder and photonics have long been seen as natural adversaries and designers of optical systems have often driven systems to perfection by minimizing deviations from the ideal design. Especially in the field of photonic crystals and metamaterials but also for optical circuits, disorder has been avoided as a nuisance for many years. However, starting from the very robust structural colors found in nature, scientists learn to analyze and tailor disorder to achieve functionalities beyond what is possible with perfectly ordered or ideal systems alone. This review article covers theoretical and materials aspects of tailored disorder as well as experimental results. Furthermore selected examples are highlighted in greater detail, for which the intentional use of disorder adds additional functionality or provides novel functionality impossible without disorder. © 2021 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH