Intense plasma waves are generated by an HF pump wave in an ionospheric heating experiment at the Arecibo Observatory. These plasma waves can be observed as enhancements to the ion and plasma lines of the incoherent backscatter echo. The enhancements can be 3 or 4 orders of magnitude more intense than the unenhanced lines and tend to fluctuate wildly. Both the purely growing and the decay mode parametric instabilities are present. When the pump wave is turned on abruptly, the enhancements develop in time in a repeatable manner. A rather remarkable feature on time scales of seconds is an overshoot in instability power. Overshoots occur frequently but not universally and last for 1-6 s. They can have a magnitude from 10 to hundreds of times the average instability level. The growth of field-aligned irregularities may be the cause of the overshoots. The overshoots appear to be definitely related to an unusually rapid rise in measured electron temperature that cannot be understood in terms of ohmic energy deposition. On time scales of milliseconds th.•re is a 'miniovershoot' before the growth of the instability to a large value. The spectral details also cl•ange in a striking manner. The instabilities can first be detected 2-4 ms after pump wave turn-on. The decay mode is present as well as a broad featureless 'noise bump,' which partially sharpens into a line as time progresses. These changes of the spectra in time seem to run counter to the currently accepted theories of plasma wave saturation.

The observed performances of carbon nanotube field-effect transistors are examined using first-principles quantum transport calculations. We focus on the nature and role of the electrical contact of Au and Pd electrodes to open-ended semiconducting nanotubes, allowing the chemical contact at the surface to fully develop through large-scale relaxation of the contacting atomic configuration. As expected from their respective work functions, the Schottky barrier heights for Au and Pd turn out to be fairly similar for realistic contact models. We present, however, direct numerical evidence of Pd contacts exhibiting perfect transparency for hole injection as opposed to that of Au contacts. These findings support experimental data reported to date.The superb performances of carbon nanotube ͑CNT͒ field-effect transistors have been demonstrated over the past decade. 1-8 However, a number of factors determining the current-voltage ͑I-V͒ characteristics have to be clarified for extensive device applications. A key factor in this context is the Schottky barrier ͑SB͒ formed at the interface between the source and drain metallic electrodes and the CNT. Traditionally, Schottky contacts have been introduced to serve as passive ohmic contacts. However, Schottky contacts in CNT field-effect transistors play an active role in affecting the transistor action. For example, the drastic disparity of reported performances of CNT transistors has generally been attributed to the difficulty in controlling the position of the Fermi energy E F with respect to the valence and conduction bands of the CNTs, when they are brought into contact to metal electrodes via different fabrication processes. 9,10 The different E F locations should in turn give rise to different SB's and hence different I-V behaviors.In the simplest Mott and/or Schottky picture for metalsemiconductor interfaces, the potential barrier of electrons is dictated primarily by the difference between the metal E F and the CNT electron affinity. Accordingly, the gap of the semiconducting CNT, which is roughly inversely proportional to its diameter, is an important factor for determining the barrier for holes. Recently, a correlation has been shown between the diameter of the CNT and the on current for negative gate voltage of the device fabricated therein. Specifically, the on current increases with increasing CNT diameter. 11 Also, the work function of the metal electrodes has been shown to affect the device performance in a number of interesting ways. For instance, a metal electrode with a large work function, e.g., Pd, is shown to induce large on currents for holes, 12 while metal electrodes having small work functions, e.g., Al, enhance the on current of electrons. 13 When E F lies near the midgap for intermediate values of the work function, the device is shown to exhibit an ambipolar behavior. 3,7,13 Early theoretical work discussed the apparent validity of the Mott-Schottky picture in CNT transistors. 14 However, this simple picture cannot account for the general features ...

Examined in this paper is the role of the metal electrode influencing the structure and electronic properties of semiconducting carbon nanotubes near the interface at low bias. Specifically, we present quantum-chemical calculations of finite sections of a (8,0) semiconducting single wall nanotube contacted with gold and palladium clusters. The calculations at the density functional level of theory, which included full geometry optimizations, indicate the formation of bonds between the metal atoms of the electrode and the carbon atoms of the nanotube. The local work function of the metal electrode can be expected to exhibit significant variations as a result of this bond formation. Compared to the gold-contacted nanotubes, the palladium-contacted nanotubes have a small but interesting increase in both length and diameter. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the gold-contacted nanotube are shown localized at the edges. In contrast, the HOMO and LUMO of the palladium-contacted nanotube are extended over the entire nanotube and the metal cluster contacted to it, providing thereby a better conduction path in the contact region of the electrode and the nanotube. The involvement of the highly directional d orbitals in the interactions involving the palladium cluster leads to an enhanced pi electron density in the nanotube. This enhanced pi electron density is synonymous with an improved electron transmission.

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A Q-switched frequency-doubled Nd : YAG laser was used to record holograms in LiNbO3 : Fe. For 1% diffraction efficiency, only 50 mJ/cm2 was needed, as compared to 500 mJ/cm2 for cw recording. Induced absorption, comparable in magnitude with the linear absorption at 647 nm, was observed during hologram recording. This indicates the existence of secondary absorption centers in LiNbO3 : Fe. Their contribution to the enhanced photorefractive effect is discussed.