Faculty Bibliography

We report an experimental evaluation of the "input-output surface" for a biochemical AND gate. The obtained data are modeled within the rate-equation approach, with the aim to map out the gate function and cast it in the language of logic variables appropriate for analysis of Boolean logic for scalability. In order to minimize "analog" noise, we consider a theoretical approach for determining an optimal set for the process parameters to minimize "analog" noise amplification for gate concatenation. We establish that under optimized conditions, presently studied biochemical gates can be concatenated for up to order 10 processing steps. Beyond that, new paradigms for avoiding noise buildup will have to be developed. We offer a general discussion of the ideas and possible future challenges for both experimental and theoretical research for advancing scalable biochemical computing.

We study the self-healing process of materials with embedded "glue"-carrying cells, in the regime of the onset of the initial fatigue. Three-dimensional numerical simulations within the percolation-model approach are reported. The main numerical challenge taken up in the present work has been to extend the calculation of the conductance to three-dimensional lattices. Our results confirm the general features of the process: The onset of material fatigue is delayed, by development of a plateaulike time dependence of the material quality. We demonstrate that, in this low-damage regime, the changes in the conductance and thus in similar transport and response properties of the material can be used as measures of the material quality degradation. A new feature found for three dimensions, where it is much more profound than in earlier-studied two-dimensional systems, is the competition between the healing cells. Even for low initial densities of the healing cells, they interfere with each other and reduce each other's effective healing efficiency.

We present results of computational modeling of the formation of uniform spherical silver particles prepared by rapid mixing of ascorbic acid and silver-amine complex solutions in the absence of a dispersing agent. Using an accelerated integration scheme to speed up the calculation of particle size distributions in the latter stages, we find that the recently reported experimental results-some of which are summarized here-can be modeled effectively by the two-stage formation mechanism used previously to model the preparation of uniform gold spheres. We treat both the equilibrium concentration of silver atoms and the surface tension of silver precursor nanocrystals as free parameters, and find that the experimental reaction time scale is fit by a narrow region of this two-parameter space. The kinetic parameter required to quantitatively match the final particle size is found to be very close to that used previously in modeling the formation of gold particles, suggesting that similar kinetics governs the aggregation process and providing evidence that the two-stage model of burst nucleation of nanocrystalline precursors followed by their aggregation to form the final colloids can be applied to systems both with and without dispersing agents. The model also reproduced semiquantitatively the effects of solvent viscosity and temperature on the particle preparation.

This report describes a unique case of prolonged hypoglycemia in a nondiabetic patient with end-stage renal disease and chronic liver disease. Following a less-than-24-hour period of being NPO (nothing per oral), the patient developed hypercapnic respiratory failure. Severe hypoglycemia in such a patient leading to respiratory failure provides major challenges in identification and management of his illness. To our knowledge, this is the first ever reported case of severe hypoglycemia leading to hypercapnic respiratory failure. We believe that the pathogenic basis for this patient's severe hypoglycemia is failure of contribution by the kidneys and liver to glucose production.

We devise an approach to measure the polarization of nuclear spins via conductance measurements. Specifically, we study the combined effect of external magnetic field, nuclear spin polarization, and Rashba spin-orbit interaction on the conductance of a quantum wire. Nonequilibrium nuclear spin polarization affects the electron energy spectrum making it time dependent. Changes in the extremal points of the spectrum result in time dependence of the conductance. The conductance oscillation pattern can be used to obtain information about the amplitude of the nuclear spin polarization and extract the characteristic time scales of the nuclear spin subsystem.

We study the evolution and distribution of nonequilibrium electron spin polarization in n-type semiconductors within the two-component drift-diffusion model in an applied electric field. Propagation of spin-polarized electrons through a boundary between two semiconductor regions with different doping levels is considered. We assume that inhomogeneous spin polarization is created locally and driven through the boundary by the electric field. We show that an initially created narrow region of spin polarization can be further compressed and amplified near the boundary. Since the boundary involves variation of doping but no real interface between two semiconductor materials, no significant spin polarization loss is expected. The proposed mechanism will be therefore useful in designing new spintronic devices.