摘要:

A simple and inexpensive circuit to facilitate the direct measurement of capture cross section, when synchronized with a deep level transient spectroscopy system, is described. It avoids the most commonly encountered problem of loading and distortion of the bias (trap filling) pulses of nanosecond duration in the capture cross‐section measurement. The capacitance meter, whose internal circuitry is responsible for the distortion, is connected and disconnected from the rest of the apparatus with the help of simple and low‐cost reed relay switches featuring high operating speed and low contact resistance. Sharp bias pulses as small as 30 ns can successfully be applied to the sample with no observable distortion. Finally, a representative measurement is shown to demonstrate the simplicity and high performance of the circuit. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147579

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The effect of macroscopic inhomogeneities on resistivity and Hall angle measurements is studied by calculating weighting functions (the relative effect of perturbations in a local transport property on the measured global average for the object) for cross, cloverleaf, and bar‐shaped geometries. The ‘‘sweet spot,’’ the region in the center of the object that the measurement effectively samples, is smaller for crosses and cloverleafs than for the circles and squares already studied, and smaller for the cloverleaf than for the corresponding cross. Resistivity measurements for crosses and cloverleafs suffer from singularities and negative weighting, which can be eliminated by averaging two independent resistance measurements, as done in the van der Pauw technique. Resistivity and Hall measurements made on sufficiently narrow bars are shown to effectively sample only the region directly between the voltage probes. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147527

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A magneto‐optic Kerr effect system for the measurement of magnetization in thin ferromagnetic layers based on a photoelastic modulator is described. The use of a quarter wave plate allows light with a variable polarization incident on the sample to be used. The polarization of the light as it passes through the system is treated algebraically using a matrix approach. A procedure for determining both the magneto‐optic (MO) rotation presented by the sample and the traditional ellipsometric parameters is described, the consistency of the solutions being demonstrated by experiment. Typical MO rotation versus thickness curves for thin films of Permalloy (Ni79Fe21) deposited on glass for both the longitudinal and transverse field configurations are presented. These results demonstrate that the magnitude and sign (in the transverse case) of the MO rotation is strongly dependent on thickness for films thinner than 200 A˚. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147528

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The measurement setup for studying changes of electric permittivity induced in liquids by a strong electric field nonlinear dielectric effect, (NDE) is presented. The construction is based on the idea of frequency modulation of an LC generator (with an inductance L and a capacitance C in resonant circuit), proposed by Malecki [J. Chem. Soc. Faraday Trans. II72, 104 (1976)]. The strong electric field is applied in the form of rectangular pulses (typically 1–4 ms). The setup enables measurements in a broad range of frequencies (80 kHz–12 MHz) and contains a new calibrating system, minimizing the influence of systematic error on the measured NDE values. We also indicate menthol as a standard, reference liquid in NDE studies. New applications of the NDE technique for studying relaxation processes in critical solution are also presented. They are based on the time resolved analysis of NDE decay after switching off the strong electric field. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147529

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A reentrant rf cavity resonator has been developed for automated detection of phase separation of fluid mixtures contained within the cavity. Successful operation was demonstrated by redetermining the phase boundaries of a CO2+C2H6mixture in the vicinity of its critical point. We developed an accurate electrical model for the resonator and used helium to determine the deformation of the resonator under pressure. With the model and pressure compensation, the resonator was capable of very accurate dielectric measurements. We confirmed this by remeasuring the molar dielectric polarizabilityA&egr;of argon and obtained the resultA&egr;=(4.140±0.006) cm3/mol (standard uncertainty) in excellent agreement with published values. We exploited the capability for accurate dielectric measurements to determine the densities of the CO2+C2H6mixture at the phase boundaries and to determine the dipole moment of 1,1,1,2,3,3‐hexafluoropropane, a candidate replacement refrigerant. Near the operating frequency of 375 MHz the capacitor in the resonator has an impedance near 14 &OHgr;. This low impedance is more tolerant of electrical conductivity within the test fluid and in parallel paths in the support structures than comparable capacitors operating at audio frequencies. This will be an advantage for operation at high temperatures where some conductivity must be expected in all fluids. Of further value for high‐temperature applications, the present rf resonator has only two metal–insulator joints. These joints seal coaxial cables; neither joint is subjected to large mechanical stresses and neither joint is required to maintain precise dimensional tolerances. The resonator is rugged and may be operated with inexpensive electronics.

