Showing papers by "Solid State Physics Laboratory published in 1976"

Reorientation of dipoles in Sr F 2 : R 3 +

Abstract: Measurements of dipole relaxation of two types of ${R}^{3+}$-interstitial complexes in $\mathrm{Sr}{\mathrm{F}}_{2}:{R}^{3+}$ single crystals using the ionic-thermocurrents method are reported. For the light elements in the series of lanthanides we have found relaxations due to tetragonal complexes, whereas in crystals doped with heavy ${R}^{3+}$ ions such as ${\mathrm{Dy}}^{3+}$, ${\mathrm{Ho}}^{3+}$, etc., the predominant defects have trigonal symmetry. The variation of the activation energy as a function of the ${R}^{3+}$ radius is discussed. In addition, the effect of the impurity concentration on the relaxation parameters is studied.

Molecular statistical calculation of pretransitional effects in nematic liquid-crystals

Abstract: A molecular statistical treatment is given of two pretransitional effects in the isotropic phase of nematic liquid crystals, namely of the magnetically induced birefringence and the scattering of light by orientational fluctuations The calculations are based on the Maier-Saupe model, treated in teh Bethe approximation in order to account for short range order The ratio (T e - T c∗)/T c is improved considerably with respect to the mean field approximation Also the cluster variation method is applied to the Maier-Saupe model The differences between the results of the Bethe and the cluster variation method are discussed A slight modification of the Bethe method produces close agreement with the cluster variation method

ENDOR of Gd3+ in LaCl3

Abstract: The parameters of the nuclear terms in the spin Hamiltonian have been determined with ENDOR for 155Gd3+ and 157Gd3+ in LaCl3, using the natural distribution of isotopes. The quadrupole coupling data, investigated on the basis of the model of Newman and Price, indicate considerable local distortion around Gd3+. However, preliminary ligand ENDOR data for the La nucleus suggest that the distortion is small. Furthermore, a useful linear relation is found between the quadrupole coupling of the Gd nucleus with the crystal field and the anisotropy in the magnetic h.f.i. between the same nucleus and the 4f electrons.

Local-mode absorption of h- and d- in srcl2-re and srf2-re single-crystals

Abstract: IR studies on localized vibrations of hydride ions in SrCI2 and SrF2 are reported. In hydrogenated and deuterated samples doped with trivalent rare earth ions two different axially symmetric complexes giving rise to two doublets are observed. For SrCI2:RE3+, H− the positions of the absorption lines can be predicted with reasonable accuracy. Lattice relaxations appear to play more important role for C4v oscillators in SrF2 than in SrCI2 and thereforeit is more difficult to give a theoretical description of these tetragonal complexes. Es wird uber IR-Untersuchungen von lokalisierten Schwingungen von Hydrid-Ionen in SrCI2, und SrF2 berichtet. In mit Wasserstoff und Deuterium behandelten und mit SE3+ dotierten Kristallen werden zwei von axialsymmetrischen Komplexen verursachte Dubletts beobachtet. Fur SrCI2: SE3+, H− konnen die Lagen der Absorptionslinien mit befriedigender Genauigkeit vorausgesagt werden. Fur C4v−Oszillatoren in SrF2 ist der Einflus von Gitterrelaxationen wahrscheinlich groser als bei SrCl2 und deshalb ist die theoretische Beschreibung dieser tetragonalen Komplexe komplizierter.

A high pressure investigation of the conduction in ilmenite

Abstract: The effect of pressure on the conductivity and the hyperfine interactions at the 57Fe nuclei of semiconducting FeTi03 has been studied The Mossbauer spectra of ilmenite are interpreted in terms of two doublets The most intense one is due to Fe2+-ions in the Fe layers ; the second one is due to Fez+-ions in the Ti layers in the corundum structure A comparison is made with the pressure induced changes in insulating a-Fez03 which has the same structure The increase of conductivity and the decrease of the isomer shift of the Fez+-ions in the Fe-layers of FeTi03 is explained by means of a delocalization of 3d electrons in these layers of ilmenite

Electron Spin Resonance Study of Cr3+ and Fe3+ in Alums

Abstract: The angular dependence of the electron spin resonance spectrum of Cr3+ and Fe3+ ions in diluted ammonium alum has been measured, and the spin Hamiltonian parameters have been derived. In addition to the allowed fine structure lines some small-intensity lowfield lines have been observed. These lines were identified as forbidden fine structure transitions due to Cr3+ and Fe3+ in alum. A good agreement between the theoretical angular dependence with that experimentally found confirmed the lines and the local symmetry of the ions to be correctly determined. Es wurden die Winkelabhangigkeit des Elektronenspinresonanzspektrums von Cr3+-und Fe3+-Ionen in verdunntem Ammoniumalaun gemessen und die Parameter des Spin-Hamilton-Operators bestimmt. Auser den erlaubten Feinstrukturlinien wurden bei kleinen Magnetfeldern zusatzliche schwache Linien beobachtet, die als verbotene ubergange mit ΔM = ±2, ±3 identifiziert werden konnten. Die gute ubereinstimmung zwischen der theoretischen und der experimentell bestimmten Winkelabhangigkeit bestatigt, das die Deutung der Linien und die Platzsymmetrie der paramagnetischen Ionen korrekt sein mussen.

