Probe Separation Research Articles

Magnetoresistance fluctuations in shortn-type Si/SiGe heterostructure wires

Magnetoresistance fluctuations in short quasiballistic Si/SiGe wire segments have been investigated as a function of magnetic field and temperature. The segments are measured in a four-probe geometry and the voltage probe distances L are taken smaller or larger than the phase coherence length l f ~’1.5 m ma tT 50.1 K! and the electron mean free path l e ~’0.8 mm!. At magnetic fields smaller thanB51 T, the amplitude dR and the correlation fieldBc of both the symmetric and antisymmetric part of the resistance fluctuations have been studied as a function of probe distance and temperature. It is found that, despite the quasiballistic character of electron transport in our samples, the behavior of the amplitude and correlation field with probe separation is in good qualitative agreement with expressions derived for the diffusive regime. The observed magnitude of Bc , however, is much larger than expected for the diffusive transport regime. A better agreement for Bc is obtained using an expression adapted for the quasiballistic regime. The temperature dependence of the correlation field cannot be explained by expressions appropriate for the diffusive or the quasiballistic transport regime.

Dual-pump four-wave mixing in a double-mode distributed feedback laser

We present a comprehensive theoretical model with experimental results on four-wave mixing in a distributed feedback laser with two pump modes. Here the twin lasing modes serve as pump waves for the four-wave mixing, which takes place within the laser cavity. We show what we believe are new experimental measurements of the two-pump four-wave mixing conversion efficiency spectrum and show that, to first order, there is no fundamental dependence of the conversion efficiency on pump separation. This result indicates significant improvement in the probe and conjugate wavelength separation and in the conversion efficiency. Our theoretical model agrees quite well with the measured data and predicts the key experimentally observed phenomena, including the relative invariance of the conversion efficiency on pump separation, and the strong enhancement of certain conjugate waves when the probe detuning is close to the pump separation.

Qualification of spreading resistance probe operations. I

In this article we discuss the final results obtained from an extensive spreading resistance probe (SRP) experimental intercomparison conducted among 21 international laboratories with different levels of expertise both in the U.S. and in eight European countries. Twenty different structures representative of current SRP needs (complementary metal–oxide–semiconductor, bipolar, epi) were used. Specialized structures such as buried oxides with different thicknesses (10–40 nm) and identical implants junction isolated and nonisolated (carrier diffusion) were included. The SRP accuracy was determined according to four point probe sheet resistance mappings and secondary ion mass spectrometry depth profiles. In order for SRP to gain wider credibility an error of less than 10% (std dev) should be routinely achievable on the typical SRP results (e.g., dose, sheet, shape carrier profile) irrespective of where, when, or by whom the measurement is performed. The round robin that was conducted showed that present American Society for Testing and Measurement standards are not detailed enough to obtain this target accuracy. In order to achieve this accuracy, precise quantitative specifications on as many as 14 primary qualification parameters must be satisfied simultaneously. Of these, some of the more important ones are low surface roughness, sharp bevel edge, low probe penetration, stable probe contact, small contact size, accurate bevel angle, accurate starting point, sufficient data point density, low probe separation, good probe alignment, and a standardized contact model. The required specifications will be illustrated with results from the round robin. Also, other topics investigated in the round robin will be discussed briefly such as the on-bevel stepsize reproducibility, the microcontact distribution, the different contact radii involved, the amount of barrier resistance, rectification, and noise.

Errors of Dual Thermal Probes Due to Soil Heterogeneity across a Plane Interface

The dual thermal probe enables estimation of the thermal diffusivity, volumetric heat capacity (C), and thermal conductivity of soils. It has been much employed for moisture estimation because of the strong dependence of C on soil water content. The present study investigates the effectiveness of dual probes near heterogeneities. Exact solutions are found for four probe/soil configurations involving heterogeneity. In one configuration the heating probe is on the plane interface between different soils. For the three other configurations it is located close to three different discontinuities: near a soil surface and outside, and behind, a wetting front. Heterogeneity errors are found to be small provided the heterogeneity is no closer to the probes than probe separation (typically 0.006 m). Estimates of C may provide good resolution of soil water content in critical regions such as near surfaces and fronts.

