The influence of both geometric and offset charge disorder of two-dimensional quantum dot arrays ͑also known as network tunnel junctions͒ on their Coulomb blockade voltage V b is studied using extensive Monte-Carlo simulations. A general increase of V b with increasing disorder is confirmed, but an exception to the rule is found for intermediate degrees of offset charge disorder. Detailed studies of the V b distribution reveal a stability of its minimal value against geometric disorder, whereas this figure is considerably increased for high offset charge disorder. Implications of our results for single electron device design are discussed.

Abstract-A silicon-based memory cell utilizing Coulomb blockade is analyzed for use as a high-speed RAM. Operation principles and design guidelines are given by simple analytical modeling and simulations. By performing transient waveform Monte Carlo simulations, high-speed write operation is demonstrated with a time shorter than 10 ns. The memory node voltage of less than 0.1 V is detected by a newly proposed split-gate cell structure with a minimum disturbance to/from nonselected cells, which indicates the compatibility of this structure with conventional field effect transistors.

Nonlinear evolution of three-dimensional magnetohydrodynamic (MHD) instabilities of a toroidal spheromak in a cylindrical flux conserver has been studied by numerical simulations for various initial equilibrium states with different q profiles. In spheromaks with qa>1, where qa is the safety factor on the magnetic axis, nonlinear evolution of the resonant internal kink mode dominates with a poloidal mode number m=1 and a toroidal mode number n=1 that causes the poloidal flux amplification. This process corresponds to that of the internal disruption model for tokamaks by Kadomtsev [Sov. J. Plasma Phys. 1, 389 (1975)]. In spheromaks with a very high qa, namely qa≳3, the gross n=1 kink mode grows extensively in the region including the major axis of the torus, which causes the flux conversion from the toroidal to poloidal directions. For spheromaks with a low qa, namely qa≲0.5, the internal kink mode with a toroidal mode number n∼1/qa is first destabilized, and the excitation of the modes with lower n numbers down to n=1 proceeds, while the n=2 mode saturates. Nonlinear coupling of various modes leads to the flux conversion from the poloidal to toroidal directions. When a center conductor is present in this case, a reversed-field pinch (RFP) configuration once formed is sustained. Relaxations through pressure-driven modes are also discussed. All final states obtained in our simulations are quite near the Taylor state with an excess magnetic energy less than 10% of that of the Taylor state.

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