ISSN:0034-6748    

DOI:10.1063/1.1147580

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A far‐infrared reflectivity set up has been developed for spectroscopy on highly reflective materials in a 20 T‐class resistive Bitter magnet. As a first application, far‐infrared reflectivity measurements on the heavy‐fermion compound URu2Si2have been performed using a silicon reflection Fabry–Pe´rot interferometer as a multiple reflection device. In a resonance, this Fabry–Pe´rot technique is one order of magnitude more sensitive than a single reflection measurement. Changes in the reflectivity as a function of magnetic field are resolved with an accuracy of 0.2% and the absolute value of the reflectivity can be obtained with an accuracy of 0.5%. With this interferometer, an excitation at about 40 cm−1in the heavy‐fermion system URu2Si2is investigated at temperatures between 2 and 20 Kelvin and in magnetic fields up to 20 T. The excitation appears to extend to lower energies under influence of a large magnetic field. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147530

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Thin film permeability measurement techniques such as stripline cavities or conventional permeameters require the sample undergoing measurement to be inserted within a fixture; typically, these samples can be no larger than a few square inches. However, certain applications require knowledge of the complex permeability of a coating or film applied to a finished, large product such as films deposited onto a continuous web for use as identification markers, the inside of a computer case for the control of EMI, or a ship mast for reduction of radar reflections. Therefore, a surface permeameter is presented that measures the rf complex permeability spectra of continuous films or coatings that are placed adjacent to the fixture over the frequency range of 10–500 MHz. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147531

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

An instrument is described which combines a glancing incidence ion beam erosion system with a scanning tunneling and an atomic force microscope. This instrument allows the ion beam polishing and surface topographic characterization of conducting and insulating, crystalline, polycrystalline, and amorphous samples under ultrahigh vacuum conditions. As an illustration of the capability of the instrument and the polishing technique, we present results demonstrating a fivefold improvement in rms roughness of a polycrystalline Cr film and a fivefold reduction in rms roughness of a vicinal, initially mechanically polished CaF2(111) sample. The final rms roughness of the latter sample of 0.12±0.04 nm measured over the bandwidth of 10–500 nm is just 75% of the Ca–F interlayer spacing for the (111) surface orientation. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147532

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Microwave digestion provides a rapid means of sample preparation in the analytical laboratory. The major disadvantage of this method is the high cost of commercial microwave digestion systems. Modifications to the magnetron timing circuits of an appliance grade oven to make it suitable for sealed vessel microwave digestion are reported. The oven was modified without alteration to the irradiation cavity, and all initial safety features remain intact. Following digestion with the modified oven, NIST Standard Reference Materials (SRMs) were analyzed by inductively coupled plasma emission spectroscopy. The analytical data are similar to the certified concentrations in the SRMs, indicating that the modified oven provides a durable, rapid, cost‐effective means of sample preparation. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147533

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The recurrent difficulties of rapidly localizing a small object in a large volume of highly scattering material such as brain and breast has been the ‘‘stumbling block’’ of optical methods for tumor detection. Amplitude cancellation of in‐ and out‐of‐phase photon diffusion patterns, used in detection of small objects containing highly absorbing and fluorescing contrast agents provides real time, two‐ and three‐dimensional localization of objects of mg size and picamole contents in models of human breast and brain tumors with positional accuracies of millimeters. A simple robust electronic circuit is described and tested to give a phase accuracy of ∼0.1°. An electro‐optical scan rapidly detects small objects in a large volume that simulates the absorption/scattering characteristics of human head or breast. ©1996 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1147534

出版商:AIP    

年代:1996

数据来源: AIP