Dual Thermal Probes near Plane Interfaces

The dual probe estimates the thermal properties of soils. Because it is strongly dependent on water content, volumetric heat capacity is useful for moisture estimation. A previous study of probe errors due to soil heterogeneity is taken further with the aid of a new solution for instantaneous line sources near a plane interface between two regions. The solution allows for different conductivities and heat capacities in the regions, but requires uniform diffusivities. This condition holds reasonably well near wetting fronts in a range of soils. Previous error bounds are confirmed and sharpened. The conclusion in the previous study that errors are small when the heterogeneity is no closer to the probes than the probe separation (typically 0.006 m) is reinforced. The expectation that the dual probe gives good resolution of water content close to fronts and interfaces is strengthened.

Direct mapping of the far and near field optical emission of nano-sized tapered glass fibers by an integrated SNOM/SF system

We describe a versatile scanning near-field optical microscope (SNOM) integrated with a shear force (SF) sample probe separation controller. The system works with special tapered optical fibers as the probing element. These fibers are fabricated by using a technique proposed earlier by Ohtsu et al. [M. Ohtsu, Optoelectronic devices and technology, 10 (2) (1995) 147]. We describe here the SNOM system we developed giving details on the fabrication of the fibers. Then we show the first direct bi-dimensional mapping of the optical emission of these tips as measured by our system in an original 2 tip configuration mode.

Field-enhanced superconductivity in disordered wire networks

The superconducting transition was studied in Al wire networks with built-in positional disorder. Application of small transverse magnetic fields increased the mean-field critical temperature in the disordered networks but not in the ordered networks. The magnitude of this Tc enhancement was independent of changes in bias current, probe separation, and measurement configuration, unlike the apparent Tc shifts associated with nonequilibrium resistance anomalies previously observed in superconducting microstructures. @S0163-1829~98!00926-6#

Optimal probe geometry for near-infrared spectroscopy of biological tissue

The results of this study clarify the influence of probe geometry on spectroscopic measurements obtained from the surface of a turbid biological tissue. We show that the transition between the measurement of the predominantly backward-propagating and the predominantly forward-propagating photon fluxes is marked by the separation between the source probe and the detector probes at which the dependence of the fluence on small changes in scattering coefficient vanishes. This is the probe separation at which a variable scattering background has the least influence on the measurement of optical absorption in turbid materials. Estimates of the optimum probe spacing for typical values of absorption and scattering coefficients of soft tissue in the near-infrared spectral region (800-2500 nm) are derived from an analytical solution of the diffusion equation. The estimates were verified by Monte Carlo simulations and experiments on particle suspensions with optical properties similar to those of skin tissue.

Space-time correlation measurements in a hypersonic transitional boundary layer

An experimental investigation of the spatial structure of second-mode instability waves was carried out in the boundary layer of a 7-deg half-angle, sharp-nosed cone at an edge Mach number of 6.8. Measurements were made at Reynolds numbers of 2.3 x 10 6 to 9.1 x 10 6 based on boundary edge conditions, spanning the range from unstable laminar flow to nearly turbulent flow. Simultaneous measurements with two hot-film probes in the boundary layer comprise the primary data set. The mean boundary-layer state was measured with pitot and total temperature surveys. Correlations were taken with circumferential, streamwise, and vertical probe separations. The second-mode waves preceding transition are relatively limited in their circumferential dimension, typically less than four boundary-layer thicknesses, based on a coherence level of 20%.

