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Journal of Applied Physics

Issue 7 • Date Oct 1984

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Displaying Results 1 - 25 of 49
  • Hybrid finite element method for scalar wave refraction into three‐dimensional bodies

    Page(s): 1909 - 1920
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    The hybrid finite element method is a combination of the finite element method in a closed domain and an analytic solution outside this domain. The analytic solution is used in the region of homogeneous dielectric constant, while the finite element method is applied to the region of heterogeneous dielectric constant. Matching conditions, expressed in the form of nonlocal boundary conditions that were described in a previous article by P. H. E. Meijer, Gregory A. H. Cowart, and Stanley M. Neuder [Appl. Sci. Res. 40, 97 (1983)], are used to obtain equations for the scalar amplitudes of the wave function refracted into a heterogeneous object of arbitrary shape. The problem is described by means of a functional, containing integrals over the volume and surface of a spherical domain which completely encloses the physical scattering object. The integrals are approximated and the variation of the functional results in a system of linear equations in the unknown wave functions at the nodal points of the domain. The new aspect of the method is the use of cubature formulas to reduce the surface integrals to sums. The formulation is tested by comparing results with the analytic solution for refraction into a homogeneous sphere. View full abstract»

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  • Direct measurement of temperature in shock‐loaded polymethylmethacrylate with very thin copper thermistors

    Page(s): 1921 - 1926
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    We present direct measurements of the temperature of shock‐loaded polymethylmethacrylate (PMMA) in the 0.5 – 3.0‐GPa range. The experimental technique is based on embedding a very thin (1 μm thick or less) copper thermistor in the shocked sample and recording its resistance changes during the 5–8 μs of a dynamic experiment. Due to the small thickness of the gauge thermal equilibrium between gauge and specimen can be reached in these short times. The measured temperatures of shock‐loaded PMMA are in good agreement with the calculated values. Moreover, the measured temperature‐time histories confirm the viscoelastic behavior of PMMA, shock loaded in the 0–2.0 GPa, which was found by others using particle velocity measurements. View full abstract»

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  • Generation of negative ions in tandem high‐density hydrogen discharges

    Page(s): 1927 - 1938
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    An optimized tandem two‐chamber negative‐ion source system is discussed. In the first chamber high‐energy (E≫20 eV) electron collisions provide for H2 vibrational excitation, while in the second chamber negative ions are formed by dissociative attachment. The gas density, electron density, and system scale length are varied as independent parameters. The extracted negative ion current density passes through a maximum as electron and gas densities are varied. This maximum scales inversely with system scale length R. The optimum extracted current densities occur for electron densities nR=1013 electrons cm-2 and gas densities N2R in the range 1014–1015 molecules cm-2. The extracted current densities are sensitive to the atomic concentration in the discharge. The atomic concentration is parametrized by the wall recombination coefficient γ and scale length R. As γ ranges from 0.1 to 1.0 and for system scale lengths of 1 cm, extracted current densities range from 8.0 to 80 mA cm-2. The relative negative‐ion yields from single‐chamber and tandem two‐chamber systems are compared. Estimates are made for the rates of polar dissociation of H2 molecules and H+3 ions, and these rates are compared with the dissociative attachment rates. View full abstract»

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  • Stark effect of atomic sodium measured in a hollow cathode plasma by Doppler‐free spectroscopy

    Page(s): 1939 - 1943
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    The Stark widths of the 5896‐Å (3 2P1/2→3 2S1/2) neutral sodium hyperfine lines in the presence of charged perturbers were measured by means of laser saturation spectroscopy. The plasma containing the sodium atoms and the perturbers was generated in a hollow cathode discharge. The Baranger and Brechot theories for the line shape of neutral atom transitions in a plasma has been extended to hyperfine structure and applied to the hyperfine components of sodium D1 line. The theoretical and experimental results were compared and the electronic density and temperature in the discharge were evaluated. View full abstract»

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  • Effect of dephasing on the molecular absorption in a two‐frequency field

    Page(s): 1944 - 1947
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    The absorption of a molecule in the field of a fixed frequency coherent wave is studied using a simple, classical one‐dimensional anharmonic oscillator model. The absorption is found to develop structure in the presence of a second mode of varying frequency. The structure is found to be sensitive to the intensity and wavelength of the tunable field and to wash out in the presence of collisions. View full abstract»

