190 research outputs found
The topological Faraday effect cannot be observed in a realistic sample
A striking feature of 3 dimensional (3D) topological insulators (TIs) is the
theoretically expected topological magneto-electric (TME) effect, which gives
rise to additional terms in Maxwell's laws of electromagnetism with an
universal quantized coefficient proportional to half-integer multiples of the
fine structure constant . In an ideal scenario one therefore expects
also quantized contributions in the magneto-optical response of TIs. We review
this premise by taking into account the trivial dielectric background of the TI
bulk and potential host substrates, and the often present contribution of
itinerant bulk carriers. We show that (i) one obtains a non-universal
magneto-optical response whenever there is impedance mismatch between different
layers and (ii) that the detectable signals due to the TME rapidly approach
vanishingly small values as the impedance mismatch is detuned from zero. We
demonstrate that it is methodologically impossible to deduce the existence of a
TME exclusively from an optical experiment in the thin film limit of 3D TIs at
high magnetic fields
Enhanced spin-orbit coupling in core/shell nanowires
The spin-orbit coupling (SOC) in semiconductors is strongly influenced by
structural asymmetries, as prominently observed in bulk crystal structures that
lack inversion symmetry. Here, we study an additional effect on the SOC: the
asymmetry induced by the large interface area between a nanowire core and its
surrounding shell. Our experiments on purely wurtzite GaAs/AlGaAs core/shell
nanowires demonstrate optical spin injection into a single free-standing
nanowire and determine the effective electron g-factor of the hexagonal GaAs
wurtzite phase. The spin relaxation is highly anisotropic in time-resolved
micro-photoluminescence measurements on single nanowires, showing a significant
increase of spin relaxation in external magnetic fields. This behavior is
counterintuitive compared to bulk wurtzite crystals. We present a model for the
observed electron spin dynamics highlighting the dominant role of the
interface-induced SOC in these core/shell nanowires. This enhanced SOC may
represent an interesting tuning parameter for the implementation of
spin-orbitronic concepts in semiconductor-based structures
Ecological selection of siderophore-producing microbial taxa in response to heavy metal contamination
Some microbial public goods can provide both individual and communityâwide benefits, and are open to exploitation by nonâproducing species. One such example is the production of metalâdetoxifying siderophores. Here, we investigate whether conflicting selection pressures on siderophore production by heavy metals â a detoxifying effect of siderophores, and exploitation of this detoxifying effect â result in a net increase or decrease. We show that the proportion of siderophoreâproducing taxa increases along a natural heavy metal gradient. A causal link between metal contamination and siderophore production was subsequently demonstrated in a microcosm experiment in compost, in which we observed changes in community composition towards taxa that produce relatively more siderophores following copper contamination. We confirmed the selective benefit of siderophores by showing that taxa producing large amounts of siderophore suffered less growth inhibition in toxic copper. Our results suggest that ecological selection will favour siderophoreâmediated decontamination, with important consequences for potential remediation strategies
How do microbiota associated with an invasive seaweed vary across scales?
Communities are shaped by scale dependent processes. To study the diversity and variation of microbial communities across scales, the invasive and widespread seaweed Agarophyton vermiculophyllum presents a unique opportunity. We characterized proâ and eukaryotic communities associated with this holobiont across its known distribution range, which stretches over the northern hemisphere. Our data reveal that community composition and diversity in the holobiont vary at local but also larger geographic scales. While processes acting at the local scale (i.e., within population) are the main structuring drivers of associated microbial communities, changes in community composition also depend on processes acting at larger geographic scales. Interestingly, the largest analysed scale (i.e., native and nonânative ranges) explained variation in the prevalence of predicted functional groups, which could suggest a functional shift in microbiota occurred over the course of the invasion process. While high variability in microbiota at the local scale supports A. vermiculophyllum to be a generalist host, we also identified a number of core taxa. These geographically independent holobiont members imply that cointroduction of specific microbiota may have additionally promoted the invasion process
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