96/06769 Phase relationships in coal ash corrosion products

Four-element hot-wire probe measurements are used to examine the structure of the spanwise vorticity in the inner region of low Reynolds number zero pressure gradient turbulent boundary layers. Single-probe measurements were made over the range 1010 ≤ Rθ ≤ 4850(Rθ = θU∞/ν, where θ is the momentum deficit thickness, and U∞ is the free-stream velocity), while two probe measurements were made at Rθ ≈ 1010. The present results indicate that for y+ < 50 statistical moment profiles of ωZ scale on inner variables. Event duration analyses indicate that a nearly logarithmic increase in inner normalized time scales of the ωZ bearing motions occurs with increasing Rθ. Outside the buffer region, this Rθ dependence is effectively removed if the Taylor time scale is used to normalize the event durations. Two-point correlations with probe separations in the spanwise as well as wall-normal direction are presented. In addition, the structure of the associated two-dimensional (2-D) probability distributions are examined to reveal the statistically most significant contributions underlying these correlations. Wall-normal probe separation measurements indicate the increasing prevalence of adjacent regions of opposing sign ωZ as the wall is approached. Spanwise probe separation experiments indicate the predominance of single-sign contributions, as well as increasing spatial coherence nearer the wall. The present results are interpreted to indicate that the organized spanwise vorticity-bearing motions are distributed in planes parallel to the wall for y+ less than about 12, and decrease to a nearly fixed scale outside the buffer region.

Spanwise vorticity structure in turbulent boundary layers

Four-element hot-wire probe measurements are used to examine the structure of the spanwise vorticity in the inner region of low Reynolds number zero pressure gradient turbulent boundary layers. Single-probe measurements were made over the range 1010 ≤ R θ ≤ 4850(R θ = θU ∞/ν, where θ is the momentum deficit thickness, and U ∞ is the free-stream velocity), while two probe measurements were made at R θ ≈ 1010. The present results indicate that for y + < 50 statistical moment profiles of ω Z scale on inner variables. Event duration analyses indicate that a nearly logarithmic increase in inner normalized time scales of the ω Z bearing motions occurs with increasing R θ. Outside the buffer region, this R θ dependence is effectively removed if the Taylor time scale is used to normalize the event durations. Two-point correlations with probe separations in the spanwise as well as wall-normal direction are presented. In addition, the structure of the associated two-dimensional (2-D) probability distributions are examined to reveal the statistically most significant contributions underlying these correlations. Wall-normal probe separation measurements indicate the increasing prevalence of adjacent regions of opposing sign ω Z as the wall is approached. Spanwise probe separation experiments indicate the predominance of single-sign contributions, as well as increasing spatial coherence nearer the wall. The present results are interpreted to indicate that the organized spanwise vorticity-bearing motions are distributed in planes parallel to the wall for y + less than about 12, and decrease to a nearly fixed scale outside the buffer region.

Plasma gradient effects on double-probe measurements in the magnetosphere

Abstract. The effects on double-probe electric field measurements induced by electron density and temperature gradients are investigated. We show that on some occasions such gradients may lead to marked spurious electric fields if the probes are assumed to lie at the same probe potential with respect to the plasma. The use of a proper bias current will decrease the magnitude of such an error. When the probes are near the plasma potential, the magnitude of these error signals, ∆E, can vary as ∆E ~ Te(∆ne/ne)+0.5∆Te, where Te is the electron temperature, ∆ne/ne the relative electron density variation between the two sensors, and ∆Te the electron temperature difference between the two sensors. This not only implies that the error signals will increase linearly with the density variations but also that such signatures grow with Te, i.e., such effects are 10 times larger in a 10-eV plasma than in a 1-eV plasma. This type of error is independent of the probe separation distance provided the gradient scale length is much larger than this distance. The largest errors occur when the probes are near to the plasma potential. At larger positive probe potentials with respect to the plasma potential, the error becomes smaller if the probes are biased, as is usually the case with spherical double-probe experiments in the tenuous magnetospheric plasmas. The crossing of a plasma boundary (like the plasmapause or magnetopause) yields an error signal of a single peak. During the crossing of a small structure (e.g., a double layer) the error signal appears as a bipolar signature. Our analysis shows that errors in double-probe measurements caused by plasma gradients are not significant at large scale (»1 km) plasma boundaries, and may only be important in cases where small-scale (&lt;1 km), internal gradient structures exist. Bias currents tailored for each plasma parameter regime (i.e., variable bias current) would o1q1improve the double-probe response to gradient effects considerably.

