Lone‐Pair‐Induced Covalency as the Cause of Temperature‐ and Field‐Induced Instabilities in Bismuth Sodium Titanate

Bismuth sodium titanate (BNT)‐derived materials have seen a flurry of research interest in recent years because of the existence of extended strain under applied electric fields, surpassing that of lead zirconate titanate (PZT), the most commonly used piezoelectric. The underlying physical and chemical mechanisms responsible for such extraordinary strain levels in BNT are still poorly understood, as is the nature of the successive phase transitions. A comprehensive explanation is proposed here, combining the short‐range chemical and structural sensitivity of in situ Raman spectroscopy (under an applied electric field and temperature) with macroscopic electrical measurements. The results presented clarify the causes for the extended strain, as well as the peculiar temperature‐dependent properties encountered in this system. The underlying cause is determined to be mediated by the complex‐like bonding of the octahedra at the center of the perovskite: a loss of hybridization of the 6s² bismuth lone pair interacting with the oxygen p‐orbitals occurs, which triggers both the field‐induced phase transition and the loss of macroscopic ferroelectric order at the depolarization temperature.     

High temperature Fabry-Perot probe interrogated with tunable fibre ring laser

A high temperature fibre-optic based probe is described based on a miniature Fabry-Perot with a maximum operating temperature of ~700degC limited by the gold coated fibre transceiver link connecting the optical source to the probe. It was interrogated by a tunable fibre laser which could be linearly or sinusoidally tuned at scanning rates of 2 and 20 kHz, respectively. The transfer function of the system is in the form of channelled spectra rather than the usual interference fringes observed for interferometric sensors. The periodicity of the channelled spectra fringes is governed by the Fabry-Perot free spectral range which is related to the inverse of the probe temperature.     

An exact TEM calculation of loss in a stripline of arbitrarydimensions

Based on a modification of a conformal mapping originally introduced by R.H.T. Bates (1956), an exact expression for the quasi-static conductive attenuation constant α_c, of a general stripline with arbitrary dimensions is obtained. The method is based on a quasi-static formula for α_c which assumes a transverse electromagnetic (TEM) mode and is thus restricted to the case where the skin depth is small compared to the strip thickness. The final formula for α_c involves an integral of only a smoothly varying function, since an analytic extraction of a term representing the singular behavior of the charge density is performed in the analysis. A comparison of the results obtained using the present method and the approximate formulas of S.B. Cohn (1955) shows that a maximum error of about 5-6% occurs in the Cohn formulas at a point where the two approximate formulas are joined     

High-power 1320-nm wafer-bonded VCSELs with tunnel junctions

A new long-wavelength vertical-cavity surface-emitting laser structure is described that utilizes AlGaAs-GaAs mirrors bonded to AlInGaAs-InP quantum wells with an intracavity buried tunnel junction. This structure offers complete wavelength flexibility in the 1250-1650 nm fiber communication bands and reduces the high free-carrier losses and bonded junction voltage drops in previous devices. The intracavity contacts electrically bypass the bonded junctions to reduce threshold voltage. N-type current spreading layers and undoped AlGaAs mirrors minimize optical losses. This has enabled 134/spl deg/C maximum continuous-wave lasing temperature, 2-mW room-temperature continuous-wave single-mode power, and 1-mW single-mode power at 80/spl deg/C, in various devices in the 1310-1340 nm wavelength range.     

Pentacene TFT with improved linear region characteristics usingchemically modified source and drain electrodes

We have fabricated pentacene active layer organic thin film transistors (OTFTs) using chemically-modified source and drain contacts with improved contact and linear region characteristics. OTFTs fabricated on heavily doped, thermally oxidized single-crystal silicon substrates have linear field-effect mobility greater than 0.5 cm² /V-s at a drain-source voltage of -0.1 V, on/off current ratio greater than 10⁷, and subthreshold slope as low as 0.7 V/decade     

Examination of gradual-junction p-MOS structures for hot carriercontrol using a new lifetime extraction method

