摘要:

Images and force measurements taken by an atomic‐force microscope (AFM) depend greatly on the properties of the spring and tip used to probe the sample’s surface. In this article, we describe a simple, nondestructive procedure for measuring the force constant, resonant frequency, and quality factor of an AFM cantilever spring and the effective radius of curvature of an AFM tip. Our procedure uses the AFM itself and does not require additional equipment.

ISSN:0034-6748    

DOI:10.1063/1.1143970

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Digital signal processors have made it possible to control scanned probe microscopes using straightforward software emulations of analog circuits. Using a system consisting of a commercially available digital signal processor board interfaced to analog I/O, we have developed algorithms for self‐optimizing feedback, raster generation (with hysteresis correction, sample tilt compensation, and scan rotation), lock‐in detection, and automatic tip‐sample approach. We also discuss an instruction parser that takes advantage of the digital architecture to allow automatic operation for extended periods.

ISSN:0034-6748    

DOI:10.1063/1.1144462

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A new low‐energy‐electron‐diffraction (LEED) instrument is described that combines resistive‐anode‐based position‐sensitive detectors with a high‐resolution electron gun capable of producing a beam having high spatial coherence (large instrument transfer width). The system produces digitized images of LEED patterns as well as high‐resolution spot profiles of individual beams using incident currents in the picoampere range, thus vastly reducing or eliminating electron beam damage to sensitive surfaces. The display resolution of both spot profile and full display images is 1024×1024 channels, and the dynamic range is 16 bits (≊104) per channel. The resistive anode pulse processing electronics limits maximum data acquisition rates to 1×106s−1. Specially developed software permits manipulation and numerical processing of the digitized two‐dimensional images at full resolution to produce spot contour maps, spot line profiles along arbitrary directions, Fourier transforms of spot profiles, integrated spot intensities, and intensity vs voltage curves with suitably subtracted background.

ISSN:0034-6748    

DOI:10.1063/1.1143971

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

In many scattering experiments, the quantity of most direct physical interest is a measure of the difference between two closely related scattering signals, each generated by a Poisson scattering process. This difference is often expressed in terms of an asymmetry statistic, that is, the difference normalized to the sum of the two signals, corrected for an additive background contribution. Typically, a propagation of errors approach is used to compute confidence intervals for asymmetry. However, these confidence intervals are not reliable in general. In this work, generally accurate confidence intervals for asymmetry are obtained using a parametric bootstrap approach. Based on the observed data, data are simulated using a Monte Carlo resampling scheme. The resampled data sets satisfy a constraint that ensures that background‐corrected count rates are not negative.

ISSN:0034-6748    

DOI:10.1063/1.1143972

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

An asymmetry statistic of physical interest is an estimate of the ratio of the difference and the sum of the Poisson rate parameters for two scattering processes. Typically, an additive background signal contributes to measurements of each signal. Background is measured in a third experiment. Data are corrected by subtracting measured background. When the measured background is larger than one of the other measurements, background‐corrected counts are negative. For this case, it would seem that no useful information can be extracted from the experiment. Here, asymmetry and an associated uncertainty interval are estimated for such cases using a bootstrap approach.  

ISSN:0034-6748    

DOI:10.1063/1.1143973

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Pendulum suspensions for laser interferometer gravitational wave detectors need to have an extremely highQfactor to minimize Brownian motion noise. In this paper we analyze the limits to theQfactor of the compound pendulum. We show that the observed acoustic loss of niobium can allow pendulumQfactors of 1010to be achieved. This should enable a 3 km terrestrial laser interferometer detector to achieve strain sensitivity of 10−22/&sqrt;Hz at frequencies as low as 10 Hz. At cryogenic temperaturesQfactors up to 1012should be achievable.

