Showing papers by "Solid State Physics Laboratory published in 1977"

Mossbauer study of atomic order in ni3fe .1. determination of long-range-order parameter

Abstract: By means of M\"ossbauer spectroscopy we studied the structural order in a series of stoichiometric ${\mathrm{Ni}}_{3}$Fe foils, which had been given different heat treatments. The long-range-order parameter $\ensuremath{\eta}$ was determined with an accuracy of \ifmmode\pm\else\textpm\fi{}0.02 by analyzing the profiles of the outer lines of $^{57}\mathrm{Fe}$ absorption spectra, recorded at room temperature. From the analysis it appeared that the hyperfine field at $^{57}\mathrm{Fe}$ nuclei depends linearly on the numbers of iron atoms in the first and second neighboring shell, and that contributions from more distant atoms are negligible. Further anisotropic hyperfine interactions in ${\mathrm{Ni}}_{3}$Fe are small. A comparison with $\ensuremath{\eta}$ as determined from x-ray diffraction indicates that the wrongly placed atoms in partly ordered ${\mathrm{Ni}}_{3}$Fe are distributed at random over the lattice sites.

EPR of Gd 3 + -metal-ion complexes in Ca F 2 , Ba F 2 , and Sr Cl 2

Abstract: EPR experiments of ${\mathrm{Gd}}^{3+}\ensuremath{-}{M}^{+}$ complexes in Ca${\mathrm{F}}_{2}$, Ba${\mathrm{F}}_{2}$, and Sr${\mathrm{Cl}}_{2}$ crystals are reported. We have studied the effect of the excess negative charge of the metal-ion impurity on the crystal field; also the relaxations of the halide ions neighboring the complex have been investigated. It will be shown that the values of the second-degree crystal-field parameters as determined from the EPR spectra can be explained in terms of an extended-point-charge model. For the calculations of the magnetic parameters we have taken into account second-order effects of odd crystal-field terms which have also been calculated using the point-ion-lattice model. Distortions of the lattice surrounding the ${\mathrm{Gd}}^{3+}\ensuremath{-}{M}^{+}$ complexes are found to have large effects on the magnitude of the second-degree crystal-field parameters. Our experimental results have also been interpreted on the basis of the superposition model in which it is assumed that the crystal-field parameters are determined by the nature and positions of the nearest neighbors of the ${\mathrm{Gd}}^{3+}$ impurity. It will be shown in this paper that for ${\mathrm{Gd}}^{3+}\ensuremath{-}{M}^{+}$ complexes in Ca${\mathrm{F}}_{2}$ the superposition model predicts unrealistically large displacements of the fluorine ions neighboring the ${\mathrm{Gd}}^{3+}$ and ${M}^{+}$ impurity.

A Monte Carlo Calculation of the Nematic-Isotropic Phase Transition

Abstract: The behaviour of a lattice version of the Maier-Saupe model near the clearing point is investigated using a biased Monte Carlo technique. The lattice version consists of an array of unit vectors, which are located at the sites of a n x n x n (n = 14, 16, 18, 20) simple-cubic lattice with periodic boundary conditions and interact by way of a nearest neighbour coupling. The system undergoes a first order phase transition with a spontaneous order S = 0.333 ± 0.009 at the clearing temperature Tc given by βce = 0.894 ± 0.001, where e denotes the maximum interaction energy between two nearest neighbours. The pretransitional light scattering in the isotropic phase can be reasonably well described by [T - Tc∗]−1, with (Tc - Tc= 0.009 ± 0.007 for n = 14 and (Tc - Tc∗)/Tc = 0.007 ± 0.004 for n = 16. This type of divergence is in agreement with experiment.

Stability of the Tetrahedral Phase in the AI2BIVCVI3 Group of Compounds

Abstract: Three compounds Cu2SnSe3, Cu2SnTe3, and Cu2GeTe3 belonging to the general formula AI2BIVCVI3 are investigated The first compound gives a single phase solid while the other two give a two phase solid with a eutectic type microstructure The first compound is found to tolerate considerable deficiency of the group IV element (Sn) The unit cell size decreases with the decrease in Sn concentration The two phase regions in Cu2GeTe3 and Cu2SnTe3 are analysed using an electron micro-probe It is found that Te distribution is uniform in the two phases but group I and IV elements show complementary distributions, that is, one phase is rich in the Group I element while the other is rich in the Group IV element The observations can be related to the effective valence character of the participating Group IV element Es werden drei Verbindungen, Cu2SnSe3, Cu2SnTe3 und Cu2GeTe3, die zur allgemeinen Gruppe AI2BIVCVI3 gehoren, untersucht Die erste Verbindung ist ein einphasiger Festkorper, wahrend die anderen beiden einen zweiphasigen Festkorper mit einer eutektischen Mikrostruktur bilden Es wird gefunden, das die erste Verbindung einen betrachtlichen Untersch us des Elements der vierten Gruppe (Sn) zulast Die Grose der Einheitszelle nimmt mit der Sn-Konzentration ab Die Zwei-Phasen-Bereiche in Cu2GeTe3 und Cu2SnTe3 werden mit einer Elektronenmikrosonde untersucht; Es wird gefunden, das die Te-Verteilung gleichformig ist in den beiden Phasen; jedoch zeigen die Elemente der I und IV Gruppe komplementare Verteilungen, d h eine Phase hat einen grosen Anteil des Elements der I Gruppe, wahrend die andere einen grosen Anteil des Elements der IV Gruppe hat Die Beobachtungen lassen sich mit dem effektiven Valenzcharakter des beteiligten Elements der IV Gruppe verknupfen

