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In this paper, the performance of the adaptive passive time-reversal method is investigated for an at-sea experiment of multiple-input/multiple-output underwater acoustic communication over a range of 13.5 km. The experiment was conducted at a depth of 200 m utilizing up to five transmitters with 4–8.5 kHz signal bandwidth for spatial division multiplexing. It was confirmed that the performance depends on the delay time from the timing of the channel response estimation to the communication signal reception, and the optimal signal length and the number of transmission channels can be determined according to the required bit error rate (BER) for the system. As a result, using the optimal signal length and a number of transmission channels, effective data rates of 30.21 and 37.53 kbps are achieved with BER ranges of less than 0.1% and 1%, respectively, with more than 90% of received communication signal packets.

We report fabrication and measurements of single-layer SnSe2/chemical vapor deposition (CVD) graphene/hexagonal boron nitride (h-BN) field-effect device. The coherent magnetotransport properties of such a hybrid system are systematically studied so as to obtain a good understanding of the structure which may find potential applications in thermoelectricity, flexible electronics, quantum coherent sensor as well as stress sensing. We observed weak localization well described by the Hikami-Larkin-Nagaoka model and the phase coherence length is around 540 nm for V BG = −20 V at 1 K. The phase coherence length could be effectively changed by controlling the temperature and gate voltage. We also obtain good field-effect dependent properties of atomic-scale SnSe2 ultrathin film/graphene system. Given the current challenges in tuning single-layer SnSe2/CVD graphene on h-BN with a suitable dielectric layer, our results suggest the potential of quantum coherent effect, an effective way for development of future quantum nano-switch device.

In this paper, the adaptive time reversal (ATR) technique combined with a single channel decision feedback equalizer (DFE) is applied to an at-sea experiment of the underwater acoustic multiple-input/multiple-output (MIMO) communication utilizing the signal bandwidth of 4.5–8.5 kHz over the distance of 12.5 km. This paper focuses on the demonstration of the effectiveness of the spatial division multiplexing MIMO using ATR-DFE schemes in time-varying real sea environment, and how much the communication rate could be achieved, with changing the signal parameters. We report that the communication quality is dependent on the duration from the channel probing for ATR process to signal reception, and we found that the output signal-to-noise ratio of the demodulated results highly correlated to the temporal coherence of the acoustic channel. Finally, the relationship between the effective data rate and the bit error rate is evaluated. As a result, the achievable data rate in the experiment of this study would be a remarkable considering the distance of the communication.

This is a continuation of our companion paper (Kida 2020b Fluid Dyn. Res.) in which the steady flow in a precessing spheroid is studied in the strong spin and weak precession limit, and the elliptic flow with uniform vorticity, the direction of which is perpendicular to the spin axis on the plane spanned by the spin and the precession axes, is shown to be excited in the interior inviscid region. In this paper we examine the boundary-layer flow connected to this elliptic flow and also the higher-order inviscid flow modified by this boundary-layer flow. The explicit expression of the velocity and pressure fields is obtained for arbitrary aspect ratiocof the polar and equatorial radii of the spheroid. The boundary-layer approximation breaks down at two critical circles on which the boundary-layer thickness as well as the normal component of velocity diverge to infinity. The flow field in the neighborhood (called the critical regions) of the critical circles is analyzed to find the same structure, up to scales, as that for a sphere. The radial component of velocity in the critical region induces the conical shear layers along the characteristic cones in the inviscid region. They reflect on the spheroid surface, and the reflection takes place endlessly in general. On the other hand, it is proved that for a spheroid of which the aspect ratio is expressed asc=3cot[(π/2)(m/n)](m/nbeing an irreducible fraction) the reflection terminates at (n − 2) times and the conical shear layers form a regular pattern. Since such special values ofcare distributed densely over all the positive numbers, we may observe practically always a regular pattern of the conical shear layers. Such closed conical shear layers are clearly visualized by the pressure field numerically calculated in the asymptotic formulation. Furthermore, the total angular momentum of the steady flow is expressed explicitly up to the non-trivial leading order for all the three components over the whole range of the aspect ratio.

