ALBERT

Notes: We have successfully built and tested a circuit designed to control a piezoelectric tube scanner having the standard single inner-electrode quartered outer-electrode configuration, using digital-to-analog (D/A) converters commercially available. To avoid noise associated with the PC, the signals transmitted by the D/A channels to the control electronics are received by instrumentation amplifiers INA 105 at the control circuit, providing 86 dB common mode rejection, thereby over four orders of magnitude of immunity to common mode noise. To prevent ground loops in the communication between the control electronics and the analog-to-digital (A/D) converters, a novel approach was used. The signals sent by the control electronics to the A/D converters were transmitted via isolation amplifiers ISO 122 followed by a 10 kHz Sallen–Key low pass filter incorporated at each output of the control circuit, providing galvanic isolation between the control electronics and the PC, thereby eliminating ground loops. The control circuit was designed to allow analog as well as digital feedback, selectable via a toggle switch. The design also incorporates the possibility of using two independent external signals to modulate the polarization of the sample and two independent external signals to modulate the piezoelectric transducer drive along the Z direction. It also incorporates the possibility of electronically canceling the slope that might occur while scanning due to the sample being tilted along the X axis (fast scan direction) and/or along the Y axis (slow scan direction). The circuit was tested using two 12 bit A/D–D/A converters DAS 1602 to control the scanner of a scanning tunneling microscope, with a home-built scanning head, electrometer, and preamplifier. With the complete system in operation but in the absence of tunneling current, the electrometer exhibits a current noise under 3 pA rms and a response time of 30 μs to a step input current, a performance that compares well with that of bulkier and more expensive commercial low noise current amplifiers. To calibrate the instrument and verify proper control operation, we obtained images of a commercial holographic grating covered with gold running the instrument in the digital feedback mode, using the algorithm described by Piner and Reifenberger [Rev. Sci. Instrum. 60, 3123 (1989)]. The control circuit and the electrometer turn out to be about one order of magnitude less expensive than commercially available control circuits and low noise current amplifiers of similar performance. © 1998 American Institute of Physics.

Notes: Parametric decays of a circularly polarized electromagnetic wave in an electron–positron unmagnetized plasma are studied. Like in the case of a plane polarized wave, there are two distinctive situations. One in which vs/c〈c/vp (vs is the electroacoustic speed, c the speed of light, and vp is the phase velocity of the electromagnetic wave), and the other situation when vs/c≥c/vp. In the first case, there is an ordinary decay instability and two modulational instabilities. One of the modulational instabilities is a resonant instability, and the other is a nonresonant, essentially electromagnetic instability in which the pump wave decays into two sideband waves. In the second situation (vs/c≥c/vp), there are two modulational instabilities similar to the previous situation, but as the intensity of the pump wave increases, the resonant modulational instability disappears and only the nonresonant electromagnetic instability remains. The effect of Landau damping on the electroacoustic modes is also studied. This effect is simulated through a collisional term in the fluid equations. © 1998 American Institute of Physics.

Notes: The integrability of multivector fields in a differentiable manifold is studied. Then, given a jet bundle J1E→E→M, it is shown that integrable multivector fields in E are equivalent to integrable connections in the bundle E→M (that is, integrable jet fields in J1E). This result is applied to the particular case of multivector fields in the manifold J1E and connections in the bundle J1E→M (that is, jet fields in the repeated jet bundle J1J1E), in order to characterize integrable multivector fields and connections whose integral manifolds are canonical lifting of sections. These results allow us to set the Lagrangian evolution equations for first-order classical field theories in three equivalent geometrical ways (in a form similar to that in which the Lagrangian dynamical equations of nonautonomous mechanical systems are usually given). Then, using multivector fields, we discuss several aspects of these evolution equations (both for the regular and singular cases); namely, the existence and nonuniqueness of solutions, the integrability problem and Noether's theorem, giving insights into the differences between mechanics and field theories. © 1998 American Institute of Physics.

Notes: We present a method for determining the thermal diffusivity in opaque solids by means of an analysis of the photoacoustic phase signal at low modulation frequencies using the open-cell photoacoustic technique. We show for f≤(π/2)2fc, where fc is the modulation frequency at which the thermal diffusion length matches the sample thickness, the photoacoustic phase signal can be written in linear form with the modulation frequency f. Then, obtaining the proportionality coefficient by fitting the experimental data, the thermal diffusivity of the sample can be determined. The advantage of this method is that it is realized in a range of modulation frequencies below those normally used, hence, the photoacoustic signal should be alone attributed to the mechanism of thermal diffusion. Moreover, the signal-to-noise ratio will be more reliable. This method was tested in some samples and it is also shown to be important in solids with high diffusivity values and thin materials. © 1998 American Institute of Physics.

Notes: Gold and nickel Schottky barrier photovoltaic detectors have been fabricated on Si-doped AlxGa1−xN layers (0≤x≤0.22) grown on sapphire by metalorganic vapor phase epitaxy. Responsivity is independent of the Schottky metal or diode size, and also of the incident power in the range measured (10 mW/m2–2 kW/m2). A higher visible rejection has been observed in the spectral response of Au photodiodes (〉103). Time response is resistance-capacitance limited, with time constants as short as 14 ns in Al0.22Ga0.78N diodes. Low frequency noise studies are also presented, and detectivities of 6.1×107 and 1.2×107 mHz1/2 W−1 are determined in GaN/Au and Al0.22Ga0.78N/Au detectors, at −2 V bias. © 1998 American Institute of Physics.

Notes: The maximum entropy method is presented in this letter as a highly interesting procedure for the investigation of high frequency noise properties of bulk semiconductors and electron devices at microscopic level. A Monte Carlo simulation of the hot electron velocity fluctuations in bulk GaAs has been performed to illustrate the efficiency and usefulness of this procedure. Comparisons with the most popular techniques presently used in Monte Carlo simulations of noise have also been performed. © 1998 American Institute of Physics.

Notes: Implantation of Au ions into Si-implanted fused quartz strongly enhances the photoluminescence (PL) intensity around 630 nm measured after subsequent sample annealing at 900 °C. This effect is attributed to the enhancement of the formation of Si nanocrystals by the presence of Au ions and not by ion-implantation-induced defects. This conclusion was deduced by monitoring the defect formation in fused silica by 2 MeV Si ion implantation with doses ranging from 2×1016 to 1×1017 Si/cm2. Some of the 4×1016 Si/cm2-implanted samples were reimplanted at a similar depth with 10 MeV Au ions at doses of 4×1016 and 1.2×1017 Au/cm2. The absorption spectroscopy, electron paramagnetic resonance and PL measurements show the presence of B2 and E′ matrix point defects in as-prepared Si-implanted samples. As these defects disappear after annealing at 600 °C, the presence of a strong PL peak in samples implanted and annealed at 900 °C strongly suggests that the observed luminescence is produced by Si nanoparticle formation. © 1998 American Institute of Physics.