Showing papers by "Varian Associates published in 1966"

Nonlinear space-charge domain dynamics in a semiconductor with negative differential mobility

Abstract: On a semiphenomenological level, the explanation of the Gunn effect is one in terms of a time-independent, negative differential, bulk conductivity. This mechanism is based on the conduction-band structure of GaAs, which provides for two kinds of electrons, light and heavy ones. Light electrons dominate at low fields, heavy ones at high fields. Since the mobility of the heavy electrons is much lower than that of the light ones, there is a range of current decrease with increasing field, i.e., a negative conductivity. This negative conductivity leads to an electrical breakup of the crystal into alternating traveling domains of high and low fields, accompanied by alternating current. In a "mathematically perfect" crystal this instability would take the form of traveling negatively charged electron accumulation layers, separating the domains of high and low fields. In real crystals the inevitable spatial fluctuations in the impurity distribution lead to the experimentally observed dipole mode, wherein both negatively charged electron accumulation layers and positively charged electron depletion layers occur.

Transport Properties of La F 3

Abstract: We report measurements of the lattice and bulk thermal expansion, fluorine nuclear magnetic resonance, and electrical conductivity of single-crystal La${\mathrm{F}}_{3}$ in the temperature range 300-1000\ifmmode^\circ\else\textdegree\fi{}K. In the lower portion of this temperature range, the measurements yield activation energies for the formation of Schottky defects of \ensuremath{\sim}0.07 eV and for fluorine ion diffusion of \ensuremath{\sim}0.45 eV. The activation energy for diffusion appears to decrease at higher temperatures. We propose a model for the crystal with low activation energy for the formation of neutral defects, and a higher energy for defect dissociation and diffusion. The apparent changes of activation energy are ascribed to the excitation of lattice vibrations near the Debye temperature.

Electron-Nuclear-Double-Resonance and Electron-Paramagnetic-Resonance Analysis of the Ytterbium-Fluorine Superhyperfine Interaction in Ca F 2 : Yb 3 +

Abstract: Superhyperfine structure in the EPR spectrum of ${\mathrm{Yb}}^{3+}$ in cubic sites of Ca${\mathrm{F}}_{2}$ has been investigated with EPR at both 9 Gc/sec and 35 Gc/sec and with ENDOR at 9 Gc/sec. The EPR measurements showed that the superhyperfine splitting is anisotropic and that the number of observable lines (17 at 9 Gc/sec along $〈100〉$) and the splitting of the lines is field-dependent. ENDOR measurements permitted complete interpretation of the EPR results. The superhyperfine structure arises from coupling to the nearest-neighbor fluorine nuclei. The Hamiltonian $\mathrm{S}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{T}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{I}}^{F}$ is used to describe the ${\mathrm{Yb}}^{3+}$-${\mathrm{F}}^{\ensuremath{-}}$ interaction where T has axial symmetry. The principal values are|${\mathrm{T}}_{\mathrm{II}}$|=36.8\ifmmode\pm\else\textpm\fi{}0.2 Mc/sec and|${\mathrm{T}}_{\mathrm{\ensuremath{\perp}}}$|=15.9\ifmmode\pm\else\textpm\fi{}0.1 Mc/sec where ${\mathrm{T}}_{\mathrm{II}}$ and ${\mathrm{T}}_{\mathrm{\ensuremath{\perp}}}$ have opposite signs. The fluorine nuclear Zeeman term and the superhyperfine interaction are of the same order, resulting in a breakdown of the usual selection rules. The number, intensity, and splitting of the EPR superhyperfine lines have been calculated with these interaction parameters and they agree well with experiment. Small shifts in the fluorine ENDOR for $^{171}\mathrm{Yb}(I=\frac{1}{2})$ and $^{173}\mathrm{Yb}(I=\frac{5}{2})$ relative to the ENDOR for the even isotopes were observed which are due to the strong hyperfine coupling in the odd isotopes. Interpretation of ${\mathrm{T}}_{\mathrm{II}}$ and ${\mathrm{T}}_{\mathrm{\ensuremath{\perp}}}$ in terms of an isotropic and an anisotropic interaction is given, and results are compared with those of other workers.

Short-term stability of passive atomic frequency standards

Abstract: The instability of passive atomic frequency standards caused by shot noise introduced by the atomic reference is calculated. This instability can be characterized by a figure of merit for the atomic reference, and the asymptotic functions for the rms frequency fluctuation for long and short averaging times expressed in terms of this quantity. Measurements of the rms frequency fluctuation of cesium atomic beam and rubidium vapor frequency standards are compared with theory, and the predicted performances of various existing and proposed atomic standards are tabulated. It is feasible to build atomic beam frequency standards whose stability for averaging times less than one second is limited by noise in the crystal oscillator rather than by shot noise, a criterion generally met by rubidium vapor frequency standards. For long averaging times, shot noise will always be the fundamental limiting factor in atomic frequency standard stability.