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  • Electromagnetic effects of a moving metal sheet in the presence of alternating‐current conductors

    Page(s): 1948 - 1952
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    Exact solutions of the quasistationary magnetic vector potential, forces, power loss, and reactive power for an infinite plane metal sheet moving in the proximity of a distribution of parallel straight wires carrying ac have been obtained by assuming a two‐dimensional field. The solutions are valid for both nonmagnetic and magnetic metal sheets, within the linear material characteristics, and for any ratio of the plate thickness to the depth of penetration. Integral expressions are derived for the transfer impedance between filamentary conductors and also more practical thick conductors separated by such moving metal sheets. Quantitative results were obtained for a wide range of system parameters and general conclusions are drawn relative to the dependence of forces, active and reactive powers, and transfer impedances on the ac frequency, velocity, and material properties. The analysis presented can be used for nondestructive testing of metallic sheets in motion and is related to the study of ac systems with moving solid conductors. View full abstract»

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  • Excitation of electromagnetic waves by an axial electron beam in a slow wave structure

    Page(s): 1953 - 1958
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    An axial electron beam passing through a slow wave structure is unstable to an electromagnetic perturbation whose phase velocity equals the velocity of the beam. This phenomenon of Čerenkov emission is the basis of all traveling wave tubes. In this paper a simple treatment of excitation of electromagnetic waves in a slow wave structure, viz., two parallel planes partially loaded with a dielectric is presented. When the effects of a guide field or a background plasma are ignored, the dispersion relation is quite simple. It is in the form of a coupling between a TM mode of a stationary electron plasma‐filled slow wave structure and the beam mode. In the presence of a guide magnetic field, the electron motion is predominantly one‐dimensional and the growth rate of instability is considerably modified. At relatively higher gas pressures, when a background plasma is created by the beam, the mode structure is considerably modified, leading to a reduction in the growth rate, hence to the efficiency of the device. View full abstract»

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  • Temperature dependence of the threshold current density in InP‐Ga0.28In0.72As0.6P0.4 (λ=1.3 μm) double heterostructure lasers

    Page(s): 1959 - 1964
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    The experimental threshold current density Jth(T) in InP‐Ga0.28In0.72As0.6P0.4(λ=1.3 μm) double heterostructure lasers varies with temperature as exp(T/T0) with T0≂100 K for 100 K ≪T≪250 K and with T≂65 K for 250 K≪T≪350 K. Numerous mechanisms have been suggested to explain this observed temperature dependence. The temperature dependence of gain gives T0≂100 K for T between 100 and 250 K, and T0≂200 K for T between 250 and 350 K. Inclusion of the temperature dependence of the Auger recombination and carrier concentration at threshold reduces T0 to ∼100 K in the 250 to 350 K temperature range. A leakage current of one‐third of the total current at 300 K is required to reduce T0 to 65 K for 250 K≪T≪350 K. This leakage current is due to the hot carriers that result from Auger recombination and some of these carriers do not lose sufficient energy by interactions with the lattice to be confined by the heterojunction barriers. Previous experimental measurements found that the leakage current was 10–30% of the total current at room temperature. Another suggested cause of the small T0 is intervalence‐band absorption. Extension of results for intervalence‐band absorption in InP to Ga0.28In0.72As0.6P0.4 demonstrates that intervalence‐band absorption is not significant. View full abstract»

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  • Study of the variation of refractive index and material dispersion with wavelength from attenuation data

    Page(s): 1965 - 1968
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    Variation of refractive index, extinction index, and material dispersion of a sodiumborosilicate glass with wavelength has been studied between 800 and 1800 nm from attenuation data obtained for samples of different thicknesses. Both the refractive index and the extinction index curves show that the OH stretching vibrational overtone near 1410 nm gives rise to Sellmeier‐type oscillation (anomalous dispersion) superposed on the normal dispersion curve for the glass. The estimated normal dispersion curve of the glass obtained by drawing the base line through the impurity oscillations fits well into a single oscillator Sellmeier equation. The oscillator and the dispersion energy parameters were obtained by extrapolating the (n2-1)-1 vs λ-2 straight line. The material contribution to the time delay dispersion for signal propagation was estimated from these values. View full abstract»

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  • Schottky barrier restricted AlGaAs laser with an etched mesa ohmic contact

