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Self-heating effects in organic electronic devices are investigated by impedance spectroscopy (IS). A temperature modulation of the admittance couples the gigantic imaginary part of the capacitance to its small real part, which results in negative capacitance at low frequencies with high bias. We present a model to explain the effects and verify it experimentally for hole-only devices and organic light-emitting diodes. The negative capacitance widely observed in various electronic devices can be explained, at least in the organic electronic devices, by self-heating effects. The generic effects, which smear the device details, need to be eliminated to obtain IS signals that reflect the intrinsic device-specific properties.

Polysilanes are polymers containing only silicon in the main chain and having organic substituents. One of the most interesting properties of the polysilanes is their remarkable electronic absorption These polymers with a-bonded main chains show optical absorption in the UV region. It is believed that this results from g-electron delocalization along the main Besides the electronic absorption, some polysilanes also show thermochromism, piezochromism, photoluminescence, photodegradation, conductivity, etc., and a number of new applications are e~pected.~-~5 Modern interests in polysilanes are generated by these unique electronic properties. As the electronic properties are attributed to a-electron delocalization along the main chains, it is of interest to form well-oriented polysilanes.Vacuum deposition has been of interest as a method to form thin films. The method has been utilized to form inorganic and metallic thin films in various fields. Recently, the vacuum-deposition method has been considered as one of the powerful methods to form organic thin films, and a number of studies have been done with vacuumdeposited organic It is well known that vacuumdeposited organic thin films show different structures or properties depending on the deposition conditions, such as the temperatures of the substrate and furnace, kind of substrate, deposition rate, etc. Poly(dimethylsi1ane) (PDMS) is a simple representative of the polysilanes and it is an important polymer to understand the basic properties of polysilanes. However, the insolubility of PDMS makes it difficult to characterize, to utilize the polymer as materials, or to make PDMS thin films. In this study, the vacuum-deposition method was adopted to form thin films of PDMS, and the molecular orientation of the vacuum-deposited films will be discussed in relation with their UV spectra.PDMS powder, which was commercially obtained from Nippon Soda Co., was used as an evaporation source without further purification. PDMS films were prepared with an apparatus for vacuum deposition (Ayumi Kogyo Model VE-88-11). The PDMS powder was heated in a quartz furnace around 400 OC and deposited on quartz plates for the UV measurements. A thickness monitor with a quartz oscillator was used to monitor the rate of the vacuum deposition and the amount of deposited films. PDMS was vacuum-deposited on the substrates at room temperature in a vacuum of less than 5 X Torr, and the vacuum depositions on liquid-nitrogen-cooled substrates (-150 "C) were performed in a vacuum of less than 5 X IO4 Torr. A Japan Spectroscopic Co. Model Ubest-0024-9297/94/2227-706$04.5010 55 spectrometer was used for the UV measurements. UV spectra of the vacuum-deposited films were measured in the usual alignment and in an alignment with an inclined sample. The spectra will be discussed in relation with the molecular orientation of the films.Furukawa et al. reported that a sharp absorption was observed in a UV spectrum of vacuum-deposited PDMS film. They explained the absorption as being due to an excitonic excit...

μSR measurements were performed on HgBa 2 Ca 4 Cu 5 O y (Hg-1245) samples with Tc of 108 K. Rapid enhancement of muon-spin depolarization rate and losses of initial asymmetry were found in ZF-μSR time spectra below 60 K. These indicate enhancement of magnetic correlation between Cu spins. Moreover, muons precession signals were observed below 45 K, indicating that the Hg-1245 samples were in long range ordered state. These results reveal that antiferromagnetic order is developing in the Hg-1245 superconductor below Tc.

Thin films of poly(dimethylsilane) (PDMS) and permethyldodecasilane (DM12) were prepared by the vacuum-deposition method, and the molecular orientation in the films was investigated. Welloriented films of DM12 were prepared without detectable degradation; however, degradation occurred in the films of PDMS, though well-oriented films were also obtained. Infrared and UV spectra of the vacuumdeposited films were measured with inclined sample alignment. In the spectra of films deposited to substrates at room temperature, a methyl rocking (parallel) band and a sharp UV absorption, which have the same polarity parallel to the chain axis, became stronger when the samples were inclined. Both of the absorptions also showed large dichroism when the samples were inclined. These results suggest that the PDMS and DM12 molecules in the films were oriented perpendicular to the substrate. The infrared spectra of vacuum-deposited PDMS films show Si-H and Si-O bonds formation; i.e., PDMS molecules were degraded during the deposition process. In clear contrast, the infrared spectra of the vacuum-deposited films of DM12 showed no molecular degradation.