New ultrarapid-scanning interferometer for FT-IR spectroscopy with microsecond time-resolution

A novel Fourier-transform infrared (FT-IR) rapid-scan spectrometer has been developed (patent pending EP14194520.4) which yields 1000 times higher time resolution as compared to conventional rapid-scanning spectrometers. The central element to achieve faster scanning rates is based on a sonotrode whose front face represents the movable mirror of the interferometer. A prototype spectrometer with a time resolution of 13 μs was realized, capable of fully automated long-term measurements with a flow cell for liquid samples, here a photosynthetic membrane protein in solution. The performance of this novel spectrometer is demonstrated by recording the photoreaction of bacteriorhodopsin initiated by a short laser pulse that is synchronized to the data recording. The resulting data are critically compared to those obtained by step-scan spectroscopy and demonstrate the relevance of performing experiments on proteins in solution. The spectrometer allows for future investigations of fast, non-repetitive processes, whose investigation is challenging to step-scan FT-IR spectroscopy

Calculation of coercivity of magnetic nanostructures at finite temperatures

We report a finite temperature micromagnetic method (FTM) that allows for the calculation of the coercive field of arbitrary shaped magnetic nanostructures at time scales of nanoseconds to years. Instead of directly solving the Landau-Lifshitz-Gilbert equation, the coercive field is obtained without any free parameter by solving a non linear equation, which arises from the transition state theory. The method is applicable to magnetic structures where coercivity is determined by one thermally activated reversal or nucleation process. The method shows excellent agreement with experimentally obtained coercive fields of magnetic nanostructures and provides a deeper understanding of the mechanism of coercivity.Comment: submitted to Phys. Rev.

Sensitive Room-Temperature Terahertz Detection via Photothermoelectric Effect in Graphene

Terahertz (THz) radiation has uses from security to medicine; however, sensitive room-temperature detection of THz is notoriously difficult. The hot-electron photothermoelectric effect in graphene is a promising detection mechanism: photoexcited carriers rapidly thermalize due to strong electron-electron interactions, but lose energy to the lattice more slowly. The electron temperature gradient drives electron diffusion, and asymmetry due to local gating or dissimilar contact metals produces a net current via the thermoelectric effect. Here we demonstrate a graphene thermoelectric THz photodetector with sensitivity exceeding 10 V/W (700 V/W) at room temperature and noise equivalent power less than 1100 pW/Hz^1/2 (20 pW/Hz^1/2), referenced to the incident (absorbed) power. This implies a performance which is competitive with the best room-temperature THz detectors for an optimally coupled device, while time-resolved measurements indicate that our graphene detector is eight to nine orders of magnitude faster than those. A simple model of the response, including contact asymmetries (resistance, work function and Fermi-energy pinning) reproduces the qualitative features of the data, and indicates that orders-of-magnitude sensitivity improvements are possible.Comment: Published 07 September 2014 in Nature Nanotechnolog

Evaluating Molecular Cobalt Complexes for the Conversion of N_2 to NH_3

We report a molecular Co−N_2 complex that generates a greater-than-stoichiometric yield of NH_3 (>200% NH_3 per Co−N_2 precursor) via the direct reduction of N_2 with protons and electrons. A comparison of the featured Co−N_2 complex with structurally related Co−N_2 and Fe−N_2 species shows how remarkably sensitive the N_2 reduction performance of potential precatalysts is. As discussed, structural and electronic effects are relevant to Co/Fe−N_2 conversion activity, including π basicity, charge state, and geometric flexibility

Probing the Edge of the Solar System: Formation of an Unstable Jet-Sheet

The Voyager spacecraft is now approaching the edge of the solar system. Near the boundary between the solar system and the interstellar medium we find that an unstable ``jet-sheet'' forms. The jet-sheet oscillates up and down due to a velocity shear instability. This result is due to a novel application of a state-of-art 3D Magnetohydrodynamic (MHD) code with a highly refined grid. We assume as a first approximation that the solar magnetic and rotation axes are aligned. The effect of a tilt of the magnetic axis with respect to the rotation axis remains to be seen. We include in the model self-consistently magnetic field effects in the interaction between the solar and interstellar winds. Previous studies of this interaction had poorer spatial resolution and did not include the solar magnetic field. This instability can affect the entry of energetic particles into the solar system and the intermixing of solar and interstellar material. The same effect found here is predicted for the interaction of rotating magnetized stars possessing supersonic winds and moving with respect to the interstellar medium, such as O stars.Comment: 9 pages, 4 figures, accepted for publication in ApJ

Design principles for riboswitch function

Scientific and technological advances that enable the tuning of integrated regulatory components to match network and system requirements are critical to reliably control the function of biological systems. RNA provides a promising building block for the construction of tunable regulatory components based on its rich regulatory capacity and our current understanding of the sequence–function relationship. One prominent example of RNA-based regulatory components is riboswitches, genetic elements that mediate ligand control of gene expression through diverse regulatory mechanisms. While characterization of natural and synthetic riboswitches has revealed that riboswitch function can be modulated through sequence alteration, no quantitative frameworks exist to investigate or guide riboswitch tuning. Here, we combined mathematical modeling and experimental approaches to investigate the relationship between riboswitch function and performance. Model results demonstrated that the competition between reversible and irreversible rate constants dictates performance for different regulatory mechanisms. We also found that practical system restrictions, such as an upper limit on ligand concentration, can significantly alter the requirements for riboswitch performance, necessitating alternative tuning strategies. Previous experimental data for natural and synthetic riboswitches as well as experiments conducted in this work support model predictions. From our results, we developed a set of general design principles for synthetic riboswitches. Our results also provide a foundation from which to investigate how natural riboswitches are tuned to meet systems-level regulatory demands

Readout for intersatellite laser interferometry: Measuring low frequency phase fluctuations of HF signals with microradian precision

Precision phase readout of optical beat note signals is one of the core techniques required for intersatellite laser interferometry. Future space based gravitational wave detectors like eLISA require such a readout over a wide range of MHz frequencies, due to orbit induced Doppler shifts, with a precision in the order of $\mu \textrm{rad}/\sqrt{\textrm{Hz}}$ at frequencies between $0.1\,\textrm{mHz}$ and $1\,\textrm{Hz}$. In this paper, we present phase readout systems, so-called phasemeters, that are able to achieve such precisions and we discuss various means that have been employed to reduce noise in the analogue circuit domain and during digitisation. We also discuss the influence of some non-linear noise sources in the analogue domain of such phasemeters. And finally, we present the performance that was achieved during testing of the elegant breadboard model of the LISA phasemeter, that was developed in the scope of an ESA technology development activity.Comment: submitted to Review of Scientific Instruments on April 30th 201