Strongly nonlinear luminescence in oxidized porous silicon films

Partially oxidized free-standing porous silicon films show a strongly superlinear increase in photoluminescence (PL) intensity above a threshold cw excitation intensity of 10 W/cm². The PL-intensity increase can be expressed by a power law with n9 as a function of the excitation intensity. The PL-peak wavelength of this emission is slightly redshifted from that at low-excitation levels. These changes are fully reversible and reproducible, but not observed in samples on substrate. We attribute this behavior to thermal reexcitation of carriers trapped at the dangling bond states in initially nonluminescent Si nanocrystallites.     

Fabrication and hard X-ray photoemission analysis of photocathodes with sharp solar-blind sensitivity using AlGaN films grown on Si substrates

Photocathode devices operating in reflection-mode, where the photoemission is detected on the same side as the light irradiation, were developed for the detection of deep ultraviolet light by using p-Al_xGa_1−xN films grown on Si(1 1 1) substrates. The external quantum efficiencies were as high as 20-15% at 200 nm and 280 nm, while the value was as low as 10⁻²% at 310 nm. The on-off ratio was more than four orders of magnitude, which represents high solar-blind sensitivity. The escape probability of Al_xGa_1−xN photocathode was decreased with increase of AlN mole fraction. The effective barrier potential against the photoelectron emission near the surface was reduced due to the upward shift of conduction band of Al_xGa_1−xN. The photoemission from the Al_xGa_1−xN films terminated with Cs-O adatoms will be discussed in terms of band diagrams that were evaluated by hard X-ray photoelectron spectroscopy.     

A realistic structural model of glassy iron

The Ichikawa model with k = 2.0 (the Bennett model) for iron was relaxed using Johnson's potential and the potential by Pak and Doyama. The relaxed models reproduce the experimental pair-distribution function and structure sensitive intensity and have high packing fractions.     

Statistical properties of aggregation with injection

We study a generalized aggregation process in which charged particles diffuse and coalesce randomly on a lattice. For one-dimensional and mean-field models, we show that there exists a statistically-invariant steady state when randomly charged particles are continuously injected. The steady-state charge distribution obeys a power law with the exponent depending both on the type of the injection and on the spatial dimension. The response of the system to a perturbation (i.e., relaxation) is characterized by either a power law decay (t ^– ,1) or a compressed exponential decay [exp(–t ),>1].     

Localization transition of <Emphasis Type="Italic">d</Emphasis>-friendly walkers

 Friendly walkers is a stochastic model obtained from independent one-dimensional simple random walks {S ^k _j } _j≥0 , k=1,2,…,d by introducing ``non-crossing condition': and ``reward for collisions' characterized by parameters . Here, the reward for collisions is described as follows. If, at a given time n, a site in ℤ is occupied by exactly m≥2 walkers, then the site increases the probabilistic weight for the walkers by multiplicative factor exp (β _m )≥1. We study the localization transition of this model in terms of the positivity of the free energy and describe the location and the shape of the critical surface in the (d−1)-dimensional space for the parameters . Received: 13 June 2002 / Revised version: 24 August 2002 Published online: 28 March 2003 Mathematics Subject Classification (2000): 82B41, 82B26, 82D60, 60G50 Key words or phrases: Random walks – Random surfaces – Lattice animals – Phase transitions – Polymers – Random walks     

Triangular arbitrage as an interaction among foreign exchange rates

We first show that there are in fact triangular arbitrage opportunities in the spot foreign exchange markets, analyzing the time dependence of the yen–dollar rate, the dollar–euro rate and the yen–euro rate. Next, we propose a model of foreign exchange rates with an interaction. The model includes effects of triangular arbitrage transactions as an interaction among three rates. The model explains the actual data of the multiple foreign exchange rates well.