Eugene Mihaliuk

Experimental and theoretical studies of feedback stabilization of propagating wave segments

Experimental and theoretical studies of the excitability boundary for spiral wave behavior are presented. The boundary is defined by unstable wave segments, which are stabilized by using a negative-feedback control algorithm. A kinematic description of the constant-size, constant-shape wave segments is presented.

Using heat to control the sample spinning speed in MAS NMR

A new approach using temperature to control the spinning speed of a sample rotor in magic-angle spinning NMR is presented. Instead of an electro-mechanical valve that regulates the flow of drive gas to control the spinning speed in traditional MAS NMR systems, we use a small heater wire located directly in the stator. The sample spinning speed is controlled very accurately with a surprisingly low heating power of 1 W. Results on a benchtop unit demonstrate the capability of the system.

Optical lever for monitoring of the magic angle.

Setting the magic angle and accurately maintaining it over a full range of experimental conditions can be a frustrating experience. We present a simple optical device for immediate indication of the stator angle, free of mechanical uncertainties present in typical adjustment mechanisms.

A frequency-selective REDOR experiment for an SI2 spin system

A frequency-selective REDOR experiment is described for SI2 spin systems. The experiment causes the net dipolar dephasing of the S spin to evolve only under the influence of one of the I spins. The experiment is based on a single pair of appropriately phased 90 degrees I-spin pulses, and the I spin causing the S-spin dipolar dephasing is determined by the relative phases between the two 90 degrees pulses. The experiment is demonstrated on a sample of 15N2-L-asparagine.

Topographic organization of Hebbian neural connections by synchronous wave activity

Experimental studies have revealed that the refinement of early, imprecise connections in the developing visual system involves activity in the retina before the onset of vision. We study the evolution of initially random unidirectional connections between two excitable layers of FitzHugh-Nagumo neurons with simulated spontaneous activity in the input layer. Lateral coupling within the layers yields synchronous neural wave activity that serves as a template for the Hebbian learning process, which establishes topographically precise interlayer connections. (c) 2001 American Institute of Physics.

Adding a lens Improves spinning speed characterization.

Highly stable sample rotation is important in many solid-state NMR experiments. Whether the necessary stability is achieved is not always clear. Typically only an average frequency over some time interval (often relatively long and unknown) is available from the spinning speed controller readout, which is not representative of the short-term variations of instantaneous rotation frequency. The necessity of the relatively slow measurement of spinning speed is a consequence of phase noise in the tachometer, which prevents speed measurement to be both rapid and precise at the same time. We show that adding a lens to the tachometer, without any other changes in the probe, reduces phase noise by nearly an order of magnitude and allows improved measurement of the spinning speed.