Conductive electrique des magnetites faiblement substituees a l'aluminium ou au chrome et faiblement oxydees au voisinage de la temperature de transition de verwey

Vol. 28, No.12 ABSTRACTS OF ARTICLES TO BE PUBUSHED IN J. PHYS. CHEM. SOLIDS being the wave and plasma frequencies) and tends to a saturation value at very high values of wi,. Received 28 July 1978 Revised 16 October 1978

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RAMAN STUDY OF (NH4)2CuC14 2H20 II. ORDER—DISORDER TRANSITION A.P. Roy, V.C. Sahni and M.L. Bansal, Solid State Physics Section, Bhabha Atomic Research Centre, Bombay 400 085, India.

The results of a Raman scattering study of the order—disorder phase transition in (NH4)2CuC14 2H20 are presented. From the variation of intensity of some selected modes in the temperature range 300—100K, we have established the temperature dependence of the long range order parameter m, defined as the difference between the fractional number of NH4 ions in the two possible orientations. Since the ammonium network in (NH4)2CuC14 2H~Ois “quasi one-dimensional”, in conDynamical electron—electron correlation in pa~an~agtrast to the three-dimensional network in NH4C1, we magnetic disordered binary alloys described by the discuss the consequence of this feature on the evolution Hubbard-type Hamiltonian is studied using the T-matrix of m. approximation (1’MA) valid for low concentration of Received 31 May 1978 particles (~ 0.6/atom). We introduce the terminal-point approximation for the many-body quantities, which allows us to solve the random part of the problem within 13. THE SURFACE PLASMON MODES OF the single-site approximation (CPA). The one-centre SPHERICAL VOIDS IN IRRADIATED version of the TMA is used in numerical calculations. METALS Results are compared with those of the Hartree—Fock G.C. Aers, A.D. Boardman and B.V. Paranjape, approximation, and with the model of a pair of interTheoretical Physics Insitute, University of acting electrons in a random alloy (CPA II). Alberta, Edmonton, Canada T6G 2J1 ; and Department of Pure and Applied Physics, Received 8 August 1978 University of Salford, Salford MS 4WT, England. 11. CONDUCTIVITE ELECTRIQUE DES The surface plasmon modes of spherical voids in MAGNETITES FAIBLEMENT SUBSTITUEES A L’ALUMINIUM OU AU CHROME ET metals modeled by a free-electron gas are calculated FAIBLEMENT OXYDEES AU VOISINAGE within a hydrodynamic formulation. Spatial dispersion, DE LA TEMPERATURE DE TRANSITION retardation, and surface electron density variation are DE VERWEY included. A sharp-surface model is considered first and it is shown that the modes are non-radiative. A novel Bernard Gillot, Laboratoire de Recherches sur matrix is of used to discuss the sensitivity of the It la Réactivité des Solides associé au CNRS, results method to the use different surface density profiles. Faculté des Sciences Mirande, BP 138, 21004 is shown that the interplay between surface inhomogenDijon, Cedex, France. eity and spatial dispersion always results in a mode in La conductivité ~lectrique des magnetites faiblement the near neighbourhood of the sharp-surface mode. substituées a l’aluminium ou au chrome et faiblement There are, however, other structure-induced modes oxydées (0
ELECTRON—ELECTRON CORRELATION IN DISORDERED BINARY ALLOYS: THE T-MATRIX APPROXIMATION V. Drchal, Institute of Solid State Physics, Czechoslovak Academy of Sciences, Praha, Cukrovarnická 10, 162 53 Praha 6, Czechoslovakia.

Received 22 May 1978 Revised 8 September 1978

M. Nogues, M. MejaI and L. Goldstein, 1. A.LaboraBriand, 92190 France.1, p toire deBellevue-Meudon, Magnétisme du CNRS,