Although all-inorganic CsPbBr3 perovskites quantum dots exhibit an excellent photophysical properties, which could be applied in various applications, they still suffer from poor structural stability owing to environment factors. Therefore, TiO2 was coated on the CsPbBr3 QDs as a protecting shell to enhance their stability. Moreover, anatase phase of TiO2 shell was obtained by annealing at more than 300°C due to its most effective electron extraction properties among other crystalline forms of TiO2. However, the CsPbBr3 QDs can be degraded under high temperature. Herein, the annealing temperatures ranging from 300-500°C were optimized to obtain the anatase TiO2 shell and good crystallinity of CsPbBr3 QDs core. The morphology and optical properties of CsPbBr3 QDs, CsPbBr3/TiOx and CsPbBr3/TiO2 were studied by using transmission electron microscope (TEM), photoluminescence (PL) measurement and UV-Visible spectroscopy, respectively. Finally, the CsPbBr3/TiO2 annealed at the optimum temperature was compared with the bare CsPbBr3 QDs to prove the enhancement of their structural stability under ambient air by using X-ray diffraction (XRD).

This paper presents an investigation of a channel tracking based passive time reversal (PTR) communication method for mobile single-input/multiple-output communications in rapidly time-varying underwater acoustic channel. To investigate its performance in the non-stationary channels, the method is applied to a synthetic dataset generated by the underwater acoustic moving source model using normal mode method. The transmitted signal is modulated by the single-carrier modulation scheme using the carrier frequency of 20 kHz and bandwidth of 4, 8, and 12 kHz. From the comparisons with conventional PTR approach, the following advantages were found: (1) the quality of the equalization result hardly degrades even if the sound source moving speed increased, (2) suboptimal diversity combining is performed in the sense of the energy utilization of the multipath, (3) the dependence on the bandwidth is relatively small, and a significant performance improvement is seen especially for narrowband signals.

In underwater acoustic communication between surface and underwater vehicles, the nonuniform Doppler shift can degrade demodulation performance severely depending on the symbol rate. In this paper, the dependence of demodulation performance on the symbol rate is investigated using simulations with and without a multipath signal. The results show that symbol rate plays an important role in determining demodulation performance under an environment with a nonuniform Doppler shift. Furthermore, the simulation results without a multipath signal indicate that the effects of a nonuniform Doppler shift of a direct signal can be suppressed at a relatively low symbol rate. On the other hand, the simulation results with a multipath signal show that a high symbol rate is required to suppress the effect of the multipath signal under an environment with a nonuniform Doppler shift.

Electroactive polymer (EAP) actuators were developed using composite nanofiber electrode films fabricated with multi-walled carbon nanotubes and poly vinylidene fluoride-co-hexafluoropropylene (PVDF-HFP). Aligned nanofiber electrode films were fabricated by the electrospinning method. We evaluated the characteristics of the nanofiber electrode films, fabricating conditions of the EAP actuators and property of the EAP actuators. It was confirmed that the alignment of nanofibers fabricated by electrospinning methods was anisotropic in the electrical conductivity and tensile strength. The nanofiber electrode film had 100 times the surface area of the cast electrode film. The capacitance and displacement of the nanofiber electrode actuator were larger than that of the cast electrode actuator. It is suggested that increasing the specific surface area enlarges the capacitance and displacement of the actuator.

Research at modern light sources continues to improve our knowledge of the natural world, from the subtle workings of life to matter under extreme conditions. Free-electron lasers, for instance, have enabled the characterization of biomolecular structures with sub-ångström spatial resolution, and paved the way to controlling the molecular functions. On the other hand, attosecond temporal resolution is necessary to broaden our scope of the ultrafast world. Here we discuss attosecond pulse generation beyond present capabilities. Furthermore, we review three recently proposed methods of generating attosecond x-ray pulses. These novel methods exploit the coherent radiation of microbunched electrons in undulators and the tailoring of the emitted wavefronts. The computed pulse energy outperforms pre-existing technologies by three orders of magnitude. Specifically, our simulations of the proposed Soft X-ray Laser at MAX IV (Lund, Sweden) show that a pulse duration of 50-100 as and a pulse energy up to 5 [Formula: see text]J is feasible with the novel methods. In addition, the methods feature pulse shape control, enable the incorporation of orbital angular momentum, and can be used in combination with modern compact free-electron laser setups.