The specification of oscillator characteristics from measurements made in the frequency domain

Abstract: A cross-correlation technique for measuring the very short-term (milliseconds to seconds) properties of stable oscillators is described. Time-dependent functions representing signals from two separate oscillators are led to a function multiplier where the instantaneous product of the functions is made. The oscillators are either set to a given phase relation or allowed a small relative drift so that a slow beat frequency is observed. Short-term fluctuations superimposed upon the slow beat signal from the multiplier output will represent the instantaneous phase difference between the oscillators when the inputs are in quadrature. When the inputs are in and out of phase, the fluctuations represent amplitude fluctuations. The time averaging function is determined by a filter having a rectangular pass band from nearly zero frequency to a cutoff frequency v c . The mean square frequency deviation measured in a bandwidth ω c is obtained by differentiating, filtering, squaring, and averaging the signal from the function multiplier data being taken when the input signals are in quadrature. Mean square averages of amplitude and phase averaged over various bandwidths ω c may be obtained by bypassing the differentiator. Sample data from measurements on hydrogen masers are presented, and the effect of thermal noise is seen to be the major factor limiting the short-term frequency stability of the signals.

A survey of atomic frequency standards

Abstract: The survey reviews the historical background and development of atomic frequency standards leading to the most recent achievements. A qualitative discussion of underlying principles is also included. Modern atomic frequency standards produced in the United States are illustrated, and performance data reflecting the present state of the art is presented. The comparison of the different types of atomic frequency standards is complicated by the many quantitative factors relating to performance and physical characteristics which differ in each case. However, on a qualitative basis it may be stated that rubidium gas cell frequency standards are, at the present time, the most compact, light weight atomic oscillators, and they have a high level of short-term stability. The long-term stability in this case exceeds that of the best quartz crystal oscillators by two or more orders of magnitude without the need for long warm-up or stabilization periods. Calibration in terms of a primary standard is required. The cost of rubidium frequency standards is less than that of other atomic standards. Cesium atomic beam frequency standards are somewhat larger and heavier than rubidium standards, and the short-term stability is limited by shot noise in the cesium tube resonance signal. However, the long term stability is very high, and the limits have not yet been measured for very long averaging periods. The cesium standard also has a high degree of intrinsic reproducibility which qualifies it for service as a primary standard without the need for calibration. The atomic hydrogen maser, at present, has the highest degree of short-term stability, long-term stability, and intrinsic reproducibility when compared with the other commercially produced atomic frequency standards. It is also the largest and the cost is influenced by the more advanced performance characteristics. New developments which are in progress include the thallium atomic beam controlled frequency standard and the active rubidium maser. Along with these advances, new improvements will continue to be made in the various characteristics of the established types of atomic frequency standards. It is likely that their relative qualities will remain approximately the same, as the absolute values of the characteristics are advanced.

Landau Levels and Magneto-Absorption in InSb

Abstract: Detailed results are presented of machine calculations of the Landau level structure and transition matrix elements for InSb near the center of the Brillouin zone, for magnetic fields in the [100] direction, and under conditions typical of valence-band cyclotron resonance and interband magneto-optical experiments. Effects of inversion asymmetry arising in the zinc-blende structure are included. The theoretical conduction-band combinational resonance spectra of Rashba and Sheka are confirmed and extended to the nonparabolic-band model. The results show good agreement with the recent interband magneto-optical observations by Pidgeon and Brown. Different heavy hole "masses" are predicted for valence-band cyclotron resonance under different experimental conditions. Some of these coincide with reported experimental results. Details of the predicted spin-split zero-field band structure in the (110) plane are given.

Microwave oscillations in high-resistivity GaAs

Abstract: It has been found that Gunn-effect oscillations in high-resistivity GaAs cease when the product of the conduction electron density and the sample length drops below 1011cm-2as predicted by theory. The samples investigated exhibited impact ionization of a deep donor level lying 0.41 eV below the conduction band edge when subjected to an applied electric field equal to the Gunn-effect threshold field. Some observations concerning space-charge limited injection of electrons and concerning electrical breakdown in high-resistivity GaAs are also reported.