ISSN:0034-6748    

DOI:10.1063/1.1143974

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Gravitational radiation detectors must detect extremely small displacements, and thus require elaborate vibration isolation systems to attenuate the surrounding environmental noise. Contacting electrical leads can provide a direct path that will short circuit the vibration isolation, and degrade the antennaQfactor. Noncontacting microstrip transceivers provide a practical means of electromagnetically coupling to a resonant bar gravitational antenna when a microwave transducer is used as a signal readout system. Two microwave patch antennas now operate as transceivers on the University of Western Australia’s 1.5 tonne niobium bar gravitational radiation antenna and are described in detail in this paper. This system allows improved vibration isolation without degradation of the acousticQfactor of the niobium bar. Combined with a cryogenic amplifier, this system should achieve a reduction of 57 dB in the series noise power. This corresponds to a factor of 700 reduction in the effective displacement noise when referred to the transducer, from 2×10−16to 3×10−19m/&sqrt;Hz.

ISSN:0034-6748    

DOI:10.1063/1.1143975

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

In order to explore and develop technology required for the magnetic suspension of objects over large ranges of orientation, a small‐scale laboratory system, the large‐angle magnetic suspension test fixture (LAMSTF) has been constructed at NASA Langley Research Center. This apparatus falls into the category of large‐gap, actively stabilized magnetic levitation systems. The hardware comprises five conventional electromagnets in a circular arrangement, each driven from a separate bipolar power amplifier. Electromagnet currents are commanded by a digital control system, implemented on a microcomputer, which in turn derives the position and attitude of the suspended element from an infrared optical system. The suspended element is a cylindrical, axially magnetized, permanent magnet core, within an aluminum tube. The element is ‘‘levitated’’ by repulsive forces, with its axis horizontal, 0.1 m above the top plane of the electromagnet conductor. The element is stabilized in five degrees‐of‐freedom, with rotation about the cylinder axis not controlled. By mechanical rotation of the sensor assembly, the suspended element can be made to undergo a full 360° rotation about the vertical axis. The controller accommodates the changes in magnetic coupling between the electromagnets and the suspended element by real‐time adaptation of a decoupling matrix. This report presents a review of the background to the problem of magnetic suspension over large ranges of orientation. Next, the design and operation procedures adopted for LAMSTF, and the system hardware are described. Finally, some performance measurements are shown, together with illustration that the major design objective—the 360° rotation, has been accomplished.  

ISSN:0034-6748    

DOI:10.1063/1.1143976

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A theoretical expression for the sensitivity of a detection coil for a vibrating sample magnetometer (VSM) is derived. This expression is used to design a detection coil system for a biaxial VSM for measuring the magnetization components in the direction of the applied field (xdirection) and perpendicular to the applied field (ydirection). This detection coil system is optimized with respect to the sensitivity, cross talk, and their position dependences. Both an eight‐coil detection system and a 12‐coil detection system are described. With the eight‐coil detection system, experiments have been performed showing that, in an electromagnet VSM, images contribute significantly to the measured signals. The 12‐coil configuration has the lowest position dependences and cross talk. An example shows that the average position dependences in thexyplane can be five times better compared to the eight‐coil configuration with comparable sensitivities. For both detection directions the signal varies less than 1% within a circle in thexyplane with a radius of 3 mm. For a displacement in thezdirection and detection in thexdirection the position dependence is larger (about 2% for a displacement of 3 mm). Finally, some examples demonstrate the usefulness of a biaxial VSM for the magnetic characterization of recording materials.

ISSN:0034-6748    

DOI:10.1063/1.1143977

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A magneto‐optic small spot Kerr photometer device with a sampling spot size in the range of 4 &mgr;m is described. Key design features of the instrument include long working distance, large air‐core electromagnetic field coils, a sample holder which accommodates large samples, and a modulation detection scheme. DirectBHloops or the derivative, susceptibility signals can be obtained from a local area illuminated by the laser spot. The instrument can be used to obtain variations in magnetic properties across a large sample and is particularly suitable in wafer level testing in a manufacturing as well as research environment. Some studies done on soft magnetic films using this equipment are presented as examples of the versatility of the instrument.

ISSN:0034-6748    

DOI:10.1063/1.1143978

出版商:AIP    

年代:1993

数据来源: AIP