Induced periodic hemodynamics in skeletal muscle of anesthetized rabbits, studied with multiple laser Doppler flow probes.

Periodic fluctuations, regular slow-wave flowmotion, were induced in the skeletal muscle of six mature anesthetized New Zealand White rabbits by acute femoral artery pressure reduction from a median control value of 78 to 36 mm Hg. This phenomenon was monitored simultaneously with four laser Doppler flowmetry (LDF) probes placed over the gastrocnemius muscle in a linear array with a spacing of 5 mm. The median relative peak-to-trough amplitude of the oscillatory flow patterns was 47%, white a frequency of approximately 2.5 cycles per minute (cpm), which remained relatively stable over an observation period of 1 h (+/- 15-20%). Application of two frequency analysis methods, Welch's FFT method and Prony Spectral Line Estimation yielded similar results and showed a correlation coefficient of r = +0.84. Spectral coherence between pairs of regular slow-wave flowmotion records decreased with increasing probe separation (0.28 at 5 mm; 0.01 at 15 mm). However, patterns of frequency fluctuation over time were significantly correlated between concurrent record pairs regardless of probe separation. These results suggest that the regular slow-wave flowmotion signal originates in regions that are independently regulated by local vasoactive sites, which may be called pacemakers. These sites may also be influenced by an additional common control mechanism, which may be myogenic/metabolic or central in nature.

Random-matrix study of multiprobe mesoscopic devices. II. A four-probe one-dimensional system.

A random-matrix analysis is presented for a four-terminal device consisting of one-dimensional mesoscopic wires. Interest is focused on the potential difference v measured between the two sides of a weakly disordered conductor. Various interference effects are found in the statistical distribution w(v) of v. Both the centroid and width of w(v) oscillate as a function of the probe separation, the size of the oscillations depending on the coupling constant \ensuremath{\epsilon} between the probes and the wire. For \ensuremath{\epsilon}\ensuremath{\ll}1, w(v) is wide enough to yield a nonzero probability to find a potential rise instead of a potential drop.

Two-dimensional spreading resistance profiling: Recent understandings and applications

Conceptually the use of spreading resistance measurements for two-dimensional profiling has been introduced two years ago. In the mean time our understanding has improved towards a better interpretation of the experimental results. A simulation program has been realized which allows to simulate the spreading resistance scans along the specially beveled surface. An ‘‘in situ’’ calibration procedure for measuring with high accuracy, the probe size and separation has been developed and a new formula, with less experimental involved parameters, for calculating the lateral spread has been derived. The technique has been applied to a study of the lateral diffusion of B as a function of annealing time. The effect of different masking material (polycrystalline silicon, thick oxide) has been studied. Other recent applications of this technique include the determination of the lateral straggling of channeling implants and the lateral diffusion of transition metals.

Measurement of the correlation spectrum of electrostatic potential fluctuations in an ECRH Helimak plasma

Measurements of the length and time scales of turbulent potential fluctuations were carried out on the MIT Versatile Toroidal Facility (VTF). Plasmas were produced by ECRF heating in a novel toroidal configuration which we have termed the Helimak. The configuration consists of a toroidal fieldB φ of approximately 800 gauss and a vertical fieldB z of typically 10 gauss, produced by a Helmholtz coil.T e is approximately 10 eV. The density exhibited a peaked profile havingn max ≈ 2×1010 cm−3 and a density gradient scale length of ∼10 cm. The fluctuation experiments were conducted using a mobile vertical array of eight Langmuir probes. At major radii outside the density peak, the vertical correlation lengths λc⊥ of fluctuations were found to be on the order of 5–10 cm for fluctuation frequencies below 3–8 kHz, and on the order of 1–2 cm at higher frequencies. At major radii on the inner slope of the density peak, a new feature appears in the spatial coherence function consisting of a second peak at a probe separation which scales linearly with vertical field. This observation indicates that these fluctuations have a correlation length on the order of 2πR 0∼600 cm in the direction parallel to the helical magnetic field lines.

