Iterative Path Integral Approach to Nonlinear Stochastic Optimal Control Under Compound Poisson Noise

Nonlinear stochastic optimal control theory has played an important role in many fields. In this theory, uncertainties of dynamics have usually been represented by Brownian motion, which is Gaussian white noise. However, there are many stochastic phenomena whose probability density has a long tail, which suggests the necessity to study the effect of non‐Gaussianity. This paper employs Lévy processes, which cause outliers with a significantly higher probability than Brownian motion, to describe such uncertainties. In general, the optimal control law is obtained by solving the Hamilton–Jacobi–Bellman equation. This paper shows that the path‐integral approach combined with the policy iteration method is efficiently applicable to solve the Hamilton–Jacobi–Bellman equation in the Lévy problem setting. Finally, numerical simulations illustrate the usefulness of this method.     

Thermal conductivity of non-stoichiometric americium oxide: A molecular dynamics study

The aim of this paper is to investigate the influence of multi-valency of americium in its oxide for the lowering of the thermal conductivity and the uncertainty in measurement. In the present study, thermal conductivity of non-stoichiometric americium oxide was evaluated up to 2000 K by the non-equilibrium molecular dynamics calculations using the Born-Mayer-Huggins interatomic potential with the partially ionic model. The oxygen-to-americium ratio (O/Am) was varied from 1.6 to 1.9, which corresponded to the variation of the ratio of Am³⁺/Am⁴⁺. So, we prepared potential parameters for both Am³⁺ and Am⁴⁺. The calculated thermal conductivity of non-stoichiometric americium oxide decreased with an increase of temperature, and the degree of the temperature dependence became smaller with a decrease of the O/Am ratio. This was mainly caused by the phonon-scattering due to oxygen vacancies induced with Am³⁺ ions. Comparing two supercells in which (1) short-range ordered Am³⁺ clusters were contained and (2) Am³⁺ ions were randomly distributed, the thermal conductivity of the former seemed to be somewhat larger than that of the latter.     

Simple equation for elastohydrodynamic film thickness under acceleration

A simple approximation of EHD film thickness under varying speed conditions is proposed. The equation is based on continuity of flow, by which the film formed at the contact inlet moves downstream within the contact with little subsequent change in its thickness even though the boundary velocities are changing. The approximation is supported by experimental results of non-steady state film thickness measurement using ultra-thin film interferometry. It is also shown by numerical simulation that the approximation holds for film thickness in the rigid piezoviscous regime under line contact so long as the squeeze film effect is insignificant.     

Calcium isotope separation by band chromatography using 18-crown-6-ether resin

To develop the ⁴⁸Ca enrichment process, a feasibility study on a band chromatography was made using 9 M HCl solution and crown ether resin synthesized in porous silica beads. Prior to the chromatographic experiments, distribution coefficients, Kd, of Ca²⁺ and Sr²⁺ were measured at different concentrations of these ionic species. The frontal boundary of the chromatography was made by a usual manner of the breakthrough mode of calcium feeding, and the rear boundary was made by introducing strontium as a following ion on the basis of the observed Kd values. It was confirmed that the heavy isotope ⁴⁸Ca was depleted in the rear boundary region, while ⁴⁸Ca was enriched in the front boundary region. The values of separation coefficient ε (= α – 1) in three chromatographic operations at different temperatures were observed as 2 × 10^?3 ~ 3 × 10^?3. The separation coefficients observed in the front boundary regions, where ⁴⁸Ca was enriched, agreed with those observed in the rear boundary regions, where ⁴⁰Ca was enriched.     

Reduction of heat leak in cryogenic system using Peltier current leads

Peltier current leads (PCLs) for cryogenic systems are investigated in regard to temperature dependence of thermoelectric materials. Due to the Peltier effect on the thermoelectric parts of the current lead, PCLs act as heat pumps. It is expected that PCLs will reduce the amount of heat leak from the room temperature side to the low temperature side of a cryogenic system. Six (three each for p and n type) hot-pressed BiTe samples for PCLs are selected to estimate PCL performance. Our experimental results and analyses indicate that PCLs show a capacity in the order of several hundred Amperes and as much as 20-30% reductions of heat leak.     

Photoluminescence of nitrogen-doped anatase

Photoluminescence depending on nitrogen concentration was investigated using anatase-type TiO₂ prepared by the calcination of a mixture of titanyl sulfate hydrate and urea. The substitutional ratio (x) of nitrogen in TiO₂ was successfully varied from 0.004 to 0.022 by changing the molar ratio of the mixture. The absorbance at 380–560 nm due to the formation of mid-gap states was proportional to the substitutional ratio of nitrogen controlled by the preparation conditions. In contrast, the fluorescent intensity at 382 nm originating from the band-to-band transition monotonically decreased with an increase in the substitutional ratio with an expansion of the anatase lattice. On the other hand, the maximum intensity of photoluminescence at 560 nm excited at 350 nm, which could be associated with the transition from the conduction band to the mid-gap states, was observed at x = 0.01. The optimal substitutional ratio for the emission was almost agreed with that for the photocatalytic decomposition of methylene blue and acetaldehyde under visible-light illumination. The photoluminescence was fundamentally determined by the balance between photoexcitation originating from a sufficient number of mid-gap states and deactivation of excited electrons and holes due to lattice distortion or defective states induced with the nitrogen doping.     

N-channel MOSFETs fabricated on homoepitaxy-grown 3C-SiC films

We present results of the enhancement mode, n-channel 3C-silicon carbide (SiC) MOSFETs fabricated on homoepitaxy 3C-SiC films. The fabricated devices exhibit excellent gate-controlled linear and saturation regimes of operation. The average effective channel mobility is found to be 229 cm/sup 2//Vs. The breakdown field of the gate oxide is observed at be 11 MV/cm and the subthreshold swing is determined to be 280 mV/decade.     

State-of-the-Art Molecular Dynamics Simulation Studies of RNA-Dependent RNA Polymerase of SARS-CoV-2

Molecular dynamics (MD) simulations are powerful theoretical methods that can reveal biomolecular properties, such as structure, fluctuations, and ligand binding, at the level of atomic detail. In this review article, recent MD simulation studies on these biomolecular properties of the RNA-dependent RNA polymerase (RdRp), which is a multidomain protein, of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are presented. Although the tertiary structures of RdRps in SARS-CoV-2 and SARS-CoV are almost identical, the RNA synthesis activity of RdRp of SARS-CoV is higher than SARS-CoV-2. Recent MD simulations observed a difference in the dynamic properties of the two RdRps, which may cause activity differences. RdRp is also a drug target for Coronavirus disease 2019 (COVID-19). Nucleotide analogs, such as remdesivir and favipiravir, are considered to be taken up by RdRp and inhibit RNA replication. Recent MD simulations revealed the recognition mechanism of RdRp for these drug molecules and adenosine triphosphate (ATP). The ligand-recognition ability of RdRp decreases in the order of remdesivir, favipiravir, and ATP. As a typical recognition process, it was found that several lysine residues of RdRp transfer these ligand molecules to the binding site such as a “bucket brigade.” This finding will contribute to understanding the mechanism of the efficient ligand recognition by RdRp. In addition, various simulation studies on the complexes of SARS-CoV-2 RdRp with several nucleotide analogs are reviewed, and the molecular mechanisms by which these compounds inhibit the function of RdRp are discussed. The simulation studies presented in this review will provide useful insights into how nucleotide analogs are recognized by RdRp and inhibit the RNA replication.