Positron annihilation in rare-earth metals

Positron annihilation in rare-earth metals

Vol. 2, No. 2 ABSTRACTS OF PAPERS TO APPEAR IN J. PHYS. CHEM. SOLIDS above, but not considered) an Overhauser-type enhancement of nuclear polarizati...

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Vol. 2, No. 2

ABSTRACTS OF PAPERS TO APPEAR IN J. PHYS. CHEM. SOLIDS

above, but not considered) an Overhauser-type enhancement of nuclear polarization is shown to occur. This may take place via two- and three-magnon processes. In antiferromagnets the former processes are shown to lead to an antiferromagnetically polarized nuclear array, the latter to ferromagnetic nuclear arrays which, depending upon which antiferromagnetic resonance mode is excited, will be polarized either up or down. (Received 4 October 1963) 7. IONIC CONDUCTIVITY OF K! COLORED WITH 12’? 8 H. L.Forbes III and D. W. Lynch (Institute for Atomic Research and Department of Physics, Iowa State University, Ames, Iowa). When V2 and V3 bands are present in KI crystals additively colored with iodine, the ionic conductivity in the region from 90 to 200”C is found to be enhanced, but with an unchanged activation energy. A stoichiometric excess of iodine presumably gives rise to two cation vacancies and two holes per excess iodine molecule. The enhanced conductivity indicates that at least one vacancy per iodine molecule is not bound in the V centers, but is free. The V2 and V3 center models of Seitz are not compatible with this result, but those Hersh of and Effenberger are, although the of addition one vacancy to their models would also be consistent with the conductivity increase. An interpretation of several earlier experiments on V centers is made in terms of the above models. (Received 7 October 1963)

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Some features of previous theories are used but including explicitly the long range magnetic couplings and a statistical study near to 0’~K. The hypothesis: Born approximation and Rudermann Kittol Yosida coupling, limit the full application of this study to the case of rare earth impurities. The scattering of conduction electrons by all the pairs of coupled spins shows up an effective resistivity minimum. The scattering of conduction electrons by all the pairs of coupled spins leads to the existence of a resistivity minimum. Some of these predictions seem in agreement with recent observations. A similar model probably explains the more complicated case of transition impurities. (Received 16 October 1963) 9. TRANSIENT SPACE-CHARGE-LIMITED CURRENTS IN IODINE SINGLE CRYSTALS IN VARIOUS CRYSTALLOGRAPHIC DIRECTIONS A. Many, M. Simhony, S. Z. Weisz, and Y. Teucher (Department of Physics, The Hebrew University, Jerusalem, Israel). Transient space-charge-limited currents in iodine single crystals are studied along the three principal crystallographic directions. The mobility of the injected holes is found to be anisotropic, 2having ~ ±0.2, (at room temperature) theand values = 0. 72 ±0.05 ~=2. ~a1 ±0. 02 cm2 /V~sec. In the range 260 330 K, the temperature dependence of the mobility ca~be expressed’by a power law ~ T-r where r assumes the values 1. 5, 1. 3, and 2. 1 along the a, b, and. c directions, respectively. -



(Received 17 October 1963) 8. SUR LES ANOMALIES DE RESISTIVITE D’ALLIAGES DILUES MATRICES NOBLESIMPURETES MAGNETIQUES. M. T. B~al(Facultédes Sciences, Orsay, (S. et 0.), France). 7 8

10.POSITRON ANNIIJILATION IN RAREEARTH METALS~ D. R. Gustalson and A. R. Mackintosh’0 (Institute for Atomic Research and De-

Contribution No. 1368. Work was performed in the Ames Laboratory of the U. S. Atomic Energy Commission.

Present address: U. S. Army Cold I~gionsResearch and Engineering Laboratory, Hanover, N.H. 9 Contribution No. 1284. Work was performed in the Ames Laboratory of the U. S. Atomic Energy Commission. 10 Currently on leave from the Ames Laboratory. Address: AEC Research Establishment, Riso Roskilde, Denmark.

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ABSTRACTS OF PAPERS TO APPEAR IN J. PHYS. CHEM. SOLIDS partment of Physics, Iowa State University, Ames, Iowa).

The electronic structure of three rareearth metals has been studied by measuring the angular correlations of the photons emitted when positrons annihilate with the electrons in the metal. From the results it is deduced that gadolinium and cerium at room temperature each has approximately three conduction electrons per atom, while ytterbium has approximately two as expected from the localized f-electron model. Qualitative information is obtained about the distortion of the Fermi surfaces of these three metals and it is concluded that the polarization of the conduction electrons in the magnetically ordered state of gadolinium is small. Annihilations with ion core electrons comprise a significant proportion of the total in each metal, the number relative to the conduction electron annihilations decreasing with increasing atomic number. (Received 29 July 1963) (Revised 24 October 1963) 11 .CONDUCTION BAND MINIMUM OF CdTe W. G. Spitzer (Electrical Engineering Department, University of Southern California, Los Angeles, California); and C. A. Mead (California Institute of Technology, Pasadena, California). The spectral dependence of the photovoltaic effect for Au n type CdTe surface barrier cells is measured for photon energies near the CdTe intrinsic absorption edge at 298~K, 77K, and -5K. The results are viewed in terms of phonon assisted direct exciton absorption, phonon assisted exciton creation at a previously proposed lattice defect site, and indirect optical transitions. The data appears to favor the third mechanism although the second one is not ruled out as a possibility. The values for the direct transition energy gap obtained at the different temperatures compare favorably with those given by other work. -

(Received 28 October 1963)

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12.CLASSICAL GROUND STATE SPIN CONFIGURATIONS IN THE CORUNDUM LATTICE N. Menyuk and K. Dwight (Lincoln Laboratory1’, Massachusetts Institute of Technology, Lexington 73, Massachusetts). By means of the Luttinger-Tisza method, it is proven that a simple coplanar spiral always constitutes the classical ground state of the Heisenberg exchange Hamiltonian in magnetic corundum-type structures (i. e. Cr 2O3 and a-Fe2O3). This proof is shown to be independent of the values of the various exchange interactions. It therefore follows that the treatment of mixed systems in terms of averaged interactions cannot yield any other result. This conclusion is discussed in connection with the conical spirals observed by Takei, Cox and Shirane in the System Fe2XCr2_2~O3. (Received 29 October 1963) 13.THE SHEAR STRENGTHS OF SOME 12 (University of Virginia, SOLIDIFIED CASES Charlottesville, Laird C. Towle Virginia). The shear strengths of solid argon, krypton and hydrogen have been measured at pressures up to 8, 000 kgm/cm2 in an extrusion apparatus. Argon and krypton were examined at 77K; hydrogen at 4. 2K. Indium was also tested at room temperature and served as a control. The shear strength of the solidified gases, which was initially in the range from 1 to 20 kgm/cm2, was found to increase by about an order of magnitude in the pressure range investigated. (Received 1 November 1963) 14.ELECTRICAL AND THERMAL PROPERTIES OF ALL9~SOF CdTe AND CdSe A. D. Stuckes~ and 0. Farrell (Associated Electrical Industries Research Laboratory, Manchester) The CdTe:CdSe system has been in-

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Operated with support from the U. S. Air Force

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Presently at Allis-Chalmers Manufacturing Company, Milwaukee, Wisconsin

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Royal College of Advanced Technology, Salford, England