Spectra and Transport in Almost Periodic Dimers

We study spectral properties of discrete Schrödinger operators with potentials obtained via dimerization of a class of aperiodic sequences. It is shown that both the nature of the autocorrelation measure of a regular sequence and the presence of generic (full probability) singular continuous spectrum in the hull of primitive and palindromic (four block substitution) potentials are robust under dimerization. Generic results also hold for circle potentials. We illustrate these results with numerical studies of the quantum mean square displacement as a function of time. The numerical techniques provide a very fast algorithm for the time evolution of wave packets.     

Flicker noise in gate overlapped polycrystalline silicon thin-filmtransistors

A study of the noise performance of gate overlapped polycrystalline silicon thin-film transistors (TFTs) is presented. Low-frequency noise measurements were carried out on n- and p-type samples fabricated by excimer laser crystallization. It is shown that the carrier number fluctuation model applies not only to n-type but also to p-type devices. The density of oxide traps was extracted from the noise measurements and was of the order of 10¹⁸-10¹⁹ eV^-1 cm^-3     

Capacity-achieving sequences for the erasure channel

This paper starts a systematic study of capacity-achieving (c.a.) sequences of low-density parity-check codes for the erasure channel. We introduce a class A of analytic functions and develop a procedure to obtain degree distributions for the codes. We show various properties of this class which help us to construct new distributions from old ones. We then study certain types of capacity-achieving sequences and introduce new measures for their optimality. For instance, it turns out that the right-regular sequence is c.a. in a much stronger sense than, e.g., the Tornado sequence. This also explains why numerical optimization techniques tend to favor graphs with only one degree of check nodes.     

Retrievals of column water vapor using millimeter-wave radiometric measurements

The airborne Millimeter-wave Imaging Radiometer (MIR) measurements conducted over the Midwest region of the continental United States during January/February 1997 and over the Alaska-Arctic region during May 1998 are used to estimate column water vapor W<0.8 g/cm/sup 2/ under a clear sky. On board the same aircraft are two other instruments, the Cloud Lidar System (CLS) and MODerate-resolution Imaging Spectrometer (MODIS) Airborne Simulator (MAS), which provide cloud cover information and independent measurements of W, respectively. The MIR-estimated W values are compared and found to be in very good agreement with those measured by rawinsondes at near concurrence. A close correlation is found between the MIR-estimated W and that estimated from the MAS near-IR reflectance ratios. Water surface emissivities at several MIR frequencies are obtained in the process of the W retrieval from several flights over the Midwest lakes. These estimated emissivities compared favorably with values calculated for a calm water surface, which are based on a di-electric permittivity model and MAS-measured surface temperatures. The results from all comparisons strongly demonstrate the soundness of the technique for estimating W.     

On Series of Ordinals and Combinatorics

This paper deals mainly with generalizations of results in finitary combinatorics to infinite ordinals. It is well-known that for finite ordinals ∑_bT<αβ is the number of 2-element subsets of an α-element set. It is shown here that for any well-ordered set of arbitrary infinite order type α, ∑_bT<αβ is the ordinal of the set M of 2-element subsets, where M is ordered in some natural way. The result is then extended to evaluating the ordinal of the set of all n-element subsets for each natural number n ≥ 2. Moreover, series ∑_β<αf(β) are investigated and evaluated, where α is a limit ordinal and the function f belongs to a certain class of functions containing polynomials with natural number coefficients. The tools developed for this result can be extended to cover all infinite α, but the case of finite α appears to be quite problematic.     

Densification involved in the UV-based photosensitivity of silicaglasses and optical fibers

A comprehensive survey of photosensitivity in silica glasses and optical fiber is reviewed. Recent work on understanding the mechanisms contributing to germanium or aluminum doped fiber photosensitivity is discussed within the framework of photoelastic densification models     

Influence of quantum capture and tunneling mechanisms on the dark current of bound-to-continuum quantum-well infrared photodetectors

We present a theoretical model for the dark current of bound-to-continuum quantum-well infrared photodetectors (QWIPs), by considering the field-induced mixing effect, tunneling rate and phonon scattering rate between bound and continuum states. Using this model, we can see clearly how these mechanisms significantly influence the Fermi levels of bound and continuum electrons, and thus, the dark current. Nonlinear temperature dependence of the dark current at low temperature is predicted and discussed in detail. The simulated dark currents exhibit good agreement with the experimental results, without use of parameter fitting techniques.