    Page(s): 1969 - 1971
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    A gain‐guided AlGaAs double heterostructure laser which employs a Schottky barrier in conjunction with a p‐GaAs etched mesa contact for current isolation has been fabricated. This Schottky barrier restricted structure provides a device whose electrical and optical characteristics are comparable to that of other gain‐guided lasers and which is simple to fabricate. Preliminary aging data indicate high reliability of the Schottky barrier restriction. View full abstract»

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  • Calculation of electromagnetic field strength, reflectivity, transmission, and absorption of n‐layer thin metallic films

    Page(s): 1972 - 1975
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    We present a calculation of the fields within a metallic heterostructure system for the purpose of optimizing the scattering efficiency of one component. The example chosen is a Ge‐Bi heterostructure. The calculation, carried out by a new technique which results in considerable simplification for the case of absorbing media, yields as well the field value within each layer of the heterostructure. View full abstract»

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  • Advances in superfluid helium acoustic microscopy

    Page(s): 1976 - 1980
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    An acoustic microscope utilizing low‐temperature (0.1 K) superfluid helium as the coupling medium has been operated at 8 GHz. At this frequency the sound wavelength in helium is 300 Å. Pulse compression techniques have been used to improve the signal‐to‐noise ratio of the system. A theoretical and experimental analysis of acoustic transducers operating at 8 GHz is presented. The transducer performance has been optimized for operation in liquid helium with the pulse compression system. The images formed with the microscope demonstrate a lateral resolution of 200 Å with a signal‐to‐noise ratio of 15 dB. A comparison of acoustic and scanning electron microscope or transmission electron microscope images is given. View full abstract»

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  • Estimating effects of processing conditions and variable properties upon pool shape, cooling rates, and absorption coefficient in laser welding

    Page(s): 1981 - 1986
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    This paper examines the role of traverse speed, Beer–Lambert absorption coefficient β, surface reflectivity, and changing liquid thermal conductivity upon the shape of the melt pool and the cooling rates that occur. The dependence of β upon processing conditions is also examined. A three‐dimensional variable property, moving heat source, quasi‐steady‐state, finite difference model for heat conduction into the substrate during laser welding is used. With an increase in traverse speed, the pool flattens out and is swept back, and cooling rates increase. An increase in β sharply decreases the depth of penetration. With the onset of melting, changes in reflectivity did not change pool shape significantly. An increase in effective liquid metal thermal conductivity increases melt pool aspect ratio. Cooling rates increased as the energy density in the pool decreased. A dimensionless melt front velocity Φ is defined such that cooling rates exceed 1000 K/s as Φ approaches unity. The product βz, where z is the depth of penetration, is shown to vary linearly with the natural logarithm of Φ. These results imply that β affects depth of penetration more than the width, that an upper bound for β may be deduced from Φ, that variations in surface reflectivity are less critical in laser welding, that the maximum thickness that can be welded in a single pass decreases as fluid flow becomes more dominant in the melt pool, and that cooling rates increase as pool energy density decreases, especially for values of Φ≪100. View full abstract»

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  • Integral equation formulation for heat conduction in closed regions with arbitrarily shaped boundary surfaces

    Page(s): 1987 - 1991
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    An integral equation formulation is given for investigating time‐dependent temperature distribution in the regions enclosed with arbitrarily shaped boundary surfaces. Only the temperature on the boundary surfaces is taken as the unknown which is determined by solving the surface integral equations numerically. Once the temperature on the boundary surfaces becomes known, that in the volume regions can be calculated through the integral expressions. Surface integral equations are derived for the two cases of surface boundary condition; one is the ‘‘radiation’’ boundary condition and the other is the condition of intimate contact of two media. The application of the first case is given for the problem of heat conduction in the human abdomen region subject to both electromagnetic heating and surface cooling from outside the body during hyperthermia therapy. View full abstract»

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  • Radiation‐induced thermal noise in optoacoustic detection cells

    Page(s): 1992 - 1996
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    When radiation is strongly absorbed in an optoacoustic detection cell, fluctuations in the radiation intensity give rise to thermal noise within the cell. The spectral density of the thermal noise thus generated is dependent on both the frequency response of the cell and the power spectral density of the radiation source. The frequency response function for the spectrophone cell is determined by application of elementary linear response theory to a solution of the heat equation for a uniformly heated cylinder. The power spectral density of coherent and incoherent radiation sources can be determined from their respective intensity autocorrelation functions. Radiation sources are discussed with regard to obtaining the smallest root‐mean‐square temperature fluctuations in the spectrophone cell. It is shown that optoacoustic noise is minimized by using a stabilized laser operating well above threshold. View full abstract»

