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Physics Faculty Publications

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Articles 2131 - 2156 of 2156

Full-Text Articles in Physical Sciences and Mathematics

The Nitrogen Dayglow On Mars, Jane L. Fox, Alexander Dalgarno, E. R. Constantinides, G. A. Victor Jan 1977

The Nitrogen Dayglow On Mars, Jane L. Fox, Alexander Dalgarno, E. R. Constantinides, G. A. Victor

Physics Faculty Publications

The ultraviolet dayglow spectrum arising from emission of N2 in the Mars atmosphere is predicted. The intensities are such that a high-resolution spectrum should readily reveal the presence of N2 at the abundances suggested by the Viking experiments.

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Radiative Transition Probabilities Of The 1S2P22P And 1S2P22D States Of The Lithium Isoelectronic Sequence, Jane L. Fox, Alexander Dalgarno Jan 1977

Radiative Transition Probabilities Of The 1S2P22P And 1S2P22D States Of The Lithium Isoelectronic Sequence, Jane L. Fox, Alexander Dalgarno

Physics Faculty Publications

A Z-expansion procedure is used to obtain eigenvalues and eigenfunctions for the 1s22p2Po, 1s2p22P, and 1s2p22D states of the lithium sequence. Expansions in inverse powers of the nuclear charge Z are obtained also for the dipole-matrix elements and oscillator strengths of the 1s22p2Po-1s2p22P and 1s22p2Po-1s2p22D transitions, valid for Z > 3. …

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Multiple Rainbows From Single Drops Of Water And Other Liquids, Jearl D. Walker May 1976

Multiple Rainbows From Single Drops Of Water And Other Liquids, Jearl D. Walker

Physics Faculty Publications

The scattering of light by spherical drops and the theory of rainbows are reviewed in detail to predict the angular positions of rainbows from single drops. The angular positions of the first 13 rainbows of water, observed from a drop suspended in a student spectrometer, were compared to these theoretical positions. Since the rainbows were dependent on the refractive index, the positions shifted with drops of other fluids having other refractive indices. Light corn syrup yielded 11 rainbows, including the 17th-order one. The light emerging from the drops was white except for the multicolors of the rainbows and the faint …

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Hall-Effect Measurements In Cd-Implanted Gaas, B. K. Shin, David C. Look, Y. S. Park, J. E. Ehret Apr 1976

Hall-Effect Measurements In Cd-Implanted Gaas, B. K. Shin, David C. Look, Y. S. Park, J. E. Ehret

Physics Faculty Publications

Cadmium ions were implanted in GaAs crystals at 135 keV to doses ranging from 1012 to 1016 ion/cm2 at room temperature. Sheet‐resistivity and Hall‐effect measurements were carried out as a function of temperature, 4.2–300°K, after annealings at 700, 800, or 900°C in an Ar ambient. The sample surfaces were protected with pyrolytically deposited Si3N4. Significant p‐type conduction was observed when samples with doses ≳1013cm−2 were annealed at ≳700°C. For doses below 1014 cm−2 nearly complete electrical activity was attained after an 800–900°C anneal. The Cd profiles were …

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Electron-Phonon Interaction In Impure Cadmium, Paul D. Hambourger Jan 1976

Electron-Phonon Interaction In Impure Cadmium, Paul D. Hambourger

Physics Faculty Publications

No abstract provided.

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Karate Strikes, Jearl D. Walker Oct 1975

Karate Strikes, Jearl D. Walker

Physics Faculty Publications

Analysis of a forward karate punch is made as an example of collision mechanics. The energy lost to deformation of an opponent is evaluated, and the deformation is shown to be maximized if contact is made when the fist has the greatest speed. In karate fighting the maximum deformation is obtained by focusing a punch to terminate several centimeters inside an opponent’s body. The average impact force is calculated and compared with that neeeded to break a human bone by stressing it beyond its ultimate bending stress value. Similar breaking forces needed for bricks and wooden boards are also computed. …

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Double Rainbow And Dark Band In Searchlight Beam, J. Harsch, Jearl D. Walker May 1975

Double Rainbow And Dark Band In Searchlight Beam, J. Harsch, Jearl D. Walker

Physics Faculty Publications

No abstract provided.

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Temperature Dependence Of Magnetoresistance In Ni3al System, Paul D. Hambourger, R. J. Olwert, C. W. Chu May 1975

Temperature Dependence Of Magnetoresistance In Ni3al System, Paul D. Hambourger, R. J. Olwert, C. W. Chu

Physics Faculty Publications

The transverse magnetoresistance of stoichiometric and off-stoichiometric polycrystalline samples of the Ni3Al system has been measured over the temperature range 1.2C. The data are in qualitative agreement with the theoretical predictions of Chang et al. based on a model of the magnetoresistance of an itinerant ferromagnet, but the observed magnetoresistance is considerably larger than predicted.

