Joule heating and spin-transfer torque investigated on the atomic scale using a spin-polarized scanning tunneling microscope

The influence of a high spin-polarized tunnel current onto the switching behavior of a superparamagnetic nanoisland on a nonmagnetic substrate is investigated by means of spin-polarized scanning tunneling microscopy. A detailed lifetime analysis allows for a quantification of the effective temperature rise of the nanoisland and the modification of the activation energy barrier for magnetization reversal, thereby using the nanoisland as a local thermometer and spin-transfer torque analyzer. Both the Joule heating and spin-transfer torque are found to scale linearly with the tunnel current. The results are compared to experiments performed on lithographically fabricated magneto-tunnel junctions, revealing a very high spin-transfer torque switching efficiency in our experiments.     

Periodic nanostructures: spatial dispersion mimics chirality

Polarization rotation in isotropic materials is commonly associated with chirality, i.e., structures with a handedness which are not identical with their mirror image. We observe this effect in the visible and near-IR regions at oblique incidence in the optical response of a subwavelength square array of holes. Mapping the complete k space via Mueller-matrix spectroscopic ellipsometry, we find that in specific directions the rotary power is orders of magnitude larger than that observed for chiral molecules. Although experimentally indistinguishable, the physics behind the two phenomena is fundamentally different: While optical activity is a consequence of magnetic interactions, nanostructures on a square lattice rotate the polarization due to spatial dispersion.     

Ultrafast transport of laser-excited spin-polarized carriers in Au/Fe/MgO(001)

Hot carrier-induced spin dynamics is analyzed in epitaxial Au/Fe/MgO(001) by a time domain approach. We excite a spin current pulse in Fe by 35 fs laser pulses. The transient spin polarization, which is probed at the Au surface by optical second harmonic generation, changes its sign after a few hundred femtoseconds. This is explained by a competition of ballistic and diffusive propagation considering energy-dependent hot carrier relaxation rates. In addition, we observe the decay of the spin polarization within 1 ps, which is associated with the hot carrier spin relaxation time in Au.     

Gradient enhanced third harmonic generation in a femtosecond filament

The third harmonic generated during femtosecond filamentation in air is studied. By establishing a gradient from atmospheric pressure to vacuum conditions, we truncate the filament abruptly at defined positions. The introduction of the pressure gradient leads to an enhancement of the generated third harmonic radiation by 3 orders of magnitude. This effect is attributed to an improved on-axis phase-matching condition. We investigate the spectral shape and the conversion efficiency of the third harmonic during the propagation in the filament.     

Initiation and inhibition of dealloying of single crystalline Cu3Au (111) surfaces

Dealloying is widely utilized but is a dangerous corrosion process as well. Here we report an atomistic picture of the initial stages of electrochemical dealloying of the model system Cu(3)Au (111). We illuminate the structural and chemical changes during the early stages of dissolution up to the critical potential, using a unique combination of advanced surface-analytical tools. Scanning tunneling microscopy images indicate an interlayer exchange of topmost surface atoms during initial dealloying, while scanning Auger-electron microscopy data clearly reveal that the surface is fully covered by a continuous Au-rich layer at an early stage. Initiating below this first layer a transformation from stacking-reversed toward substrate-oriented Au surface structures is observed close to the critical potential. We further use the observed structural transitions as a reference process to evaluate the mechanistic changes induced by a thiol-based model-inhibition layer applied to suppress surface diffusion. The initial ultrathin Au layer is stabilized with the intermediate island morphology completely suppressed, along an anodic shift of the breakdown potential. Thiol-modification induces a peculiar surface microstructure in the form of microcracks exhibiting a nanoporous core. On the basis of the presented atomic-scale observations, an interlayer exchange mechanism next to pure surface diffusion becomes obvious which may be controlling the layer thickness and its later change in orientation.     

An all-photonic molecule-based D flip-flop

The photochromic fluorescence switching of a fulgimide derivative was used to implement the first molecule-based D (delay) flip-flop device, which works based on the principles of sequential logic. The device operates exclusively with photonic signals and can be conveniently switched in repeated cycles.     

