Method for atomic-layer-resolved measurement of polarization fields by nuclear magnetic resonance

A nuclear magnetic resonance (NMR) method of probing the dielectric response to an alternating electric field is described, which is applicable to noncentrosymmetric sites with nuclear spin I>1/2. A radio-frequency electric field induces a linear quadrupole Stark effect at a multiple of the nuclear Larmor frequency. This perturbation is applied in the windows of an NMR multiple-pulse line-narrowing sequence in such a way that the resulting nonsecular spin interactions are observed as first-order quadrupole satellites, free of line broadening by the usual dominant static interactions. A simulation of the 69Ga spectrum for the nuclei within the two-dimensional electron gas of a 10 nm quantum well predicts resolution of individual atomic layers in single devices due to the spatial dependence of the polarization response of the quantum-confined carriers to the applied field. This method is part of a more general strategy, perturbations observed with enhanced resolution NMR. Experimentally realized examples in GaAs include spectrally resolving electron probability densities surrounding optically relevant point defects and probing the changes in radial electric field associated with the light-on and light-off states of these shallow traps. Adequate sensitivity for such experiments in individual epitaxial structures is achieved by optical nuclear polarization followed by time-domain NMR observed via nuclear Larmor-beat detection of luminescence

An Efficient Sliding Mesh Interface Method for High-Order Discontinuous Galerkin Schemes

Sliding meshes are a powerful method to treat deformed domains in computational fluid dynamics, where different parts of the domain are in relative motion. In this paper, we present an efficient implementation of a sliding mesh method into a discontinuous Galerkin compressible Navier-Stokes solver and its application to a large eddy simulation of a 1-1/2 stage turbine. The method is based on the mortar method and is high-order accurate. It can handle three-dimensional sliding mesh interfaces with various interface shapes. For plane interfaces, which are the most common case, conservativity and free-stream preservation are ensured. We put an emphasis on efficient parallel implementation. Our implementation generates little computational and storage overhead. Inter-node communication via MPI in a dynamically changing mesh topology is reduced to a bare minimum by ensuring a priori information about communication partners and data sorting. We provide performance and scaling results showing the capability of the implementation strategy. Apart from analytical validation computations and convergence results, we present a wall-resolved implicit LES of the 1-1/2 stage Aachen turbine test case as a large scale practical application example

Towards Exascale CFD Simulations Using the Discontinuous Galerkin Solver FLEXI

Modern high-order discretizations bear considerable potential for the exascale era due to their high fidelity and the high, local computational load that allows for computational efficiency in massively parallel simulations. To this end, the discontinuous Galerkin (DG) framework FLEXI was selected to demonstrate exascale readiness within the Center of Excellence for Exascale CFD (CEEC) by simulating shock buffet on a three-dimensional wing segment at transsonic flight conditions. This paper summarizes the recent progress made to enable the simulation of this challenging exascale problem. For this, it is first demonstrated that FLEXI scales excellently to over 500 000 CPU cores on HAWK at the HLRS. To tackle the considerable resolution requirements near the wall, a novel wall model is proposed that takes compressibility effects into account and yields decent results for the simulation of a NACA 64A-110 airfoil. To address the shocks in the domain, a finite-volume-based shock capturing method was implemented in FLEXI, which is validated here using the simulation of a linear compressor cascade at supersonic flow conditions, where the method is demonstrated to yield efficient, robust and accurate results. Lastly, we present the TensorFlow-Fortran-Binding (TFFB) as an easy-to-use library to deploy trained machine learning models in Fortran solvers such as FLEXI.Comment: 15 pages, 5 figure

Preparation and reactivity of biomass-derived dihydro-dioxins

This work was supported by EPSRC PhD studentships EP/1654168 (JRDM) and EP/1518175 (DMMB) and the Industrial Biotechnology Innovation Centre (DMMB).The depolymerisation of the biopolymer lignin can give pure aromatic monomers but selective catalytic approaches remain scarce. Here, an approach was re-routed to deliver an unusual phenolic monomer. This monomer’s instability proved challenging but a degradation study identified strategies to overcome this. Heterocycles and a useful synthetic intermediate were prepared. The range of aromatics available from the b-O-4 unit in lignin was extended.PostprintPeer reviewe

Evaluation of an immunomagnetic separation method to capture Candida yeasts cells in blood

BackgroundCandida species have become the fourth most-frequent cause of nosocomial bloodstream infections in immunocompromised patients. Therefore, rapid identification of pathogenic fungi to species level has been considered critical for treatment. Conventional diagnostic procedures such as blood culture or biochemical tests are lacking both sensitivity and species specificity, so development of rapid diagnostic is essential. Results An immunomagnetic method involving anti-Candida monoclonal antibodies was developed to capture and concentrate in human blood four different species of Candida cells responsible for invasive yeast infections. In comparison with an automated blood culture, processing time of immunomagnetic separation is shorter, saving at least 24 hours to obtain colonies before identification. Conclusion Thus, this easy to use method provides a promising basis for concentrating all Candida species in blood to improve sensitivity before identification

Flux ramp modulation based MHz frequency-division dc-SQUID multiplexer

We present a MHz frequency-division dc-SQUID multiplexer that is based on flux ramp modulation and a series array of $N$ identical current-sensing dc-SQUIDs with tightly coupled input coil. By running a periodic, sawtooth-shaped current signal through an additional modulation coil being tightly, but non-uniformly coupled to the individual SQUIDs, the voltage drop across the array changes according to the superposition of the flux-to-voltage characteristics of the individual SQUIDs within each cycle of the modulation signal. In this mode of operation, an input signal injected in the input coil of one of the SQUIDs and being quasi-static within a time frame adds a constant flux offset and leads to a phase shift of the associated SQUID characteristics. The latter is inherently proportional to the input signal and can be inferred by channelizing and down-converting the sampled array output voltage. Using a prototype multiplexer as well as a self-developed high-speed readout electronics for real-time phase determination, we demonstrate the simultaneous readout of four signal sources with MHz bandwidth per channel.Comment: The article has been submitted to Applied Physics Letter

A First-Quantized Formalism for Cosmological Particle Production

We show that the amount of particle production in an arbitrary cosmological background can be determined using only the late-time positive-frequency modes. We don't refer to modes at early times, so there is no need for a Bogolubov transformation. We also show that particle production can be extracted from the Feynman propagator in an auxiliary spacetime. This provides a first-quantized formalism for computing particle production which, unlike conventional Bogolubov transformations, may be amenable to a string-theoretic generalization.Comment: 18 pages, LaTeX; v2: significantly revised for clarity; conclusions unchange