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Ohmic Contacts To Al‐Implanted Znse, B. K. Shin, David C. Look, Y. S. Park Mar 1975

Ohmic Contacts To Al‐Implanted Znse, B. K. Shin, David C. Look, Y. S. Park

Physics Faculty Publications

No abstract provided.

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Temperature-Dependence Of Electron Mean Free Paths In Tungsten, Paul D. Hambourger Jan 1975

Temperature-Dependence Of Electron Mean Free Paths In Tungsten, Paul D. Hambourger

Physics Faculty Publications

No abstract provided.

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Electrical Characteristics Of Al‐Implanted Znse, B. K. Shin, Y. S. Park, David C. Look May 1974

Electrical Characteristics Of Al‐Implanted Znse, B. K. Shin, Y. S. Park, David C. Look

Physics Faculty Publications

Aluminum was implanted in ZnSe at 90 keV to a dose of 1015 ions/cm2 at room temperature. Hall‐effect and sheet‐resistivity measurements were made on the samples for various annealing conditions. The implanted layer is found to be degenerate n‐type having a sheet resistivity of ∼103 Ω/□, after annealing at 900°C for 4 h.

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High-Temperature Annealing In Electron-Irradiated Cds, David C. Look Jan 1974

High-Temperature Annealing In Electron-Irradiated Cds, David C. Look

Physics Faculty Publications

The room‐temperature irradiation of as‐grown n‐type CdS by 1‐MeV electrons results in an increase in carrier concentration. The damage anneals in the temperature range 250–300°C, and a study of the annealing kinetics reveals an activation energy of about 1.3 eV which is attributed to sulfur‐vacancy motion.

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Self‐Focusing Of Laser Light In The Isotropic Phase Of A Nematic Liquid Crystal, D.V.G.L.N. Rao, S. Jayaraman Nov 1973

Self‐Focusing Of Laser Light In The Isotropic Phase Of A Nematic Liquid Crystal, D.V.G.L.N. Rao, S. Jayaraman

Physics Faculty Publications

Self‐focusing of ruby laser light is studied as a function of sample length and temperature in the isotropic phase of nematic liquid‐crystal MBBA. The critical power for self‐focusing, 0.36 kW, observed near the phase transition temperature is a factor of 20 times less than that for CS2. The corresponding nonlinear index 4.4 × 10−10 esu is the largest value known so far for any material.

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Proton-Scattering On (29)Si In Range Ep = 2.5-3.4 Mev*, Joseph W. Hemsky, S. C. Ling, Paul J. Wolfe Jul 1973

Proton-Scattering On (29)Si In Range Ep = 2.5-3.4 Mev*, Joseph W. Hemsky, S. C. Ling, Paul J. Wolfe

Physics Faculty Publications

Elastic and inelastic proton scattering measurements on Elastic and inelastic proton scattering measurements on 29Si have been obtained in the energy range Ep = 2.5-3.4 MeV. 30 elastic and 32 inelastic resonances were observed. 14 prominent resonances from elastic scattering were analyzed. Results of the analysis for proton orbital momenta, spin and parity assignments, and proton partial widths are given. The inelastic resonance strengths measured relative to the known strength of the 1302-keV resonance in 29Si(p,y)30P are also reported.

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The Howard University Reviews Of Science, Vol 1 No 3, Department Of Physics Nov 1972

The Howard University Reviews Of Science, Vol 1 No 3, Department Of Physics

Physics Faculty Publications

No abstract provided.

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Nuclear-Magnetic-Resonance Study Of Co2+ In Cds, David C. Look, Donald R. Locker Aug 1972

Nuclear-Magnetic-Resonance Study Of Co2+ In Cds, David C. Look, Donald R. Locker

Physics Faculty Publications

Nuclear spin-lattice relaxation times, T1, have been measured over a temperature range T=(1.1-300) °K and frequency range ν=2-15 MHz in a single crystal of CdS doped with 13-ppm cobalt. Minima in T1 vs T are observed, and absolute values of the effective electron relaxation time, τe, may be calculated at the temperatures of the minima, e.g., at 14 °K, τe=8.0×10-8 sec. At low temperatures, T≲5 °K, τe is dominated by a resonant Orbach process involving the two ground-state Kramers doublets (S=±3/2 and S=±1/2) which, according to our measurements, are split by …

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Nuclear-Magnetic-Resonance Measurement Of The Conduction-Electron G Factor In Cdte, David C. Look, D. L. Moore May 1972

Nuclear-Magnetic-Resonance Measurement Of The Conduction-Electron G Factor In Cdte, David C. Look, D. L. Moore

Physics Faculty Publications

The effective g factor g* of conduction electrons in degenerate CdTe has been determined by using measurements of the Cd113 nuclear spin-lattice relaxation time T1 and the Knight shift K. It is shown that the magnitude of g* is given by the Korringa product T1TK2=C(g*)2, where C is a known constant and T is the absolute temperature, and that the sign of g* is given by the sign of K for a spherically symmetric conduction band. The measured value, g*=-1.1±0.1, is within the range allowed …

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The Howard University Reviews Of Science, Vol 1 No 2, Department Of Physics Jan 1972

The Howard University Reviews Of Science, Vol 1 No 2, Department Of Physics

Physics Faculty Publications

No abstract provided.

