Random walk with barriers: Diffusion restricted by permeable membranes

Restrictions to molecular motion by barriers (membranes) are ubiquitous in biological tissues, porous media and composite materials. A major challenge is to characterize the microstructure of a material or an organism nondestructively using a bulk transport measurement. Here we demonstrate how the long-range structural correlations introduced by permeable membranes give rise to distinct features of transport. We consider Brownian motion restricted by randomly placed and oriented permeable membranes and focus on the disorder-averaged diffusion propagator using a scattering approach. The renormalization group solution reveals a scaling behavior of the diffusion coefficient for large times, with a characteristically slow inverse square root time dependence. The predicted time dependence of the diffusion coefficient agrees well with Monte Carlo simulations in two dimensions. Our results can be used to identify permeable membranes as restrictions to transport in disordered materials and in biological tissues, and to quantify their permeability and surface area.Comment: 8 pages, 3 figures; origin of dispersion clarified, refs adde

Direct Evidence of Two-component Ejecta in Supernova 2016gkg from Nebular Spectroscopy*

Spectral observations of the type-IIb supernova (SN) 2016gkg at 300-800 days are reported. The spectra show nebular characteristics, revealing emission from the progenitor star's metal-rich core and providing clues to the kinematics and physical conditions of the explosion. The nebular spectra are dominated by emission lines of [O i] lambda lambda 6300, 6364 and [Ca ii] lambda lambda 7292, 7324. Other notable, albeit weaker, emission lines include Mg I] lambda 4571, [Fe ii] lambda 7155, O I lambda 7774, Ca II triplet, and a broad, boxy feature at the location of H alpha. Unlike in other stripped-envelope SNe, the [O i] doublet is clearly resolved due to the presence of strong narrow components. The doublet shows an unprecedented emission line profile consisting of at least three components for each [O i]lambda 6300, 6364 line: a broad component (width similar to 2000 km s(-1)), and a pair of narrow blue and red components (width similar to 300 km s(-1)) mirrored against the rest velocity. The narrow component appears also in other lines, and is conspicuous in [O i]. This indicates the presence of multiple distinct kinematic components of material at low and high velocities. The low-velocity components are likely to be produced by a dense, slow-moving emitting region near the center, while the broad components are emitted over a larger volume. These observations suggest an asymmetric explosion, supporting the idea of two-component ejecta that influence the resulting late-time spectra and light curves. SN 2016gkg thus presents striking evidence for significant asymmetry in a standard-energy SN explosion. The presence of material at low velocity, which is not predicted in 1D simulations, emphasizes the importance of multidimensional explosion modeling of SNe

The broad-lined Type-Ic supernova SN 2022xxf and its extraordinary two-humped light curves: I. Signatures of H/He-free interaction in the first four months

We report on our study of supernova (SN) 2022xxf based on observations obtained during the first four months of its evolution. The light curves (LCs) display two humps of similar maximum brightness separated by 75 days, unprecedented for a broad-lined (BL) Type Ic supernova (SN IcBL). SN 2022xxf is the most nearby SN IcBL to date (in NGC 3705, z = 0.0037, at a distance of about 20 Mpc). Optical and near-infrared photometry and spectroscopy are used to identify the energy source powering the LC. Nearly 50 epochs of high signal-to-noise-ratio spectroscopy were obtained within 130 days, comprising an unparalleled dataset for a SN IcBL, and one of the best-sampled SN datasets to date. The global spectral appearance and evolution of SN 2022xxf points to typical SN Ic/IcBL, with broad features (up to ~1400 km s-1 and a gradual transition from the photospheric to the nebular phase. However, narrow emission lines (corresponding to ~1000-2500 km s-1 are present in the spectra from the time of the second rise, suggesting slower-moving circumstellar material (CSM). These lines are subtle, in comparison to the typical strong narrow lines of CSM-interacting SNe, for example, Type IIn, Ibn, and Icn, but some are readily noticeable at late times such as in Mg I \5170 and [O I] \l5577. Unusually, the near-infrared spectra show narrow line peaks in a number of features formed by ions of O and Mg. We infer the presence of CSM that is free of H and He. We propose that the radiative energy from the ejecta-CSM interaction is a plausible explanation for the second LC hump. This interaction scenario is supported by the color evolution, which progresses to the blue as the light curve evolves along the second hump, and the slow second rise and subsequent rapid LC drop. SN2022xxf may be related to an emerging number of CSM-interacting SNeIc, which show slow, peculiar LCs, bluecolors,and subtle CSM interaction lines.The progenitor stars of these SNe likely experienced an episode of mass loss consistingof H/He-free material shortly prio rto explosion

Power transfer function tailoring in a highly Ge-doped nonlinear interferometer-based all-optical thresholder using offset-spectral filtering

We experimentally investigate and characterize the improvement in thresholding capability of a compact highly Ge-doped nonlinear interferometer-based all-optical thresholder using optical offset spectral filtering. The thresholder we study has an in-loop nonlinearity requirement lower than that of a classical nonlinear loop mirror scheme. Therefore, only 15 m of nonholey silica-based fiber is used as a nonlinear element. Although the nonlinear interferometer-based thresholder has been useful for signal regeneration and thresholding, it has several limitations, including severe pulse distortion due to pulse bifurcation at high input powers and a fixed power transfer function. In this paper, we propose and demonstrate the use of offset spectral filtering at the output of this Ge-doped low nonlinearity interferometer-based thresholder to adjust the power transfer function and thresholding slope, as well as reducing pulse distortion due to pulse bifurcation. To the best of our knowledge, this is the first experimental demonstration of power transfer function tailoring, which makes the thresholder more flexible and allows customization of thresholding parameters in meeting requirements in various systems