TRANSFORMATION OF SOME AMORPHOUS FePC ALLOYS DURING ISOTHERMAL AGING

Abstract: A Mossbauer effect study of the liquid quenched alloys Fe75P18C7, Fe80P13C7 and Fe83P11C6 as a function of aging time at 350 °C is reported. Crystallization occurs after a certain aging time which depends on the alloy composition; Fe80P13C7 being the most stable. The crystallization process starts probably with the formation of clusters of iron atoms. The most important crystalline phases in the final decomposed state are identified. During the amorphous to crystalline transition the mean f-factor of the alloy remains constant to within about five percent.

MOSSBAUER EFFECT OF 57Fe IN Ti2O3

Abstract: The Ti2O3 system has been investigated with the Mossbauer effect technique using 57Fe. It is found that iron exists in divalent as well as in trivalent oxidation states, the Fe3+/Fe2+ intensity ratio increasing with temperature. An explanation for this is given in terms of bound states associated with the Fe2+ ions. The temperature dependence of the Fe2+ quadrupole splitting is in agreement with the results of a crystal field calculation. No evidence for magnetic ordering in Ti2O3 is found, in agreement with neutron diffraction results.

Laser-induced shock waves in liquids

Abstract: The interaction of TEA-CO2-laser pulses with liquids has been investigated using shadowgraphy and high-speed pressure transducers. It is shown that the absorption coefficient of the liquid at the laser wavelength defines the geometry of the shock wave. Peak pressures in the kbar range have been observed.

MÖSSBAUER STUDY OF DILUTE FeX ALLOYS WITH X THE NON-TRANSITION ELEMENTS Al, Si, Ga, Ge, As, Sn AND Sb

Abstract: By temperature dependent Mossbauer experiments magnetic hyperfine fields H((m, n) T) at different iron sites are determined in the alloys Fe - 1 a/0 Ge, Fe - 3 a/0 Ge, Fe - 39 a/o Sn, Fe - 5 a/o Ga, Fe - 51 a/o Sb, Fe - 3 a/o Sb and Fe - 1 a/o Sb The differences of the reduced hyperfine fields are small indicating that itinerant effects are of major importance The Curie temperatures are determined There appears to be a rough correspondence between Tc and H(0,0) of these alloys compared with pure iron, indicating that variation of Tc is also governed by non local interactions

Effect of permanent dipoles on nematic-isotropic phase-transition

Abstract: 2014 The influence of permanent dipoles on the nematic isotropic transition is studied by adding a simple polar interaction to the Maier-Saupe model of nematic liquid crystals. The model is treated in the Bethe-Peierls approximation in order to account for short range order. We have also included the possibility of (anti) ferroelectric long range order. This ferroelectric order strongly affects the stability of the nematic phase. LE JOURNAL DE PHYSIQUE TOME 37, NOVEMBRE 1976, : Classification Physics Abstracts 7.130 One of the characteristic properties of nematics is the indistinguishability of states with director n and n. When individual molecules carry a permanent electric dipole moment, just as many dipoles point up as down. Otherwise the system would be ferroelectric. Ferroelectric nematics have, as yet, not been found [1]. There are a number of nematogenic materials [2] which do not carry a permanent dipole moment, i.e. the influence of permanent dipoles in establishing nematic order cannot be dominant. In the model of Maier and Saupe [3] the Van der Waals interaction between mutually induced dipole moments gives rise to a phase transition from the isotropic to the nematic phase. The interaction energy can be written as where ; specifies the orientation of the long axis of a molecule i, and P2 denotes the second Legendre polynomial. In the mean field approximation, as used by Maier and Saupe, the energy of a molecule is given by where y is the number of nearest neighbours, and S = ( p 2 (aiz) > is the long range order parameter, to be determined self consistently. In two recent papers [4] we studied the interaction (1) in the approximation of Bethe and Peierls in order to account for short range order. The long range properties appeared to be very similar to Maier and Saupe’s mean field results. The description of pretransitional effects in the isotropic phase, namely the magnetically induced birefringence and the scattering of light by orientational fluctuations, was improved considerably. Although permanent dipoles should play a minor role in establishing nematic order, the effects of a polar interaction on the transition temperature, the jump in the order parameter, etc., is still open to question. If we add a polar interaction in its simplest form, the interaction energy reads The first term could result from the interaction between permanent dipoles which, for cylindrically symmetric molecules, point effectively along the long molecular Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0197600370110133100

La-139 endor of lacl3-gd3+

Abstract: The magnetic transferred hfi and the electric quadropole interaction of the two 139La nuclei nearest to Gd3+ in LaCl3 have been determined with ENDOR, using Ka-band (345 GHz) microwaves For both lanthanum positions it was found that A s A p ≈ 1 and Ap − Ad ≈ 0 The ENDOR data give no evidence for significant local distortion around Gd3+ This implies that ligand displacements due to the difference in size between Gd3+ and La3+ cannot account for the disagreement between the observed and calculated B02 of Gd3+ [1]

Design Considerations of Cordwood Module for Electronic Assembly

Abstract: The paper describes the design procedure of a sandwich or cordwood module from a given circuit. It compares the edge loaded cordwood module with a general cordwood module and enumerates the advantages of the edge loaded cordwood module. The module is suitable for adoption in any shape including rectangular sandwich in order to suit stringent volumetric packaging requirements.