Probing charge pumping and relaxation of the chiral anomaly in a Dirac semimetal.

The linear band crossings of 3D Dirac and Weyl semimetals are characterized by a charge chirality, the parallel or antiparallel locking of electron spin to its momentum. These materials are believed to exhibit an E · B chiral magnetic effect that is associated with the near conservation of chiral charge. Here, we use magneto-terahertz spectroscopy to study epitaxial Cd3As2 films and extract their conductivities σ(ω) as a function of E · B. As field is applied, we observe a markedly sharp Drude response that rises out of the broader background. Its appearance is a definitive signature of a new transport channel and consistent with the chiral response, with its spectral weight a measure of the net chiral charge and width a measure of the scattering rate between chiral species. The field independence of the chiral relaxation establishes that it is set by the approximate conservation of the isospin that labels the crystalline point-group representations.

Room-Temperature Spin Transport in Cd3As2.

As the need for ever greater transistor density increases, the commensurate decrease in device size approaches the atomic limit, leading to increased energy loss and leakage currents, reducing energy efficiencies. Alternative state variables, such as electronic spin rather than electronic charge, have the potential to enable more energy-efficient and higher performance devices. These spintronic devices require materials capable of efficiently harnessing the electron spin. Here we show robust spin transport in Cd3As2 films up to room temperature. We demonstrate a nonlocal spin valve switch from this material, as well as inverse spin Hall effect measurements yielding spin Hall angles as high as θSH = 1.5 and spin diffusion lengths of 10-40 µm. Long spin-coherence lengths with efficient charge-to-spin conversion rates and coherent spin transport up to room temperature, as we show here in Cd3As2, are enabling steps toward realizing actual spintronic devices.

Efficient Terahertz Harmonic Generation with Coherent Acceleration of Electrons in the Dirac Semimetal Cd_{3}As_{2}.

We report strong terahertz (∼10^{12} Hz) high harmonic generation at room temperature in thin films of Cd_{3}As_{2}, a three-dimensional Dirac semimetal. Third harmonics are detectable with a tabletop light source and can be as strong as 100 V/cm by applying a fundamental field of 6.5 kV/cm inside the film, demonstrating an unprecedented efficiency for terahertz frequency conversion. Our time-resolved terahertz spectroscopy and calculations also clarify the microscopic mechanism of the nonlinearity originating in the coherent acceleration of Dirac electrons in momentum space. Our results provide clear insights for nonlinear currents of Dirac electrons driven by the terahertz field under the influence of scattering, paving the way toward novel devices for high-speed electronics and photonics based on topological semimetals.

Observation of the Quantum Hall Effect in Confined Films of the Three-Dimensional Dirac Semimetal Cd_{3}As_{2}.

The magnetotransport properties of epitaxial films of Cd_{3}As_{2}, a paradigm three-dimensional Dirac semimetal, are investigated. We show that an energy gap opens in the bulk electronic states of sufficiently thin films and, at low temperatures, carriers residing in surface states dominate the electrical transport. The carriers in these states are sufficiently mobile to give rise to a quantized Hall effect. The sharp quantization demonstrates surface transport that is virtually free of parasitic bulk conduction and paves the way for novel quantum transport studies in this class of topological materials. Our results also demonstrate that heterostructuring approaches can be used to study and engineer quantum states in topological semimetals.

Bias in recent miRBase annotations potentially associated with RNA quality issues.

Although microRNAs are supposed to be stable in-vivo, degradation processes potentially blur our knowledge on the small oligonucleotides. We set to quantify the effect of degradation on microRNAs in mouse to identify causes for distorted microRNAs patterns. In liver, we found 298, 99 and 8 microRNAs whose expression significantly correlated to RNA integrity, storage time at room temperature and storage time at 4 °C, respectively. Expression levels of 226 microRNAs significantly differed between liver samples with high RNA integrity compared to liver samples with low RNA integrity by more than two-fold. Especially the 157 microRNAs with increased expression in tissue samples with low RNA integrity were most recently added to miRBase. Testing potentially confounding sources, e.g. in-vitro degraded RNA depleted of small RNAs, we detected signals for 350 microRNAs, suggesting cross-hybridization of fragmented RNAs. Therefore, we conclude that especially microRNAs added in the latest miRBase versions might be artefacts due to RNA degradation. The results facilitate differentiation between degradation-resilient microRNAs, degradation-sensitive microRNAs, and likely erroneously annotated microRNAs. The latter were largely identified by NGS but not experimentally validated and can severely bias microRNA biomarker research and impact the value of microRNAs as diagnostic, prognostic or therapeutic tools.

Innate immunity in CKD-associated vascular diseases.

