Zur Oxydation von Metallen bei sehr tiefen Temperaturen

The formation of oxide films on various metals has been investigated in the temperature range between 1·5 and 100°K. The metal films (50 to 150Å thick) were produced by condensation of the vapor on a quartz plate at about 100°K. After cooling down the films to helium temperatures, oxygen molecules were condensed to the fresh metal surface. The electrical resistance was measured before, during and after this O₂ condensation at temperatures between 1·5 and 300°K. The formation of an oxide layer causes an increase of the electrical resistance, since the conductivity of the oxide is comparatively low. The resistance behaviour of the metals investigated indicates two different steps of oxidation each one starting at a certain “characteristic” temperature below 25°K. Below the lower characteristic temperature, no reaction takes place. Increasing the temperature slowly, the oxide layer grows up to a final thickness. No further growing is detectable between 50 and 150°K for most of the metals. The results are discussed with regard to theoretical considerations ofMott, Cabrera andHauffe.     

Optische Untersuchungen an CdS-dotierten AgBr-Kristallen bei tiefen Temperaturen

The paper reports on the results of studying the absorption and fluorescence of AgBr crystals doped with CdS. The results are discussed by means of a one-dimensional model of configuration curves.     

Thermische Ausdehnung von Metallen bei tiefen Temperaturen

The thermal expansion coefficients of the following metals in the temperature region between 1,5° and 12°K have been measured: Al, Pb, Pt, Mo, Ta, W, Mg, Cd, Re, Ti, La, Ce, Nd, Gd, Yb. Except for tantalum all the specimens were polycrystalline. Is is found in accordance with theoretical prediction that the coefficient of thermal expansionβ=1/V (?V/?T) [whereV = volume,T = temperature] at sufficiently low temperatures is composed of an electronic component varying linearly with temperature, and a lattice component varying as the cube of the temperature. The electronic component is strongly modified in the superconducting state (Pb, Ta, La). The rare earth metals Gd, Ce and Nd have negative anomalies in their expansion. These are connected with the ferro- and antiferromagnetism of these substances. The results are discussed on the basis of lattice dynamics and the theory of electrons in metals.     

Zur Widerstandsanomalie von Legierungen bei tiefen Temperaturen

Dilute alloys of Cu-Fe, Au-Fe, Zn-Mn, and Ag with V, Cr, Mn, Fe, Co are produced by simultaneous evaporation of both components on a substrate at 77 °K. By this method one succeeds in solving all systems, especially Cu alloys with high Fe concentration, which alloys normally show precipitations after cooling from the molten state. For these alloys we observe resistance minima up to 150 °K. Besides this, Cu-Fe shows a resistance maximum which depends on concentration. For Au-Fe and Ag-Mn we measure resistance anomalies which are even bigger for quenched films than for annealed films and bulk material. Apparently the lattice defects have an influence on the resistance anomaly. The increase of the resistance with decreasing temperature can be described by the functionρ _s /ρ _s0 =f(T/T _k ) for all systems. The result does not agree with the theories basing on Kondo's model. Here is tried an explanation by means of spin-orbit interaction. This interaction results in a temperature dependent cross-section for a molecule consisting of a magnetic impurity and a lattice defect.     

Die Szintillations-Anisotropie von Anthrazen bei tiefen Temperaturen

An anthracene crystal is cooled down to liquid hydrogen temperature and is bombarded with alpha particles in two directions parallel to theb andc′ crystal axes. The ratio of maximum to minimm light yield (for alpha particles parallel toc′ andb, respectively) is reduced to 1.23 at 20.4° K. This ratio is 1.46 for ThC′ and 1.53 for ThC alpha particles at room temperature and is measured as a function of temperature between 20.4 and about 90° K.     