Structural fine-tuning of a multifunctional cytochrome P450 monooxygenase

AurH is a unique cytochrome P450 monooxygenase catalyzing the stepwise formation of a homochiral oxygen heterocycle, a key structural and pharmacophoric component of the antibiotic aureothin. The exceptional enzymatic reaction involves a tandem oxygenation process including a regio- and stereospecific hydroxylation, followed by heterocyclization. For the structural and biochemical basis of this unparalleled sequence, four crystal structures of AurH variants in different conformational states and in complex with the P450 inhibitor ancymidol were solved, which represent the first structures of the CYP151A group. Structural data in conjunction with computational docking, site-directed mutagenesis, and chemical analyses unveiled a switch function when recognizing the two substrates, deoxyaureothin and the hydroxylated intermediate, thus allowing the second oxygenation-heterocyclization step. Furthermore, we were able to modify the chemo- and regioselectivity of AurH, yielding mutants that catalyze the regioselective six-electron transfer of a nonactivated methyl group to a carboxylic acid via hydroxyl and aldehyde intermediates.     

Colorimetric bacteria sensing using a supramolecular enzyme-nanoparticle biosensor

Rapid and sensitive detection of pathogens is a key requirement for both environmental and clinical settings. We report here a colorimetric enzyme-nanoparticle conjugate system for detection of microbial contamination. In this approach, cationic gold nanoparticles (NPs) featuring quaternary amine headgroups are electrostatically bound to an enzyme [β-galactosidase (β-Gal)], inhibiting enzyme activity. Analyte bacteria bind to the NP, which releases the β-Gal and restores its activity, providing an enzyme-amplified colorimetric readout of the binding event. Using this strategy, we have been able to quantify bacteria at concentrations of 1 × 10(2) bacteria/mL in solution and 1 × 10(4) bacteria/mL in a field-friendly test strip format.     

Biosynthesis of the siderophore rhodochelin requires the coordinated expression of three independent gene clusters in Rhodococcus jostii RHA1

In this work we report the isolation, structural characterization, and the genetic analysis of the biosynthetic origin of rhodochelin, a unique mixed-type catecholate-hydroxamate siderophore isolated from Rhodococcus jostii RHA1. Rhodochelin structural elucidation was accomplished via MS(n)- and NMR-analysis and revealed the tetrapeptide to contain an unusual ester bond between an L-δ-N-formyl-δ-N-hydroxyornithine moiety and the side chain of a threonine residue. Gene deletions within three putative biosynthetic gene clusters abolish rhodochelin production, proving that the ORFs responsible for rhodochelin biosynthesis are located in different chromosomal loci. These results demonstrate the efficient cross-talk between distantly located secondary metabolite gene clusters and outline new insights into the comprehension of natural product biosynthesis.     

Effective and Reversible Carbon Dioxide Insertion into Cerium Pyrazolates

The homoleptic pyrazolate complexes [Ce^III₄(Me₂pz)₁₂] and [Ce^IV(Me₂pz)₄]₂ quantitatively insert CO₂ to give [Ce^III₄(Me₂pz?CO₂)₁₂] and [Ce^IV(Me₂pz?CO₂)₄], respectively (Me₂pz=3,5‐dimethylpyrazolato). This process is reversible for both complexes, as observed by in situ IR and NMR spectroscopy in solution and by TGA in the solid state. By adjusting the molar ratio, one molecule of CO₂ per [Ce^IV(Me₂pz)₄] complex could be inserted to give trimetallic [Ce₃(Me₂pz)₉(Me₂pz?CO₂)₃(thf)]. Both the cerous and ceric insertion products catalyze the formation of cyclic carbonates from epoxides and CO₂ under mild conditions. In the absence of epoxide, the ceric catalyst is prone to reduction by the co‐catalyst tetra‐n‐butylammonium bromide (TBAB).