Thin-film MOM-diodes for infrared detection

Abstract: Thin-film metal-oxide-metal (MOM) diodes were fabricated by a positivephotoresist process using iron oxide and chromium masks. Reproducible junctions were obtained with areas of 2 μm2. The diodes exhibit stable, highly nonlinear I–V characteristics. Several hundred diodes were printed on the same substrate with nonlinearity parameters varying less than 15%. Infrared rectification was tested with a stabilized 10.6 μm CO2 laser.

Composition and temperature-dependence of pitch in cholesteric binary-mixtures

Abstract: In the cholesteric phase of mixtures of cholesteric cholesteryl chloride (CC) with nematic p-ethoxy benzylidene-p-n butylaniline (EBBA) as well as with nematic p-[(p-ethoxy-benzyl-idene)amino]benzonitrile (PEBAB), the pitch has been determined asl a function of temperature and composition. For a CC weight fraction x≲ 0.8 we used the “variable wedge” method for x ≳ 0.8 we determined the wavelength of maximum reflection λo and the average refractive index n and employed the rlation λo = n p. We show that n can vary appreciably as a function of composition in a cholesteric binaryo system. For each composition the reciprocal pitch can be fitted reasonably well to a linear function of the temperature, except in the vicinity of phase transitions. It appears that the slope of the linear function, i.e. d(p−1/dt, is always positive. We dicuss the results in relation to recently developecd expressions for the pitch as a function of temperature and composition.

Waveguides for submillimeter-wave lasers

Abstract: A review of the development and the present state of the art of waveguides for submillimeter-wave lasers is presented. Theoretical and experimental aspects of waveguides for longitudinally and transversely excited as well as for optically pumped lasers are discussed. This is followed by a comment on investigations of periodic waveguide structures for distributed feeback submillimeter-wave lasers.

Transversely excited waveguide HCN laser with UV preionization

Abstract: In order to obtain high-power submillimeter-wave pulses, UV preionization was applied to the transverse discharges in HCN-laser gas mixtures. Homogeneous discharges were obtained at pressures up to 1 atm. An increase of laser output and working pressure was achieved for the 337-μm emission.

Structural order and homogeneity in ni3al

Abstract: The process of ordering in mechanically disordered Ni3Al has been followed by means of X-ray, susceptibility and Mossbauer measurements. A short anneal at 800° is sufficient to restore the long range order completely. An anneal of at least ten days appears to be necessary for achieving homogeneity.

Electron-excess centers in calciumchlorapatite

Abstract: Flux-grown synthetic calciumchlorapatite (Ca10 (PO4)6Cl2) single crystals are known to show significant off-stoichiometry; the CaCl2 deficiency can be as large as 3 to 5%. This has severe implications for many physical properties of these materials; especially the defects as produced by means of X-rays will be strongly modified by the presence of large concentrations of vacancies. It is found that at 300 K predominantly complex colour centres are created. Below 50 K rather simple electron-excess centres are formed during X-irradiation. A systematic study of the photo-chemical bleaching and annealing properties of the colour centres enables one to develop models for the electron-excess centres observed at different temperatures. Synthetische Einkristalle von Calciumchlorapatit (Ca10(PO4)6Cl2) sind bekanntlich nicht-stochiometrisch; es fehlen im Durchschnitt 3 bis 5% CaCl2-Molekule in den Kristallen, die aus einer Schmelzlosung gewachsen sind. Das bedeutet, das die durch Rontgenstrahlen erzeugten Defekte stark durch die grose Menge von Leerstellen in dem Kristall beeinflust werden. Es wird gefunden, das die Farbzentren die bei 300 K erzeugt werden, ziemlich komplex sind. Bei nied-rigen Temperaturen (< 50 K) werden nur einfachere Elektronenuberschuszentren erzeugt. Modelle fur diese Farbzentren werden nach einer systematischen Untersuchung der Eigenschaften dieser Farbzentren bei verschiedenen Temperaturen aufgestellt.