Chronic kidney disease (CKD) is associated with an increased risk for cardiovascular events. Therefore, the activation of the innate immune system plays an important role. In contrast to the adaptive immunity, unspecific recognition of conserved endogenous and exogenous structures by pattern recognition receptors (PRRs) represents a key feature of the innate immunity. Of these PRRs, Toll-like receptors (TLRs) as well as the inflammasome complex have been documented to be involved in the pathogenesis of cardiovascular diseases (CVDs). They are not only expressed in leukocytes but also in a variety of cell types such as endothelial cells or fibroblasts. While activation of TLRs on the cell surface leads to nuclear factor κB-dependent expression of pro-inflammatory mediators, the inflammasome is a cytosolic multimeric protein complex, which cleaves cytokines such as interleukin-1ß into their biologically active forms. Several endogenous ligands for these PRRs have been identified as contributing to the development of a CKD-specific pro-inflammatory microenvironment. Notably, activation of TLRs as well as the inflammasome is associated with arterial hypertension, formation of atherosclerotic vascular lesions and vascular calcification. However, detailed molecular mechanisms on how the innate immune system contributes to CKD-associated CVDs are as yet poorly understood. Currently, several agents modulating the activation of the innate immune system are the focus of cardiovascular research. Large clinical studies will provide further information on the therapeutic applicability of these substances to reduce cardiovascular morbidity and mortality in the general population. Further trials including patients with CKD will be necessary to assess their effects on CKD-associated CVD.

Synthesis of quasi-free-standing bilayer graphene nanoribbons on SiC surfaces.

Scaling graphene down to nanoribbons is a promising route for the implementation of this material into devices. Quantum confinement of charge carriers in such nanostructures, combined with the electric field-induced break of symmetry in AB-stacked bilayer graphene, leads to a band gap wider than that obtained solely by this symmetry breaking. Consequently, the possibility of fabricating AB-stacked bilayer graphene nanoribbons with high precision is very attractive for the purposes of applied and basic science. Here we show a method, which includes a straightforward air annealing, for the preparation of quasi-free-standing AB-bilayer nanoribbons with different widths on SiC(0001). Furthermore, the experiments reveal that the degree of disorder at the edges increases with the width, indicating that the narrower nanoribbons are more ordered in their edge termination. In general, the reported approach is a viable route towards the large-scale fabrication of bilayer graphene nanostructures with tailored dimensions and properties for specific applications.

Recognition of bacterial signal peptides by mammalian formyl peptide receptors: a new mechanism for sensing pathogens.

Formyl peptide receptors (FPRs) are G-protein-coupled receptors that function as chemoattractant receptors in innate immune responses. Here we perform systematic structure-function analyses of FPRs from six mammalian species using structurally diverse FPR peptide agonists and identify a common set of conserved agonist properties with typical features of pathogen-associated molecular patterns. Guided by these results, we discover that bacterial signal peptides, normally used to translocate proteins across cytoplasmic membranes, are a vast family of natural FPR agonists. N-terminally formylated signal peptide fragments with variable sequence and length activate human and mouse FPR1 and FPR2 at low nanomolar concentrations, thus establishing FPR1 and FPR2 as sensitive and broad signal peptide receptors. The vomeronasal receptor mFpr-rs1 and its sequence orthologue hFPR3 also react to signal peptides but are much more narrowly tuned in signal peptide recognition. Furthermore, all signal peptides examined here function as potent activators of the innate immune system. They elicit robust, FPR-dependent calcium mobilization in human and mouse leukocytes and trigger a range of classical innate defense mechanisms, such as the production of reactive oxygen species, metalloprotease release, and chemotaxis. Thus, bacterial signal peptides constitute a novel class of immune activators that are likely to contribute to mammalian immune defense against bacteria. This evolutionarily conserved detection mechanism combines structural promiscuity with high specificity and enables discrimination between bacterial and eukaryotic signal sequences. With at least 175,542 predicted sequences, bacterial signal peptides represent the largest and structurally most heterogeneous class of G-protein-coupled receptor agonists currently known for the innate immune system.

Formyl peptide receptors from immune and vomeronasal system exhibit distinct agonist properties.

The formyl peptide receptor (Fpr) family is well known for its contribution to immune defense against pathogens in human and rodent leukocytes. Recently, several structurally related members of these receptors were discovered in sensory neurons of the mouse vomeronasal organ (VNO), key detectors of pheromones and related semiochemicals. Although the biological role of vomeronasal Fprs is not yet clear, the known contribution of other Fprs to host immune defense suggested that they could contribute to vomeronasal pathogen sensing. Precise knowledge about the agonist properties of mouse Fprs is required to determine their function. We expressed all seven mouse and three human Fprs using an in vitro system and tested their activation with 32 selected compounds by conducting high throughput calcium measurements. We found an intriguing functional conservation between human and mouse immune Fprs that is most likely a consequence of closely similar biological constraints. By contrast, our data suggest a neofunctionalization of the vomeronasal Fprs. We show that the vomeronasal receptor mFpr-rs1 can be activated robustly by W-peptide and structural derivatives but not by other typical ligands of immune Fprs. mFpr-rs1 exhibits a stereo-selective preference for peptides containing d-amino acids. The same peptide motifs are contained in pathogenic microorganisms. Thus, the ligand profile of mFpr-rs1 is consistent with a role in vomeronasal pathogen sensing.