Method and apparatus for chromatography

Abstract: 1,165,468. Chromatography apparatus; proportionately mixing flowing fluids. VARIAN ASSOCIATES. 10 Oct., 1967 [14 Oct., 1966], No. 46247/67. Headings B1C, B1L and B1X. A chromatographic system with a programmed mobile phase flow and mixing control means comprises a plurality of reservoirs 21, 22 each containing a mobile phase, a plurality of flow conduits 23, 24, flow control means 16 and 17 to control the flow rates in conduits 23, 24 means for generating a primary control signal 11 whose value is variable in response to an input signal from programmed primary function generator means 12, means 13 for producing a plurality of secondary control signals from said primary control signal, said secondary control signals controlling the flow control means 16, 17, and being variable in response to an input signal from a programmed secondary function generator means. Although the use of voltage signals are indicated in the embodiment of Fig. 1, current frequencies or phase differences could be used instead. Gas and liquid chromatographic systems are described. The system can be used to control the flow rates in conduits 23, 24 to provide simultaneous or independent variation of flow rate and composition in conduit 29. Fig. 1 shows a system suitable for liquid chromatography, and Fig. 2 (not shown) are suitable for gas chromatography.

Calculations of chemical shifts

Abstract: Electric field theory is used to predict the 1H, 13C and 19F chemical shifts of a series of halogenated ethanes and the 19F shifts of some cyclobutanes. The quantity 3PI/r 6 is used to describe the time-averaged square of the electric field at the nucleus from fluctuating point-dipoles elsewhere in the molecule. The results are compared with predictions made by an alternative method, using P/Ir 3, to define the electric fields. The calculations show that, whatever the theoretical justification or lack of it, the former quantity is often more widely applicable than the latter.

Persistent mode superconductive orthogonal gradient cancelling coils

Abstract: A superconductive magnet is disclosed together with superconductive gradient cancelling coils for homogenizing the magnetic field produced by the magnet. A gyromagnetic spectrometer is also disclosed which employs the magnetic field produced by the superconductive magnet. The gradient cancelling coils include a plurality of turns of a superconductor arranged adjacent the region of the magnetic field to be corrected, such magnetic field having certain residual magnetic field gradients therein to be cancelled. Means are provided for energizing the gradient cancelling coils with electrical currents in such a configuration as to define orthogonal superconductive gradient cancelling coils. A superconductive connection is formed across the ends of the coil structure for closing the superconductive circuit across the ends of the coil to form a closed superconductive circuit, whereby the superconductive gradient cancelling coils may be operated in a persistent mode for enhanced stability and reduced power consumption. In one embodiment, a DC transformer is provided which has a primary winding coupled to the main magnetic field of the magnet and a secondary winding which serves to energize the gradient cancelling coils. In this manner, the gradient cancelling field components change their intensity in proportion to changes in intensity of the main magnetic field to maintain a corrected field.

TEMPERATURE VARIATION OF $sup 139$La NUCLEAR QUADRUPOLE RESONANCE IN LaF$sub 3$

Abstract: The temperature dependence of the pure quadrupole transitions of ${\mathrm{La}}^{139}$ in La${\mathrm{F}}_{3}$ has been studied from 88 to 447\ifmmode^\circ\else\textdegree\fi{}K. ${\mathrm{La}}^{139}$ has a nuclear spin of $\frac{7}{2}$, so three $|\ensuremath{\Delta}m|=1$ transitions were observed. A spectrometer of crossed-coil geometry was used. The Bayer torsional-motion mechanism and an Einstein phonon spectrum are used to interpret the measurements. The characteristic (or Einstein) temperatures are 465 and 641\ifmmode^\circ\else\textdegree\fi{}K. Values for the quadrupole coupling constant, asymmetry parameter, and electric-field-gradient components at the La sites are tabulated. The observed temperature variation is approximately two orders of magnitude larger than predicted by the simple theory employed. Mechanisms are discussed to account for this discrepancy.

Thermionic detector for gas chromatography

Abstract: 1,160,828. Gas analysis. VARIAN ASSOCIATES. June 1, 1967 [June 6, 1966], No.25410/67. Heading G1N. Effluent gas from a chromatographic column is mixed with hydrogen and the mixture is burnt in air at the upper surface of a homogeneous ion source block 29 so that the heat of the resulting flame can ionize material from the surface of the ion source to introduce charged particles into the space between electrodes 32, 34. The ion source block 29 preferably consists of a salt of an alkali, or alkaline earth, metal (examples given) containing 2 to 5% refractory binder, e.g. silica. In the presence of an organic constituent containing phosphorus or a halogen, the rate of ionization of material from the ion source block is increased. The relative sensitivity to phosphorus and halogens depends on the air and hydrogen input flow rates so that by proper adjustment of the flow rates, the detector can be made virtually insensitive to halogens, responding only to phosphorus. A bucking voltage is used to oppose the effect of ions produced in the absence of phosphorus or a halogen. Electrode 32 also serves as an ignition coil.