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  • Plastic instability and particulation in stretching metal jets

    Page(s): 1997 - 2006
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    Metal jets (or rods) that are plastically stretching eventually undergo a particulation process that arrests the plastic elongation and thus limits the target penetration that is to be achieved by the jet. The particulation process is found to depend upon the perturbation structure and upon a single dimensionless flow parameter ϕ. Statistical definitions are introduced for the perturbation magnitudes (jet roughness magnitude, velocity irregularity function, yield strength irregularity function). Then the jet length Lb at breakup is determined as a function of ϕ0 and these perturbation magnitudes. Jet yield strength σ, density ρ, and initial velocity gradient Ux occur in the definition of ϕ0 so that the effects of σ, ρ, and Ux upon Lb are also determined. View full abstract»

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  • Two‐photon laser‐induced fluorescence monitoring of O atoms in a plasma etching environment

    Page(s): 2007 - 2011
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    Atomic radicals are usually the most important reactants in plasma processing. For example, in dry etching or development of organic photoresists, O2 plasmas are used to generate O atoms which can etch the resist spontaneously. However, concentration measurements of these reactive atoms have been limited largely to indirect and often unverifiable methods such as emission spectroscopy, making process optimization difficult. We demonstrate the applicability of two‐photon laser‐induced fluorescence to monitor atom concentrations in situ by exciting O atoms at 226 nm and detecting fluorescence at 845 nm. A detection limit of ≪1013 atoms cm-3 is determined using downstream electron paramagnetic resonance spectroscopy. Noise from background plasma‐induced emission is eliminated by firing the laser at a time when the applied voltage crosses zero and the emission is extinguished. From the relative intensities of the O(2p43P) fine‐structure components, the plasma temperature is found to be thermalized with the ambient walls. This technique should also be applicable to detection of other atoms found in plasma reactors, such as N, Cl, S, and H. View full abstract»

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  • Forbidden (M1) lines in the spectra of titanium, vanadium, chromium, iron, and nickel observed in a tokamak plasma

    Page(s): 2012 - 2016
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    A time‐resolving spectrograph has been used to survey the forbidden magnetic dipole emissions of titanium, vanadium, chromium, iron, and nickel ions injected into the TFR (Tokamak de Fontenay‐aux‐Roses) plasma. A number of magnetic dipole transitions within the n=2 ground configuration of the isoelectronic sequences from FI to BI were identified in the tokamak spectra; in some cases, experimental values of the wavelength were measured for the first time. The spectrograph had a photoelectric multispectral detector and a calibrated linear photometric sensitivity; thus, reliable determinations of the brightnesses in typical tokamak discharges could be made for these lines. View full abstract»

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  • Magnetic field perturbations due to a hole in a conducting wall on the ZT‐40M reversed‐field pinch

    Page(s): 2017 - 2022
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    Radial magnetic field components were measured near a pump port on the ZT‐40M Reversed‐Field Pinch experiment. The measurements show radial components which are about 20% of the equilibrium field at the wall during the setup portion of the discharge. During the flat top of the toroidal current, radial components, ∼8% of the total field at the wall, are measured. A two‐dimensional field diffusion calculation was done to understand the measurements. This gave results which were in basic agreement with the measurements at the position of the probe. View full abstract»

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  • Enhanced penetration of field‐aligned currents into a Lorentz plasma in a stochastic magnetic field

    Page(s): 2023 - 2026
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    We show that anomalously fast current penetration into a plasma can result from the nonlocal electrical conductivity caused by a stochastic magnetic field. We calculate the admittance of a plasma slab subjected to an oscillating surface electric field along B. The effect of increasing stochasticity (or of increasing electron mean free path) is to make the admittance less dissipative and more inductive. View full abstract»

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  • 〈ρR〉 measurements in laser‐produced implosions using elastically scattered ions