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The Howard University Reviews Of Science, Vol 1 No 1, Department Of Physics Nov 1971

The Howard University Reviews Of Science, Vol 1 No 1, Department Of Physics

Physics Faculty Publications

No abstract provided.

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Nuclear-Magnetic-Resonance Studies Of The Semiconductor-To-Metal Transition In Chlorine-Doped Cadmium Sulfide, Frank D. Adams, David C. Look, Carlton L. Brown, Donald R. Locker Oct 1971

Nuclear-Magnetic-Resonance Studies Of The Semiconductor-To-Metal Transition In Chlorine-Doped Cadmium Sulfide, Frank D. Adams, David C. Look, Carlton L. Brown, Donald R. Locker

Physics Faculty Publications

Spin-lattice relaxation times (T1) and Knight shifts were measured for Cd113 nuclei in 12 CdS crystals doped with various amounts of chlorine. Hall coefficients were measured in order to estimate conduction-electron concentrations. Data were obtained for all samples at 300 °K and for some highly doped samples at 77, 4.2, and 2.13 °K. Metallic properties were observed in all samples having electron concentrations n>2×1018 cm-3. At 300 °K, we find 1/T1∝n for nonmetallic samples and 1/T1∝n2/3 when samples are metallic. The latter proportionality continues to hold at lower …

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Nuclear Magnetic Resonance Investigation Of The Metal-To-Semiconductor Transition In Crystalline Cdo, R. P. Benedict, David C. Look Dec 1970

Nuclear Magnetic Resonance Investigation Of The Metal-To-Semiconductor Transition In Crystalline Cdo, R. P. Benedict, David C. Look

Physics Faculty Publications

Measurements of the Cd113 nuclear-spin-lattice relaxation time (T1) and resonance frequency shift (K) were carried out in several samples of polycrystalline n-type CdO over the temperature (T) range 1.4-300°K, frequency (ν) range 2-10 MHz, and carrier concentration (Ne) range 3.3-44×1018 cm-3. For Ne>1019 cm-3, we find that T1∝Ne-2/3T-1 and K∝Ne1/3, showing that the nuclei are interacting with degenerate conduction electrons. Furthermore, the Korringa relationship holds and the calculated electron density at a nucleus is almost the same as …

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Time Saving In Measurement Of Nmr And Epr Relaxation Times, David C. Look, Donald R. Locker Feb 1970

Time Saving In Measurement Of Nmr And Epr Relaxation Times, David C. Look, Donald R. Locker

Physics Faculty Publications

By producing a train of absorption or dispersion signals (continuous‐wave magnetic resonance) or free induction decays (pulsed magnetic resonance) it is possible to save time in spin‐lattice relaxation measurements due to the fact that it is not necessary to wait for equilibrium magnetization before initiating the train. The relaxation time may be calculated from the train according to a simple rapidly converging iteration.

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Nmr Determination Of The Conduction-Electron Hyperfine Interaction In Crystalline Cdo, David C. Look Aug 1969

Nmr Determination Of The Conduction-Electron Hyperfine Interaction In Crystalline Cdo, David C. Look

Physics Faculty Publications

Measurements of the Cd113 nuclear-spin-lattice relaxation time T1 and Hall effect in crystalline CdO, a degenerate semiconductor, have yielded the contact hyperfine strength of the conduction electrons at the nuclei. The product T1T=168 sec °K, independent of temperature T and frequency ν for T=1.4,4.2, and 77-350 °K, and for ν=2-10 MHz. Taking the carrier concentration N=2.6×1019 cm-3 independent of temperature to within 3% at 4.2, 77, and 300°K, and using an effective electron mass me*=0.2me, we calculate an averaged electron probability density at the nucleus, 〈|φF(0)|2 …

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Pulsed Nmr By Tone-Burst Generation, David C. Look, Donald R. Locker Mar 1969

Pulsed Nmr By Tone-Burst Generation, David C. Look, Donald R. Locker

Physics Faculty Publications

No abstract provided.

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Measurement Of Electron Spin-Lattice Relaxation Times Using Ordinary Epr Spectrometers, David C. Look, Donald R. Locker Dec 1968

Measurement Of Electron Spin-Lattice Relaxation Times Using Ordinary Epr Spectrometers, David C. Look, Donald R. Locker

Physics Faculty Publications

No abstract provided.

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Nuclear Spin-Lattice Relaxation Measurements By Tone-Burst Modulation, David C. Look, Donald R. Locker Jan 1968

Nuclear Spin-Lattice Relaxation Measurements By Tone-Burst Modulation, David C. Look, Donald R. Locker

Physics Faculty Publications

No abstract provided.

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