Fabrication of quantum point contacts by engraving GaAs/AlGaAs-heterostructures with a diamond tip

We use the all-diamond tip of an atomic force microscope for the direct engraving of high quality quantum point contacts in GaAs/AlGaAs-heterostructures. The processing time is shortened by two orders of magnitude compared to standard silicon tips. Together with a reduction of the line width to below 90 nm the depletion length of insulating lines is reduced by a factor of two with the diamond probes. The such fabricated defect free ballistic constrictions show well resolved conductance plateaus and the 0.7 anomaly in electronic transport measurements.Comment: 3 pages, 3 figure

Certification of a class of industrial predictive controllers without terminal conditions

Three decades have passed encompassing a flurry of research and commercial activities in model predictive control (MPC). However, the massive strides made by the academic community in guaranteeing stability through a state- space framework have not always been directly applicable in an industrial setting. This paper is concerned with a priori and/or a posteriori certification of persistent feasibility, boundedness of industrial MPC controllers (i) based on input-output formu- lation (ii) using shorter control than prediction horizon (iii) and without terminal conditions.This work has been supported by FDOC, UGent

Evaluation of the synoptic and mesoscale predictive capabilities of a mesoscale atmospheric simulation system

The overall performance characteristics of a limited area, hydrostatic, fine (52 km) mesh, primitive equation, numerical weather prediction model are determined in anticipation of satellite data assimilations with the model. The synoptic and mesoscale predictive capabilities of version 2.0 of this model, the Mesoscale Atmospheric Simulation System (MASS 2.0), were evaluated. The two part study is based on a sample of approximately thirty 12h and 24h forecasts of atmospheric flow patterns during spring and early summer. The synoptic scale evaluation results benchmark the performance of MASS 2.0 against that of an operational, synoptic scale weather prediction model, the Limited area Fine Mesh (LFM). The large sample allows for the calculation of statistically significant measures of forecast accuracy and the determination of systematic model errors. The synoptic scale benchmark is required before unsmoothed mesoscale forecast fields can be seriously considered

A label-free microfluidic assay to quantitatively study antibiotic diffusion through lipid membranes

PublishedJournal ArticleResearch Support, Non-U.S. Gov'tWith the rise in antibiotic resistance amongst pathogenic bacteria, the study of antibiotic activity and transport across cell membranes is gaining widespread importance. We present a novel, label-free microfluidic assay that quantifies the permeability coefficient of a broad spectrum fluoroquinolone antibiotic, norfloxacin, across lipid membranes using the UV autofluorescence of the drug. We use giant lipid vesicles as highly controlled model systems to study the diffusion through lipid membranes. Our technique directly determines the permeability coefficient without requiring the measurement of the partition coefficient of the antibiotic.This work was supported by a European Research Council (ERC) grant (261101 PassMembrane) to UFK. JC acknowledges support from an Internal Graduate Studentship, Trinity College, Cambridge. CC is supported by the ERC. SP acknowledges the support of the Leverhulme Trust and the Newton Trust through an Early Career Fellowship. AJ is supported by the Mexican National Council of Science and Technology. We thank Thomas Muller for help with the lithography and Tuomas Knowles for the use of his lithography facilitie

Interacting electrons on a quantum ring: exact and variational approach

We study a system of interacting electrons on a one-dimensional quantum ring using exact diagonalization and the variational quantum Monte Carlo method. We examine the accuracy of the Slater-Jastrow -type many-body wave function and compare energies and pair distribution functions obtained from the two approaches. Our results show that this wave function captures most correlation effects. We then study the smooth transition to a regime where the electrons localize in the rotating frame, which for the ultrathin quantum ring system happens at quite high electron density.Comment: 19 pages, 10 figures. Accepted for publication in the New Journal of Physic

Critical shoe contact area ratio for sliding on a tennis hard court

Dimples have been used in the design of some modern tennis shoe outsoles to enhance sliding ability on hard courts. Experiments were performed with bespoke rubber samples possessing various numbers of holes, which served to simulate dimples in tennis shoe treads. The aim of the research was to assess the effect of contact area on sliding friction. As the ratio of holes to solid rubber increased, a critical ratio was reached whereby the static friction coefficient decreased by more than 11% for tread-to-court pressures comparable to real tennis play. Although this study analyzed bespoke rubber samples and not actual tennis shoe treads, shoe manufacturers should be interested in the existence of a critical dimple ratio that could aid them in the creation of tennis shoes suited for sliding on hard courts

The detection of ultra-relativistic electrons in low Earth orbit

Aims. To better understand the radiation environment in low Earth orbit (LEO), the analysis of in-situ observations of a variety of particles, at different atmospheric heights, and in a wide range of energies, is needed. Methods. We present an analysis of energetic particles, indirectly detected by the Large Yield RAdiometer (LYRA) instrument on board ESA's Project for On-board Autonomy 2 (PROBA2) satellite as background signal. Combining Energetic Particle Telescope (EPT) observations with LYRA data for an overlapping period of time, we identified these particles as electrons with an energy range of 2 to 8 MeV. Results. The observed events are strongly correlated to geo-magnetic activity and appear even during modest disturbances. They are also well confined geographically within the L=4-6 McIlwain zone, which makes it possible to identify their source. Conclusions. Although highly energetic particles are commonly perturbing data acquisition of space instruments, we show in this work that ultra-relativistic electrons with energies in the range of 2-8 MeV are detected only at high latitudes, while not present in the South Atlantic Anomaly region.Comment: Topical Issue: Flares, CMEs and SEPs and their space weather impacts; 20 pages; 7 figures; Presented during 13th European Space Weather Week, 201

Direct visualization of magnetic vortex pinning in superconductors

We study the vortex structure in a Pb film deposited on top of a periodic array of ferromagnetic square microrings by combining two high resolution imaging techniques: Bitter decoration and scanning Hall probe microscopy (SHPM). The periodicity and strength of the magnetic pinning potential generated by the square microrings are controlled by the magnetic history of the template. When the square rings are in the magnetized dipolar state, known as the onion state, the strong stray field generated at the domain walls prevents the decoration of vortices. SHPM images show that the stray field generated by the dipoles is much stronger than the vortex field in agreement with the results of simulations. Real space vortex imaging has revealed that, in the onion state, the corners of the square rings act as effective pinning centers for vortices.Comment: To be published in Phys. Rev.