摘要:

The minimum scanning speed of atomic force microscopes for improved atomic resolution has been measured in liquid media, and shown to be equal to 100 nm/s for mica immersed in water corresponding to the time spent scanning the distance between two neighbor ions (∼0.52 nm) of ∼5 ms. The scanning velocity dependence of the force acting on the tip in the double-layer region (∼135 nm) when it approaches the surface was also measured. The stationary component of this force, for scanning speeds up to 30 &mgr;m/s, was identified as the exchange of the liquid media with&egr;≈80by the tip with&egr;≈6.As the tip approaches the surface and as well as when the tip images atomic features, this repulsive force shows a relaxation time of a few milliseconds, corresponding to the shielding of the surface charge by the solution, i.e., the double-layer relaxation time. Scanning surfaces at speeds higher than the ratio of the atomic features distance and this relaxation time results in a variable repulsive force acting on the tip, as a function the scanning speed, which might be used to improve the atomic imaging resolution. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149143

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Scanning probe microscopes use piezoelectric actuators (piezoscanners) as extremely sensitive and exact positioners for small scaled sample scannings. They are capable of making fine movements along theX,Y, andZaxes, with displacements that can vary from several hundreds of angstroms to some micrometers. For high amplitude sweeps, the displacement response results in visible distortions due to the intrinsic nonlinearity effects of thepiezoelectric tube. This work proposes the nonlinearity correction by means of a microcontroller-based system which produces (in real time) the correction values using the piezoscanner quadratic transfer function, adapted to an atomic force microscope (AFM) of its own construction. The AFM is intended to be used with biological samples. Other correction methods are described. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149144

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A unique system to nonintrusively track a particle in three dimensions is presented. It is based on the principle of magnetic induction coupling and consists of small transmitters mounted inside the particle being tracked, and a set of receiving antennae surrounding the experimental apparatus. In Part I of the sequence of two papers, the focus is on the theoretical aspects, in particular, on developing a computational technique to solve the inverse problem, i.e., find the three-dimensional position as well as orientation of the particle from the voltages induced in the antennae. The computational technique is based on the Levenberg–Marquardt algorithm, along with a scheme for providing good initializations. Through simulated experiments that include various levels of added noise in the voltage readings, the success of this algorithm is demonstrated and the feasibility of the overall technique is established. It is also shown that a system with three mutually orthogonal transmitters provides accurate results even with the noise in the voltage data. Although this system is intended for use in experimental studies of dry granular flows, it has wide applicability due to its nonintrusive nature, and is particularly useful when optical tracking techniques are not feasible. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149145

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A system, based on the principle of magnetic induction coupling, to track a particle nonintrusively in three dimensions is presented. It consists of small transmitters mounted inside the particle being tracked, and a set of receiving antennae surrounding the experimental apparatus. In Part I of the sequence of two papers, the focus is on the theoretical aspects, in particular, on developing a computational technique to solve the inverse problem of finding the three-dimensional position as well as orientation of the particle from the voltages induced in the antennae. In Part II the focus is on the actual system development, including all hardware and data acquisition aspects. The results presented here are comprehensive, as they include details of hardware/electronics, comparison of induced voltage model and actual measurements, and a set of actual tracking results. Through test experiments that include a variety of real-time trajectories, this system is tested and its success is demonstrated. Results of real trajectories of a single ball rising in a mass of other spheres in a vibrated bed are also shown. Although this system is intended for use in experimental studies of dry granular flows, it has wide applicability due to its nonintrusive nature, and is particularly useful when optical tracking techniques are not feasible. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149146

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A near field optical microscope with a scanning area up to1 mm×1 mmwas demonstrated. The scanning element is the tapered fiber probe and a cylindrical lens is used to focus the feedback laser. Instead of the vibrational amplitude, the vibrational angle signal is used to regulate the distance between the probe and sample. The microscope was employed to map out the evanescent wave on a channel waveguide. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149147