Paramagnetic color centers in calciumchlorapatite doped with F− ions

Abstract: The results are given of EPR experiments on electron excess color centers in X-rayed synthetic calciumchlorapatite which is doped with fluoride ions. A comparison of the results obtained with those given in earlier papers on color centers in calciumchlorapatite and fluorapatite enables one to propose models for some of the electron excess centers observed. The centers observed in the material- consist of a) an electron trapped at an anion vacancy neighbored by two equivalent (or approximately equivalent) Cl− ions, b) an electron trapped at an anion vacancy neighbored by one Cl− and one F− ion, c) an electron trapped at an anion divacancy neighbored by two (nearly) equivalent F− ions. Es werden Ergebnisse von Elektronenspinresonanzmessungen an Elektronenuberschuszentren in rontgenbestrahlten synthetischen, mit F−-Ionen dotierten Kalziumchlorapatitkristallen mitgeteilt. Ein Vergleich mit fruheren Ergebnissen fur Kalziumchlorapatit und Fluorapatit ergibt Modelle fur einige Elektronenuberschuszentren: Die beobachteten Zentren sind a) ein in einer Cl−-Anionlucke eingefangenes Elektron; zwei (fast) aquivalente Chlorionen sind die Nachbarn, b) ein in einer Cl−-Anionlucke eingefangenes Elektron; die Nachbarn sind ein Cl−- und ein F−-Ion, und c) ein in einer Cl−-Doppellucke eingefangenes Elektron; die Nachbarn sind zwei (fast) aquivalente F−-Ionen.

A new approach to overlap repulsion

Abstract: Introducing the concept of interorbital distance (δi), a new form of overlap repulsion (ΨR = Aδ) for ionic crystals is suggested. Alkali halide crystals are considered for the present purpose. The theory is based on the assumption that the ions possess a structure like the Bohr atom and this assumption along with other basic considerations exhibits a gap effect which leads to the formation of the present potential energy function. To test the validity of the new approach the compressibility (β) and the elastic constant (C‖) of alkali halide crystals are evaluated. The closeness of theoretical and experimental values strongly supports the present approach. The ionic radii of alkali and halogen ions are also evaluated and discussed in detail in the light of Pauling's values. Durch Einfuhrung der Vorstellung des interorbitalen Abstands (δi) wird eine neue Form der Uberlappungsabstosung (ΨR = Aδ) fur lonenkristalle vorgeschlagen. Zu diesem Zweck werden Alkalihalogenidkristalle betrachtet. Die Theorie beruht auf der Annahme, das die Ionen eine Struktur besitzen, die dem Bohrschen Atom ahnlich ist; diese Annahme zusammen mit anderen grundlegenden Betrachtungen liefert einen Gapeffekt, der zur Bildung der vorliegenden Potential-funktion fuhrt. Um die Gultigkeit des neuen Verfahrens zu prufen, wird die Kompressibilitat (β) und die elastische Konstante (C‖) der Alkalihalogenidkristalle berechnet. Die Ubereinstimmung zwischen theoretischen und experimentellen Werten bestatigt im hohen Mase das vorliegende Verfahren. Die lonenradien der Alkali- und Halogenionen werden ebenfalls berechnet und im Liehte der Paulingschen Werte im Detail diskutiert.

Xps-study of fe (4-5a/o) x-alloys with x=al, si, ga, ge, as, sn, sb

Abstract: An XPS-study of Fe (4−5 a/o) X-alloys, X = Al , Si , Ga , Ge , As , Sn , and Sb , is reported. The binding energy shifts in FeX are −0.9 eV and −0.6 eV for Al and Ga, respectively, −0.3 eV for As and Sb and zero for Si, Ge, Sn and Fe. The shifts for Al and Ga are interpreted as a volume effect. No difference was found between the valence bands of the dilute alloys and pure Fe.

Stress wave generation in liquids by laser impact

Abstract: The generation of stress transients in liquids by laser impact has been investigated using special stress transducers. The observed time dependence and amplitudes of the pressure pulses are strongly dependent on the incident laser energy density and on physical properties of the liquid. This is explained by various interaction mechanisms involved. The results are in good agreement with theoretical models. For the pure thermoelastic process an analytical solution has been found where the laser impact is represented by a three dimensional heat pole.

Anomalies in the temperature dependence of the hyperfine fields in Sn doped and pure magnetite

Abstract: A Mossbauer study of the behavior of the hyperfine interactions at the 57Fe and 119Sn nuclei in Fe3O4 and Sn doped Fe3O4 has revealed anomalies in the temperature dependence of these hyperfine fields in the Verwey transition temperature region.

Low-Temperature Deposition of SiO2 Films on GaAs Using Tetraethoxysilane

Abstract: A chemical vapour deposition method for the deposition of SiO2 films on GaAs is reported in this paper. It is found that decomposition by oxidation of tetraethoxysilane in oxygen at a temperature of 450°C yields uniform, good quality, adhesive films of SiO2. The films have been characterized by various techniques. The advantages of this method over others are also discussed.

Some aspects of a study of dilute fex alloys, with x non-transition elements al, si, ga, ge, as, sn and sb

Abstract: By temperature dependent Mossbauer measurements the magnetic hyperfine fields at different iron sites are determined. The Curie temperatures are also determined. These results are interpreted in terms of a modified Zener-Vonsovskii model.