Computations were performed to determine the optimal conditions for the suppression of vortex shedding from circular cylinders at high Reynolds number by means of splitter plates. Previous studies of this mechanism for vortex-shedding control have largely been confined to two-dimensional flows at low Reynolds number, and to a single ratio of splitter plate width to cylinder diameter. In this study, we consider fully-turbulent, three-dimensional flows at high Reynolds number, and investigate the dependence of vortex-shedding suppression on two geometric parameters; namely the ratio of splitter-plate width and height to cylinder diameter and length. The computations were performed with the OpenFOAM software using a well-validated turbulence closure that considers the effects of the organized mean-flow unsteadiness on the random turbulent fluctuations. Comparisons were made with experimental data, and the validated method is used to perform a systematic study to determine the effectiveness of vortex suppression. To aid in the analysis, two-point correlations of forces along span of cylinder were obtained to determine uncoupling conditions of sectional oscillations. The results obtained indicate that splitter plates provide a practical method for vortex suppression at high Reynolds number, and the degree of suppression can be maximized by optimal geometric configuration.

While electrons moving perpendicular to a magnetic field are confined to cyclotron orbits, they can move freely parallel to the field. This simple fact leads to complex current flow in clean, low carrier density semi-metals, such as long-ranged current jets forming along the magnetic field when currents pass through point-like constrictions. Occurring accidentally at imperfect current injection contacts, the phenomenon of "current jetting" plagues the research of longitudinal magneto-resistance, which is particularly important in topological conductors. Here we demonstrate the controlled generation of tightly focused electron beams in a new class of micro-devices machined from crystals of the Dirac semi-metal Cd3As2. The current beams can be guided by tilting a magnetic field and their range tuned by the field strength. Finite element simulations quantitatively capture the voltage induced at faraway contacts when the beams are steered towards them, supporting the picture of controlled electron jets. These experiments demonstrate direct control over the highly non-local signal propagation unique to 3D semi-metals in the current jetting regime, and may lead to applications akin to electron optics in free space.

The impact of nonmagnetic and magnetic impurities on topological insulators is a central focus concerning their fundamental physics and possible spintronics and quantum computing applications. Combining scanning tunneling spectroscopy with transport measurements, we investigate, both locally and globally, the effect of nonmagnetic and magnetic substituents in SmB₆, a predicted topological Kondo insulator. Around the so-introduced substitutents and in accord with theoretical predictions, the surface states are locally suppressed with different length scales depending on the substituent's magnetic properties. For sufficiently high substituent concentrations, these states are globally destroyed. Similarly, using a magnetic tip in tunneling spectroscopy also resulted in largely suppressed surface states. Hence, a destruction of the surface states is always observed close to atoms with substantial magnetic moment. This points to the topological nature of the surface states in SmB₆ and illustrates how magnetic impurities destroy the surface states from microscopic to macroscopic length scales.