Gyromagnetic resonance spectrometer having selectable internal and external resonantcontrol groups

Abstract: 1,170, 523. Gyromagnetic spectrometers. VARIAN ASSOCIATES. Feb, 13, 1967 [Feb. 21, 1966], No.6825/67. Heading G1N. In a gyromagnetic side-band resonance spectrometer, in which the applied R.F. or D.C. fields are audio frequency modulated, the resonance condition of the spectrometer is controlled using either an "internal" or "external" reference sample, both being provided. By "internal" is meant a reference sample situated in the same field as the sample under investigation and by "external" a reference sample situated in a different field. The internal reference sample may be intermixed with the sample or may be contained in one chamber of a vial 3, Fig. 2 (not shown), having coaxial chambers or an axial or transverse partition providing the two chambers. It is preferably formed of a substance, e.g. the protons in tetramethylsilane, having a narrow resonance line. The external control sample (13, Fig. 2 not shown), is placed to one side of the internal sample and is surrounded by coaxial coils (14), whose axis is parallel to the D.C. field direction, by which this field may be locally altered. It is preferably composed of the protons in water, having a wide resonance line. In operation, the output of either the internal or external reference sample is fed to an oscilloscope 57, Fig. 3 on which it can be seen whether it is in resonance or not, and to a phase detector 54, the output of which, representative of the dispersion mode resonance signal, is fed to a control 55 by which the D.C. field is regulated to maintain the resonance condition of the spectrometer. Initially the current in the coils (14) is adjusted until the frequencies of resonance of the two reference samples coincide so that, in switching between the two reference samples, the control of the D.C. field is not altered. Thus while the internal reference sample is being removed with the sample under investigation and another sample put in its place, the D.C. field is automatically controlled by the external reference sample, the more exact control provided by the internal reference sample taking effect as soon as the system is switched over without the need to relocate its resonance line.

Calculations of chemical shifts: Part III. 19F shifts of interconverting fluorinated cyclohexenes

Abstract: The use of electric field theory to predict the chemical shifts of fluorine nuclei in molecules containing localized double bonds has been justified in a previous publication [1]. This approach is ...

Coaxial receiver coil and capacitor structure for probes of uhf gyromagnetic spectrometers

Abstract: 1,150,248. Gyromagnetic spectrometers. VARIAN ASSOCIATES. April 7, 1967 [April 22, 1966], No. 16132/67. Heading G1N. To improve the signal to noise ratio in a gyromagnetic spectrometer probe, the length of the leads connecting the receiver coil 22, Fig.4, to its tuning capacitor 24 is decreased by placing the capacitor in the probe body, the capacitor having the form of two coaxial cylindrical or arcuate members arranged coaxial with the receiver coil former 23. In one embodiment the receiver coil 22 is mounted on the glass cylinder 23 which also carries a silver cylindrical or arcuate member 36 or (36 1 ), Figs.4, 7 (latter not shown), forming the fixed plate of the tuning capacitor and connected to one end of the coil by lead 35. The other end of the coil is connected both via lead 41 and coaxial cable 25 to external circuitry and via lead 37 to an r.f. earthed screw threaded brass plug 47 which may be screwed in and out of the member 36 and which forms the second plate of the capacitor. Alternatively, the plug 47 may be omitted, the second capacitor plate being provided by an outer screw threaded earthed metal cylinder (55, Fig. 6 not shown), or a semi-cylindrical screw threaded metal member (56, Fig.7 not shown). In all the embodiments a sample 1, Fig.3, in a vial 2, supported at its upper end by a spinner 21 by which the sample is rotated about the longitudinal axis, is placed in the tube 23, which is surrounded by a dielectric tube 28 on which the transmitter coil 27 is mounted with its axis perpendicular to that of the receiver coil. A dielectric tube 26, between the tubes 23, 38, coated with closely spaced longitudinal conducting strips, serves as a Faraday shield between the two coils 22, 27. The whole of this assembly is surrounded by the double walls 29, 31 of a dewar flask whose outer wall is in turn surrounded by a cylindrical assembly of magnetic field gradient cancelling coils 32, covered with an aluminium cylinder. In this spectrometer the unidirection magnetic field H 0 is provided by a superconducting solenoid (6, Fig. 1 not shown), surrounding the probe and immersed in liquid helium in a dewar flask (5) to keep the solenoid at superconducting temperatures. The spectrometer is operated using the side band technique by audio frequency modulation of the D.C. field H 0 which is also scanned and the recorder synchronized with the scan, Fig. 2 (not shown).