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When grown under constant illumination, Gloeothece ATCC 27152, a unicellular cyanobacterium, can use nitrate, nitrite, dinitrogen or ammonium as the sole N-source for growth. The uptake systems for nitrate and nitrite were fully active and ammonium-repressible in N₂-fixing cultures. Nitrite uptake was mediated via two pH-dependent systems: passive diffusion of HNO₂ and active transport of nitrite. Nitrate uptake was highly light-dependent. Ammonium was also transported by passive diffusion of ammonia and active uptake of ammonium, depending on the pH of the medium. Ammonium inhibited nitrate uptake almost completely, but inhibited nitrite uptake only partially. The true inhibitor was a product of ammonium assimilation, possibly glutamine, rather than ammonium itself, since L-methionine-DL-sulphoximine, an inhibitor of glutamine synthetase blocked GS activity very quickly and decreased the inhibitory effect of ammonium. On the other hand, the inhibitory effect of nitrite on ammonium uptake was stronger than that of nitrate. Nitrate and nitrite competitively inhibited each other's assimilation, occurring at the uptake stage. This suggested that nitrate and nitrite were transported by a common transporter in Gloeothece. The rates of nitrate and nitrite uptake were similar, but the uptake of ammonium was much faster than that of either nitrate of nitrite. In contrast to the uptake systems, systems of nitrate and nitrite reductions were substrate-inducible. Ammonium, either generated intracellularly or supplied exogenously, was assimilated via the GS-GOGAT pathway. Nitrite and ammonium inhibited N₂ fixation rapidly. Nitrate inhibited N₂ fixation less rapidly and less extensively, and often temporarily stimulated nitrogenase activity. The inhibitory effects of nitrate and ammonium could be prevented by L-methionine-DL-sulphoximine, suggesting that the true inhibitor of N₂ fixation was an assimilatory product of ammonium rather than ammonium or nitrate itself.

Abstract: As transistors scale to ever smaller dimensions, power density becomes an increasingly important issue in integrated circuit (IC) design. Recently, negative-capacitance field-effect transistors (NCFETs), realized by stacking ferroelectric material on top of conventional gate oxides, have been proposed to reduce power consumption in modern aggressively scaled devices. The negative capacitance of these ferroelectric materials provide voltage amplification to the transistor in order to reduce the subthreshold-swing (SS), which would reduce the active power consumption of the device via a reduction of the supply voltage. Beyond reduction of power consumption for individual transistors, the unique negative capacitance behavior in these ferroelectric materials also offers a vast array of options in modern IC design. As a result, ferroelectric materials are an exciting area of research. The current state-of-the-art modeling approach for the dynamics of ferroelectric materials is via the Landau-Khalatnikov (LK) equation. In this work, we implement a multi-domain improvement upon the LK equation and combine it with Cadence circuit simulations to model and predict the characteristics of NCFETs and other ferroelectric devices. In the first stage of this work, we calibrate our multi-domain LK model to experimental results to show a very strong match. Using this calibrated model, we examine the potential speed limitations of NCFETs and identify the requirement on the viscosity parameter of the ferroelectric materials to provide sub-picosecond rise time required for modern transistors. In the second stage, we propose a new measurement technique for extracting the LK parameters of a ferroelectric material. We demonstrate via Cadence circuit simulation that this new measurement technique is able to accurately extract all LK parameters, including the viscosity parameter, which is difficult to extract using standard techniques. In the third stage, we propose a new application for ferroelectric materials to increase the unity-current-gain frequency f_T of a transistor. By placing the ferroelectric in parallel with the FET gate, the negative capacitance of the ferroelectric cancels the positive gate capacitance of the FET, which in turn increases the f_T. This new application of ferroelectrics opens new possibilities for IC design. Overall, this work improves the understanding of ferroelectric materials pertaining to their applications in IC design, providing critical information for the electron device community as it continues to explore methods to advance the performance of nanoscale electronics into the 2030s and beyond, the current horizon of the International Roadmap for Devices and Systems.