    Page(s): 2027 - 2032
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    The measurement of the number of elastically scattered deuterium and tritium ions is shown experimentally to be an effective means of determining the 〈ρR〉 of the fuel in a laser fusion implosion. A method is described for clearly separating the unwanted proton background signal from the desired d and t knockon signals. The details of the analysis are presented for an experiment resulting in a measured 〈ρR〉 of 1.2×10-3 g/cm2. View full abstract»

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  • Thermionically electron emitting matrix cathodes for magnetohydrodynamic generators: Analytical model

    Page(s): 2033 - 2040
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    Current and voltage drop calculations have been made for matrix cathodes having a number of sharp pins projecting out from its surface in open‐cycle magnetohydrodynamic plasmas. It is assumed that from the cathode surface to the cathode sheath edge, the current flows in current tubes formed around the projected pins. The effect of high electric field on the work function of the tip has been taken into account and calculations are made for a variable number of tips and tip area. Work functions of the tip and the slant surface are taken to be different. The current and voltage characteristics have been obtained by solving the current continuity and Poisson’s equations in spherical coordinate geometry. It is observed that the current increases with an increase in the number of pins per unit area and tip area. The theoretical results have been compared with the experimental observations. View full abstract»

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  • Two‐stream instability in a self‐pinched relativistic electron beam

    Page(s): 2041 - 2046
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    The two‐stream stability properties are investigated for a self‐pinched relativistic electron beam propagating through a weakly ionized plasma channel. The analysis is carried out within the framework of a macroscopic cold‐fluid model, and electrostatic stability properties are investigated for the case of rectangular beam and channel density profiles. Depending upon the ionization condition of the channel, either the beam electron‐ion two‐stream or the beam electron‐plasma electron two‐stream instability is excited. In either case, the critical beam current for instability is obtained. In contrast with the one‐dimensional analysis of the two‐stream instability, the stabilizing influence of a self‐pinched beam originates from the finite beam radius. From the critical current, it is shown that for an appropriate choice of physical parameters, the electron beam is free from the two‐stream instability. Moreover, the growth rate of instability under unstable conditions can be drastically reduced for a small axial wave number. View full abstract»

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  • Optogalvanic effects in a hollow cathode glow discharge plasma

    Page(s): 2047 - 2055
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    The hollow cathode glow discharge is a very suitable source to measure the optogalvanic effect (OGE). A detailed study of this effect in neon has been made using a tunable, single‐frequency dye laser. It was found that the lower level of the irradiated transition together with the plasma conditions determine the sign of the effect. Three situations occur: the lower level can either be metastable, resonant, or highly excited. With a five‐level model taking into account electronic collisions and radiative decay a qualitative description of all the cases found in the experiment can be given. Further, it is shown that from a frequency scan of the OGE, line profiles can be obtained which, under certain conditions, contain the gas temperature in their Doppler part. Temperatures ranging from 700 to 1300 K determined this way are consistent with other results. This method makes it possible to measure the temperature as a function of the position, showing that inside the glow the gas temperature is constant. Finally, the time‐dependent OGE is measured and analyzed in terms of ambipolar diffusion. This way an average transport temperature for the plasma electrons in the hollow cathode discharge in neon between 0.2 and 1.2 eV is found. View full abstract»

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  • Long‐wavelength lattice dynamics of In1-xGaxAsyP1-y alloys

    Page(s): 2056 - 2064
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    The first investigation of the lattice dynamics of In1-xGaxAsyP1-y quaternary semiconductor alloys lattice matched to GaAs has been made by Raman scattering. The spectra in the optical frequency range consist of two separate bands, each having a peak at the high‐frequency end. These two peaks are shown to be GaP‐like and GaAs‐like, respectively. The cell‐isodisplacement theory for the long‐wavelength lattice dynamics by Zinger, Ipatova, and Subashiev [Sov. Phys. Semicond. 10, 286 (1976)] is reexamined in detail. It is improved upon by consistently treating the renormalization of the intracell force constants and then applied to InGaAsP. This improvement gives remarkably better fitting of the calculated phonon frequencies to the existing experiments in the entire composition range 0≤x, y≤1 and enables the construction of a coherent model for the long‐wavelength lattice dynamics of InGaAsP. The results of the Raman measurements are discussed in the light of the calculation. The oscillator strengths are also calculated and compared with the infrared experiment. View full abstract»

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Aims & Scope

Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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Editor
P. James Viccaro
Argonne National Laboratory