Forcibly driven coherent soft phonons in GeTe with intense THz-rate pump fields

We propose an experimental technique to generate large amplitude coherent phonons with irradiation of THz-rate pump pulses and to study the dynamics of phase transition in GeTe ferroelectrics. When a single pump pulse irradiates the sample at various pump power densities, the frequency of the soft phonon decreases sub-linearly and saturates at higher pump powers. By contrast, when THz-rate pump pulse sequence irradiates the sample at matched time intervals to forcibly drive the oscillation, a large red-shift of the phonon frequency is observed without saturation effects. After excitation with a four pump pulse sequence, the coherent soft phonon becomes strongly damped leading to a near critical damping condition. This condition indicates that the lattice is driven to a precursor state of the phase transition.Comment: 4 pages, 3 figure

Distributed Adaptive Learning with Multiple Kernels in Diffusion Networks

We propose an adaptive scheme for distributed learning of nonlinear functions by a network of nodes. The proposed algorithm consists of a local adaptation stage utilizing multiple kernels with projections onto hyperslabs and a diffusion stage to achieve consensus on the estimates over the whole network. Multiple kernels are incorporated to enhance the approximation of functions with several high and low frequency components common in practical scenarios. We provide a thorough convergence analysis of the proposed scheme based on the metric of the Cartesian product of multiple reproducing kernel Hilbert spaces. To this end, we introduce a modified consensus matrix considering this specific metric and prove its equivalence to the ordinary consensus matrix. Besides, the use of hyperslabs enables a significant reduction of the computational demand with only a minor loss in the performance. Numerical evaluations with synthetic and real data are conducted showing the efficacy of the proposed algorithm compared to the state of the art schemes.Comment: Double-column 15 pages, 10 figures, submitted to IEEE Trans. Signal Processin

Multi-dimensional Sensitivity Analysis by Monte Carlo Method

A multi-dimensional sensitivity analysis method is proposed for evaluating the neutron flux perturbation due to the variance in the total, total scattering and differential scattering cross sections of the medium. The method is formulated based on the Monte Carlo method. The method uses the perturbation theory where the neutron random paths are not changed, and the variation in the neutron weight is estimated to describe the flux perturbation due to cross section errors. The method is applied to analyze the neutron scattering experiment in the gun-source geometry. It is shown through the analysis that a variety of the flux perturbation is caused at each scattering direction by the cross section errors

Measurements of Fission Neutron Spectra Transmitted through Iron, Heavy Concrete, Polyethylene and Graphite Slabs

Measurements of transmitted neutron spectra through slabs of iron, heavy concrete, polyethylene and graphite are shown. The experiments are done using collimated fission neutrons from a ²⁵²Cf source. To verify the results, multigroup Monte Carlo calculations are done. Fairly good agreement is shown between spectra obtained by experiments and calculations. Some discrepancies shown between them are thought to come from inaccuracies in the cross section data used in the calculations

Composite Asymmetric Dark Matter with a Dark Photon Portal

Asymmetric dark matter (ADM) is an attractive framework relating the observed baryon asymmetry of the Universe to the dark matter density. A composite particle in a new strong dynamics is a promising candidate for ADM as the strong dynamics naturally explains the ADM mass in the GeV range. Its large annihilation cross section due to the strong dynamics leaves the asymmetric component to be dominant over the symmetric component. In such composite ADM scenarios, the dark sector has a relatively large entropy density in the early Universe. The large dark sector entropy results in the overclosure of the Universe or at best contradicts with the observations of the cosmic microwave background and the successful Big-Bang Nucleosynthesis. Thus, composite ADM models generically require some portal to transfer the entropy of the dark sector into the Standard Model sector. In this paper, we consider a dark photon portal with a mass in the sub-GeV range and kinetic mixing with the Standard Model photon. We investigate the viable parameter space of the dark photon in detail, which can find broad applications to dark photon portal models. We also provide a simple working example of composite ADM with a dark photon portal. Our model is compatible with thermal leptogenesis and $B - L$ symmetry. By taking into account the derived constraints, we show that the parameter space is largely tested by direct detection experiments.Comment: 18 pages, 1 figures; presentation refined, results unchanged; matched to the JHEP versio

Three-dimensional flow visualization system by holographic interferometry

A holographic system has been developed to visualize a three-dimensional fluid flow. The system consists principally of a thermoplastic film, a monochrome video camera, a microcomputer with image-processing capability and a printer. The system makes it possible to measure a slow flow. Two examples of such, the bimodal mixed-mode convection flows within a Bénard cell, are presented