The observation of Hall plateaux in diffusive quasi-two-dimensional wires of submicron size

Plateaux in the Hall effect have been observed in heavily doped n +-GaAs wires with a mobility as low as 0.15 m 2/V s and widths of 200 nm and 800 nm. The observed plateaux are strongly dependent on sample width and electron temperature. The usual vanishing of the longitudinal resistance is not seen. Pronounced plateaux are only observed in the 200 nm sample. An explanation is suggested in terms of conduction along edge states. In this picture the quantized Hall effect is observed if the mean free path of an electron traveling in an edge state, or the backscattering mean free path, well exceeds the Hall probe separation.

Dual-beam interferometer for the accurate determination of surface-wave velocity

A novel dual-beam interferometer has been designed and constructed that enables two beams from a He-Ne laser to probe remotely the surface of a material. The separation of the two He-Ne beams is adjustable in the 15-to- 40-mm range with a spatial resolution of 2 microm. Surface-acoustic-wave measurements have been performed with two different probe separations so that the travel time for the surface waves over a known distance can be determined accurately. With the aid of autocorrelation algorithms, the Rayleigh pulse velocity on 7075-T651 aluminum has been measured to be 2888 +/- 4 m/s. The current precision of the system is limited mainly by the 10-ns sampling rate of the digital oscilloscope used. Rayleigh pulse interactions with a surface-breaking slot, machined to a nominal depth of 0.5 mm, have also been examined and the depth estimated ultrasonically to be 0.49 +/- 0.02 mm. The system may also provide a technique for direct quantitative studies of surface-wave attenuation.

The verification of the heat pulse velocity technique for estimating sap flow in Eucalyptusgrandis

The heat pulse velocity technique was verified on a diffuse porous hardwood, Eucalyptusgrandis Hill ex. Maiden, using the cut-tree method. The heat pulse velocity apparatus accurately reflected sap flow in the stems of 3-year-old trees and of a 16-year-old tree. Measured sap flow was found to be particularly sensitive to errors in the estimation of the wound size and probe separation. Four probes, each providing a point estimate of heat pulse velocity, were sufficient to estimate sap flow in young trees, but a minimum of eight was needed for trees larger than about 20 cm in diameter. For the successful field application of the heat pulse velocity technique on standing E. grandis trees, it is necessary to wait 5 days after implanting the probes to enable tyloses to form, which facilitates wound size measurement. This technique is recommended for the accurate and rapid measurement of water use by E. grandis and other similar plantation tree species.

Geoelectric stratigraphy and subsurface evaluation of quaternary stream sediments at the Cooper Basin, NE Texas

AbstractAn electrical resistivity survey of a portion of the Cooper Reservoir Basin in northeast Texas has been used to delineate the major lithostratigraphic units of Quaternary flood plain deposits and subsurface paleogeomorphic features of interest to geoarchaeologists. This investigation was conducted to evaluate the fluviatile deposits along the South Sulphur River which flows through Cooper Basin, to identify the basin morphology, and to detect buried geomorphic features. Buried stream channels were considered to be important targets since buried prehistoric site locations are thought to be related to patterns of fluvial facies. Two general techniques were used in the survey. First, soundings were conducted over borehole locations to correlate lithologies with resistivity variations. Second, multiple resistivity profiles were established along traverses across the flood plain. By repeating each traverse but with increased probe separations, a composite of profiles was developed yielding electrical cross‐sections of the subsurface. These cross‐sections were then interpreted in terms of the lithologic stratigraphy in the basin. Sounding data were evaluated using empirical methods, and profile data were evaluated qualitatively. the sounding data defined the major resistivity horizons which correlate to the primary sedimentary units in the Cooper Basin. Profile data, used to construct resistivity cross‐sections, revealed a series of buried erosional features interpreted as channel segments and allowed for the estimation of relative time boundaries within sediments buried under the flood plain as the basin evolved.