The effect of junction engineering on the hot carrier lifetimes of p-MOS transistors is examined. A normalizing method for predicting lifetimes is developed and used to show that a critical parameter controlling the lifetimes of submicrometer p-MOS devices is the size of the hot-carrier-damaged region. This is verified on conventional and gradual-junction transistors, where different implant species and energies were used to alter the source and drain junction profiles. Conventional junction devices with gate currents up to 100 times larger than those of gradual junction devices were found to have the same lifetimes as gradual junctions devices for the same effective transistor length. It is concluded that, contrary to n-MOS transistors, controlling the size of the damage region is as important as, if not more important than, reducing the hot electron gate currents by junction engineering in p-MOS devices     

Millimeter-wave radiometric observations of the troposphere: acomparison of measurements and calculations based on radiosonde andRaman lidar

A comparison of clear-air brightness temperatures is performed between radiometric measurements and atmospheric radiative transfer calculations. The measurements were made using the NASA Goddard Space Flight Center's Millimeter-wave Imaging Radiometer (MIR) in a series of airborne and ground-based atmospheric experiments at six millimeter-wave frequencies: 89; 150; 183.3±1, ±3, ±7; and 220 GHz. With the inclusion of the 220 GHz channel, these measurements are the first passive observations of the atmosphere made simultaneously at the six frequencies. The MIR was operated concurrently with supporting meteorological instruments (radiosonde and Raman lidar) to construct a paired set of both spatially and temporally coincident calibrated brightness temperatures and atmospheric profile parameters. Calculated brightness temperatures based on the measured atmospheric profile parameters were obtained using a numerical radiative transfer model. Incremental water-vapor weighting functions were used to study the impact of radiosonde hygrometer errors on the radiative transfer calculations. The aircraft-based brightness temperature comparisons are generally within 3 K for the channels sensitive to the lower atmospheric levels (89, 150, 183.3±7, and 220 GHz), but show discrepancies of up to 11 K for the opaque channels (183.3±1 and ±3 GHz) caused primarily by radiosonde bias. The ground-based calculations are similarly found to be sensitive to hygrometer errors in the lower atmosphere. Ground-based comparisons between MIR observations and lidar-based calculations are typically within ±6 K     

Resistance Compression Networks for Radio-Frequency Power Conversion

A limitation of many high-frequency resonant inverter topologies is their high sensitivity to loading conditions. This paper introduces a new class of matching networks that greatly reduces the load sensitivity of resonant inverters and radio frequency (RF) power amplifiers. These networks, which we term resistance compression networks, serve to substantially decrease the variation in effective resistance seen by a tuned RF inverter as loading conditions change. We explore the operation, performance characteristics, and design of these networks, and present experimental results demonstrating their performance. Their combination with rectifiers to form RF-to-dc converters having narrow-range resistive input characteristics is also treated. The application of resistance compression in resonant power conversion is demonstrated in a dc-dc power converter operating at 100MHz     

Applications of static and dynamic iterated rippled noise to evaluate pitch encoding in the human auditory brainstem

This paper presents a new application of the dynamic iterated rippled noise (IRN) algorithm by generating dynamic pitch contours representative of those that occur in natural speech in the context of EEG and the frequency following response (FFR). Besides IRN steady state and linear rising stimuli, curvilinear rising stimuli were modeled after pitch contours of natural productions of Mandarin Tone 2. Electrophysiological data on pitch representation at the level of the brainstem, as reflected in FFR, were evaluated for all stimuli, static or dynamic. Autocorrelation peaks were observed corresponding to the fundamental period (tau) as well as spectral bands at the fundamental and its harmonics for both a low and a high iteration step. At the higher iteration step, both spectral and temporal FFR representations were more robust, indicating that both acoustic properties may be utilized for pitch extraction at the level of the brainstem. By applying curvilinear IRN stimuli to elicit FFRs, we can evaluate the effects of temporal degradation on 1) the neural representation of linguistically-relevant pitch features in a target population (e.g., cochlear implant) and 2) the efficacy of signal processing schemes in conventional hearing aids and cochlear implants to recover these features.