No abstract

Phase-separated semiconductors containing magnetic nanostructures are relevant systems for the realization of high-density recording media. Here, the controlled strain engineering of GaδFeN layers with FeyN embedded nanocrystals (NCs) via AlxGa1−xN buffers with different Al concentration 0<xAl<41% is presented. Through the addition of Al to the buffer, the formation of predominantly prolate-shaped ε-Fe3N NCs takes place. Already at an Al concentration xAl≈ 5% the structural properties—phase, shape, orientation—as well as the spatial distribution of the embedded NCs are modified in comparison to those grown on a GaN buffer. Although the magnetic easy axis of the cubic γ’-GayFe4−yN nanocrystals in the layer on the xAl=0% buffer lies in-plane, the easy axis of the ε-Fe3N NCs in all samples with AlxGa1−xN buffers coincides with the [0001] growth direction, leading to a sizeable out-of-plane magnetic anisotropy and opening wide perspectives for perpendicular recording based on nitride-based magnetic nanocrystals.

A remarkably long-lived spin plasmon may exist in two-dimensional electron liquids with imbalanced spin up and spin down population. Predictions for this interesting mode by Agarwal et al. [Phys. Rev. B 90, 155409 (2014)] are based on the random phase approximation. We here show how to account for spin dependent correlations from known ground state pair correlation functions and study the consequences on the various spin dependent longitudinal response functions. The spin plasmon dispersion relation and its critical wave vector for Landau damping by minority spins turn out to be significantly lower. We further demonstrate that spin dependent effective interactions imply a rich structure in the excitation spectrum of the partially spin-polarized system. Most notably, we find a "magnetic antiresonance", where the imaginary part of both, the spin-spin as well as the density-spin response function vanish. The resulting minimum in the double-differential cross section is awaiting experimental confirmation.

Next‐generation high‐speed optical networks demand the development of ultrafast optical interconnects capable of Tbit s−1 data rates. By utilizing colloidal CdSe/CdS core/shell quantum dots, gated by intense THz pulses, a proof of concept of an all‐optical femtosecond electro‐absorption switch is presented in this work. Without any additional enhancement of the THz electric field, an extinction contrast of more than 6 dB and transmission changes in the visible of more than 15% are achieved, with the latter setting a new record for solution‐processed electro‐absorption materials at room temperature. The absence of physical artifacts, originating from electrodes and field enhancing structures, allows to employ a simple and intuitive numerical model, which rationalizes the large field‐induced electro‐absorption response. Supported by theoretical calculations, the importance of the energy band alignment of heterostructure quantum dots are discussed for the first time and suggest that further improvement of the modulation depth and contrast may be achieved with Type‐II quantum dots.

A spin‐sensitive linear response theory is presented that includes correlations beyond the well‐known random phase approximation. Especially for very dilute systems, such correlations play an important role. The response functions obtained give insight into both charge and longitudinal magnetic excitations. In addition to the spin‐plasmon, we propose a new regime where no magnetic excitation is possible, namely the magnetic anti‐resonance. Both effects lie in experimentally accessible ranges.