Anomales elastisches Verhalten von Eis bei tiefen Temperaturen

The temperature dependence of the longitudinal elastic constants of ice single crystals has been measured by an ultrasonic pulse-echo method. With heating, resp. cooling rates less than a critical value, deviations to lower values of the elastic constants have been observed. The time dependence of the anomaly has been studied at fixed temperatures. Limiting values of the elastic constants have been reached after sufficiently long time; the maximum of these values was observed at about 105°K. The findings are interpreted as representing a phase transition within the protonic structure.     

Die Szintillations-Anisotropie von Anthrazen bei tiefen Temperaturen. II

Continuing previous work the scintillation anisotropy of anthracene bombarded with α particles has been measured as a function of temperature between 1.9 and about 80° K. With all crystal specimens investigated a pronounced minimum has been found between 20 and 30° K. The anisotropy is constant below about 9° K, whereas the light yield still increases with decreasing temperature down to 1.9° K. The light yield has been found to be a nonlinear function of the temperature.     

Zur Theorie des elektrischen Widerstandes reiner Metalle bei tiefen Temperaturen

A method is described for the calculation of the electrical conductivity caused by the scattering of phonons which allows to take into account the correct spectrum of the lattice waves and the correct shape of the Fermi surface. The transport problem is solved using a variational method. The electrical resistance due to the scattering by normal processes yields proportionality toT ⁵ as in the fundamental treatment ofBloch, the exact factor of the relation being determined by integrals extended over the Fermi surface. If both normal and umklapp processes are effective the equation of the transport problem has to be solved for the simultaneous action of both scattering mechanisms. In general it is not possible to describe the influence of the umklapp processes on the electrical resistance by an additional term. Explicit expressions for magnitude characterizing umklapp processes are given.     

Elektrischer Widerstand von metastabilenAg,Fe- undCu,Fe-Legierungen bei tiefen Temperaturen

Ag, Fe undCu, Fe alloys of high concentration (up to 20 at %), which cannot be produced as solid solution in bulk, are produced by the simultaneous condensation of both the components on a substrate at a temperature of 4.2 or 78 K. By cooling the two alloys show a minimum and a maximum of the electric resistance, both dependent on concentration and annealing temperature. In contrast toAg, Fe theCu, Fe alloys with more than 3,5 at % show an additional minimum at temperatures between 10 and 1 K, which disappears if the samples are annealed at 380 K.     

Zur Temperaturabhängigkeit des elektrischen Widerstandes der Alkalimetalle bei tiefen Temperaturen

Investigations of the low temperature electrical resistance of the alkali metals, taking into account the actual spectrum of the lattice waves and the actual form of the wave functions to the best degree of approximation possible, are reported. It was found that for Na and K, where the approximation of free electrons is allowed, the umklapp processes augmented the temperature dependent electrical resistance only by few percents. In the case of Li, umklapp processes are more important, because of the fact that the wave functions differ from plane waves considerably. But in the whole temperature region the magnitude of the electrical resistance is determined by normal processes. Deviations from Matthiessen's rule caused by point defects are studied. In spite of the different refinements, the theory is not able to explain the experimental results in a satisfactory manner.     

Absorptions- und Fluoreszenzspektren des Systems KCl:Sm2+ bei tiefen Temperaturen

Sharp emission line spectra of Sm²⁺ in KCl single crystals are observed up to 40 °K. From these it is possible to verify the idea of a substitution of a regular cation by an impurity ion and of a simultaneous association of a cation vacancy in the nearest neighbourhood [1/2 (110)]. This means that the crystal field at the place of the bivalent Samarium has the site symmetryC _2v . A cubic symmetry does not explain the spectra. It is possible to identify the line groups with the transitions, which are permitted by reason of the selection rules, and to set up the energy level diagram of the Sm²⁺ ion in KCl. The separation of the single impurity-vacancy complexes was reached by quenching the crystals from 650 °C to room temperature. The assumption of excitation exchange between neighbouring Samarium ions explains the spectra of the non-quenched crystals. The number of emission lines is determined for two different orientations of two dipole complexes towards one another. An interpretation of the absorption spectra of Sm²⁺ in KCl is proposed.