Low-frequency Pole Research Articles

Comparison of AC and DC constant photocurrent methods for determination of defect densities

We propose an explanation for large discrepancies in the absorption spectrum of a-Si:H between measurements by DC and AC constant photocurrent methods (CPM). DC measurement gives a consistently higher value for the absorption coefficient α at low photon energies. A small-signal analysis of the photoconductive response to modulated sub-gap illumination reveals low frequency poles associated with thermal emission processes, which explain this discrepancy. We demonstrate with computer simulations that while DC CPM, which includes these transitions, gives a more accurate value for absorption, AC CPM provides a more accurate means of determining the distribution of occupied gap-states. Further, we show that combining DC and AC methods allows determination of the distribution of deep unoccupied gap-states. These concepts are applied to experimental results for several undoped a-Si:H films.

Theory of far-infrared reflectivity and surface polaritons of two-component magnetoplasmas.

The optics of a two-component magnetoplasma within the Drude model is developed and compared with the one-component case. The Voigt permittivity has an additional low-frequency pole. Consequently an additional bulk-polariton band appears, leading to a prominent low-frequency dip in the normal-incidence reflectivity. An additional surface polariton is found, and the attenuated-total-reflection spectrum is quite different from the one-component case. The frequencies of all the plasma-related spectral features are functions of the applied magnetic field. It is concluded that the densities and effective masses of both majority and minority carriers can be determined from suitable far-infrared spectra.

Dynamic Properties of a Nonsuperfluid Bose Liquid in the Random-Phase Approximation

The dynamic structure factor $S(k, \ensuremath{\omega})$ of a nonideal Bose liquid is calculated within the random-phase approximation and compared with neutron scattering data by Cowley and Woods for liquid helium in the temperature range ${T}_{\ensuremath{\lambda}}<T<~4.2\ifmmode^\circ\else\textdegree\fi{}$K, with the conclusion that the model is wholly inadequate. A low-frequency pole in the generalized susceptibility discussed by Mattis and Landovitz is shown to be overdamped in $S(k, \ensuremath{\omega})$.

Computation of vocal tract area functions

This paper describes a noniterative algorithm for synthesizing vocal tract area functions. The vocal tract is parameterized in terms of a few of the low-frequency poles and zeros of the driving point impedance at the lips and the length of the tract. Using an asymptotic relation, ship for a Sturm-Liouville system, additional critical frequencies are supplied on the basis of the length parameter only. The algorithm, which is due basically to Richards, yields a structure which is a cascade of equal length uniform tubes of differing cross sectional areas. The number of such tubes depends upon the number of critical frequencies specified. Vowel data of Fant is used to test the algorithm. It is found that the synthesized area functions are good approximations to the actual ones. Because of the noniterative nature of the algorithm, execution time is very small.

A holding technique to reduce number of bits in digital transfer functions

A novel technique for hardware implementation of low-frequency poles in sampled-data systems employs a zero-order hold function. The hold makes possible the desired coefficient accuracy with a smaller word size than theoretically possible for a given sampling rate.