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A version of a Kelvin probe that has been developed for an atomic force microscope (AFM), here referred to as a nano-Kelvin probe, is used to create maps of surface potential for study of samples that have been abraded by an AFM tip. The focus is on the wear at very low loads that involve the absence of wear debris and/or wear scars. Wear scars at higher loads, where significant damage to surface has occurred, have also been studied for reference purposes. Samples studied include single crystal aluminum, alumina, gold, and silicon. It is shown that even in cases where there is little or no damage to the surface, as observed by topography scans of an AFM, there is often a large change in the potential at the surface of the sample. The change in surface potential is believed to be the result of chemical and structural changes in the first few nanometers of the sample. We have shown that, even in the case of “zero wear” (that is, no visible deformation of the surface), there can be a significant change in the surface potential inside the wear scar. This allows for the study of the onset of wear in the ultralow wear regime that is not possible with other techniques. The study of wear precursors is particularly important for the hard disk drive industry, the microelectromechanical systems industry, and other industries where the presence of any microscopic wear or debris can represent total failure. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149148

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The authors extend their study of the effect of macroscopic impurities on resistive and Hall measurements to include objects of finite thickness. The effect of such impurities is calculated for a series of rectangular parallelepipeds with two current and two voltage contacts on the corners of one square face. The weighting functions display singularities near these contacts, but these are shown to vanish in the two-dimensional limit, in agreement with previous results. Finally, it is shown that while Hall measurements principally sample the plane of the electrodes, resistivity measurements sample more of the interior of an object of finite thickness. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149149

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A noncontact technique of measuring the changes in electrical conductivity (or resistivity) of conducting liquids is reported. The technique is based on a conducting drop that is levitated by the high-temperature electrostatic levitator in a high vacuum. This technique, which utilizes the principle of the asynchronous induction motor, measures the relative changes in torque as a function of temperature by applying a rotating magnetic field to the sample. Changes in electrical resistivity are related to the changes in measured torque using the formula developed for the induction motor. Validity of this technique was demonstrated using a pure aluminum sample around its melting temperature. When the measurement results were calibrated by a literature value of resistivity at the melting point, our resistivity data around the melting point could be expressed byrliq=24.19+1.306×10−2(T−Tm)&mgr;&OHgr; cm overTm∼1160 K,rsolid=10.77+1.421×10−2(T−Tm)&mgr;&OHgr; cm over700 K∼Tm,and the thermal conductivity as determined by the Wiedemann–Franz–Lorenz law from the resistivity data was given by&kgr;liq(T)=94.61+4.41×10−2(T−Tm)W m−1 K−1,&kgr;solid(T)=211.13−7.57×10−2(T−Tm)W m−1 K−1. Both electrical resistivity and thermal conductivity are in close agreement with the literature, confirming the validity of the present technique. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149150

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A system for the direct and continuous determination of the irreversibility line in highTcsuperconductors is described. The system, based on a standard ac susceptometer, operates by applying a controlled dc field to the sample that maximizes the imaginary (out-of-phase) component of the ac susceptibility. This control is achieved using a proportional-integral-derivative loop with the reference set to zero and with the magnetic field derivative of the imaginary component of the ac susceptibility taken as the feedback sample (error signal). To obtain a dc voltage proportional to this derivative a double lock-in detection scheme is used. This apparatus is able to carry out a continuous measurement of this line by sweeping the temperature in the range of interest while the sample is maintained, by the controller, at the irreversibility magnetic field at each temperature. Hence, the whole process is accomplished in the same time as a typical resistivity‐versus-temperature measurement, which is faster than other methods. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149167

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A novel probe head with a two-loop-one-gap resonator forX-band (9.5 GHz) time-resolved transient electron paramagnetic resonance measurements with direct detection is described. The low quality factorQof the resonator allows time resolution in the order of 10 ns while the high filling factor guarantees a sensitivity which is at least comparable to that of conventional cavity resonators. A novel feature of the resonator is the coupling mechanism which exploits the field distribution in the microwave shield. The type of the resonator and the microwave coupling scheme chosen provide high mechanical stability of the setup. This is particularly important for pulsed laser excitation and gas stream cooling of the sample. The advantages of the probe head are demonstrated by relaxation measurements on the photoexcited triplet state of zinc-tetratolylporphyrin, for which the high time resolution allows the determination of fast anisotropic relaxation. This relaxation behavior can be attributed to the dynamic Jahn–Teller effect. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149151

出版商:AIP    

年代:1998

数据来源: AIP