B.-G. Yoon

Effect of the statistical shift on the anomalous conductivities of n‐type hydrogenated amorphous silicon

We calculated the shift of Fermi energy EF with temperature, using a model density of states for hydrogenated amorphous silicon (a‐Si:H) similar to that of deep level transient spectroscopy results, by a numerical method. The conductivity σ was calculated from the calculated EF as a function of temperature. It was found that some features of anomalous transport phenomena of n‐type a‐Si:H such as kinks or the continuous bending of log σ vs 1/T curves and the Meyer–Neldel‐type preexponential factors can be explained, at least in part, by the statistical shift alone.

Carrier concentration-dependence of field emission from semiconductors

The carrier concentration-dependence of field emission is theoretically investigated calculating the conduction band contributions to emission current for several n-type semiconductors. The calculated field emission current densities j are found to increase slowly with increasing concentration n for n-type Ge, Si, GaN semiconductors. For n-type GaAs, j increases before a certain value of n and decreases after it. Such n-dependence of all semiconductors varies slightly with the applied field. It is to note that the internal voltage drop due to field penetration is a crucial quantity in determining the n-dependence. The band gap shrinkage effect is meaningful only in high carrier concentrations.

Noise effects on synchronization in systems of coupled oscillators

We study the synchronization phenomena in systems of globally coupled oscillators, each possessing finite inertia, with particular attention to the noise effects. The self-consistency equation for the order parameter as well as the probability distribution is obtained from the Smoluchowski equation, and analyzed in the presence of thermal noise. It is found that the hysteresis present in the system without noise disappears as the thermal noise comes into the system. Numerical simulations are also performed to give results generally consistent with the analytical ones.

Scale-free dynamics emerging from information transfer

The dynamics based on information transfer is proposed as an underlying mechanism for the scale-invariant dynamic critical behavior observed in a variety of systems. We apply the dynamics to the globally coupled Ising model, which is analytically tractable, and show that dynamic criticality is indeed attained. Such emergence of criticality is confirmed numerically in the two-dimensional Ising model as well as the globally coupled one and in a biological evolution model. Although criticality is precise only when information transfer is reversible, it may also be observed even in the irreversible case, during the practical time scale shorter than the relaxation time.

Dynamic model for failures in biological systems

A dynamic model for failures in biological organisms is proposed and studied both analytically and numerically. Each cell in the organism becomes dead under sufficiently strong stress, and is then allowed to be healed with some probability. It is found that unlike the case of no healing, the organism in general does not completely break down even in the presence of noise. Revealed is the characteristic time evolution that the system tends to resist the stress longer than the system without healing, followed by sudden breakdown with some fraction of cells surviving. When the noise is weak, the critical stress beyond which the system breaks down increases rapidly as the healing parameter is raised from zero, indicative of the importance of healing in biological systems.

Synchronization and resonance in a driven system of coupled oscillators

We study the noise effects in a driven system of globally coupled oscillators, with particular attention to the interplay between driving and noise. The self-consistency equation for the order parameter, which measures the collective synchronization of the system, is derived; it is found that the total order parameter decreases monotonically with noise, indicating overall suppression of synchronization. Still, for large coupling strengths, there exists an optimal noise level at which the periodic (ac) component of the order parameter reaches its maximum. The response of the phase velocity is also examined and found to display resonance behavior.

Model calculation on the Meyer–Neldel rule for the field‐effect conductance of hydrogenated amorphous silicon

A model calculation was carried out to study the Meyer–Neldel rule of the field‐effect conductance of hydrogenated amorphous silicon (a‐Si:H). It was found that the shift of Fermi level and the potential profile in the sample with temperature can explain the Meyer–Neldel rule if a model density of states of a‐Si:H is properly chosen. So it is of doubt to think that the conductivity prefactor varies in a single sample of band‐bending case, as assumed by some authors.

Effects of neuronal loss in the dynamic model of neural networks

We study the phase transitions and dynamic behavior of the dynamic model of neural networks, with an emphasis on the effects of neuronal loss due to external stress. In the absence of loss the overall results obtained numerically are found to agree excellently with the theoretical ones. When the external stress is turned on, some neurons may deteriorate and die; such loss of neurons, in general, weakens the memory in the system. As the loss increases beyond a critical value, the order parameter measuring the strength of memory decreases to zero either continuously or discontinuously, namely, the system loses its memory via a second- or a first-order transition, depending on the ratio of the refractory period to the duration of action potential.

Light-induced defect generation in hydrogenated amorphous silicon under the constant photocurrent conditions

Abstract We measured the decay of the steady state photoconductivity in a -Si : H due to light soaking under the constant photocurrent condition, in which the light intensity was increased to keep the photocurrent constant and the photoconductivity was measured at a preset light intensity. The results are discussed in terms of the Stutzmanns model and the model of Redfield and Bube. We have found that Redfield and Bubes equation in a modified form including carrier enhanced dispersion may be used to describe light-induced defect generation.

On the dynamics of traveling phase-oscillators with positive and negative couplings

We investigate numerically the dynamics of traveling clusters in systems of phase oscillators, some of which possess positive couplings and others negative couplings. The phase distribution, speed of traveling, and average separation between clusters, as well as the order parameters for positive and negative oscillators, are computed as the ratio of the two coupling constants and the fraction of positive oscillators are varied. The dependence of the traveling speed on these parameters is obtained and is observed to fit well with the numerical data of the systems. With the help of this, we describe the conditions for the traveling state to appear in the systems with and without a periodic driving field.