Die Übergangstemperatur der Supraleitung verschiedener Metalle unter dem Einfluß sehr dünner Oxydschichten

The superconducting transition of thin films of Al, In, Tl, Pb, Sn, and Ga⁺ is investigated by resistance measurements before and after the low temperature oxidation of the surface. The films are condensed onto a crystalline quartz plate to a thickness of about 100 Å or less at a temperature of mostly 105 °K. The oxidation process below 40 °K was described in a previous paper. After a theory ofMott, it may be understood by the tunneling of electrons from the metal to some acceptor levels of the oxygen molecules at the outside of the oxide layer. This way, a strong electric field arises which, on the one hand, facilitates the motion of metal ions outwards to form oxide molecules and, on the other hand, is suspected to affect the transition temperature. The experimental results show a shift of the transition to higher temperatures with the metals Al, In, and Tl, however, with the metals Sn, Pb, and Ga⁺ to lower temperatures. These results agree qualitatively very well with direct measurements of the influence of surface charges on the superconducting transition of Sn and In byGlover andSherrill. TheT _c -shift depends clearly upon the film thickness and amounts to about 0.3 to 0.4 °K for the thinnest films used. Some experiments are undertaken in order to prove the existence of the electric field. Removing the residual oxygen molecules on top of the oxide layer destroys the acceptor levels and causes a reduction of the transition temperature to the value observed with the oxygen free film. As expected, the thickness of the oxide layer is not important for the magnitude of theT _c shift.     

Zur Theorie der Übergangstemperaturen von supraleitenden Doppelschichten (Proximity-Effekt)

A theory of the transition temperature of superconducting, superposed films is given by generalizing the methods ofde Gennes andWerthamer. It is shown that the transition temperature of nonhomogeneous superconductors obeys a general rule, which is formally of the same kind as the corresponding formula for homogeneous superconductors and is valid for all mean free paths. The results ofWerthamer follow analytically in the dirty limit if the transition temperatures of the superposed films are nearly equal, but there is numerical agreement for almost all superconductor-superconductor films. In the thin film limit and for normalconductor-superconductor films there are essential deviations. The theory is in good accord with the experimental data. By calculating the unknown interaction constant of copper from the experimental transition temperatures of superposed Cu-Pb films one gets for copper a transition temperature of the order of 10^?2°K.     

Zur Änderung der übergangstemperatur verschiedener Supraleiter durch Elektronenentzug

The growing of oxid layers on pure metal films at low temperatures may be explained by acceptor levels at the oxid-oxygen interface. These surface states produce an electrical field of high strength by extracting about 10¹⁵ electrons per cm² out of the metal film. As had been shown in earlier experiments, this capturing process shifts the superconducting transition temperature to higher values for the 3-valent metals Al, In and Tl and to lower values for the 4-valent metals Sn and Pb. Extended studies on Tl show that this change in transition temperature is proportional to the relative decrease of the electron density in the metal film. The constant of proportionality has the valuek _Tl=?4.4 deg. With reasonable assumptions, this constant could be determined for Sn too:k _Sn=7.9 deg. These values agree quantitatively with the results ofGlover andSherill who investigated the charge effect on superconductors by a condenser method. They also fit the data calculated byGinsberg from experimental results on superconducting compounds with his extension of the theory ofMarkowitz andKadanoff.     

Die Temperaturabhängigkeit des magnetischen Barkhausen-Effekts. II

The size distribution of Barkhausen discontinuities was measured during “thermal idealisation” of a hard drawn nickel specimen. Thermal idealisation means the magnetization by increasing temperature in a constant magnetic field. The sensitivity of the apparatus covered a range of magnetic moment between 1.3 and 17.3·10^?6 e.m.u. The results are in good agreement with investigations ofGerlach andTemesváry on the irreversible part of thermal idealisation. The average volume of Barkhausen discontinuities of the measured size was found to be nearly independent of temperature in a range from ?150 to+200°C, while above 200°C the volume increases markedly with temperature. By means of the average Barkhausen volume the temperature dependence ofNéels thermal fluctuation field was calculated. The results are in good agreement with those ofPescetti andBarbier on the temperature dependence of magnetic after effect.     

Der Übergang zur Supraleitung in Sinterproben des Systems Nb-Sn

For samples of the system Nb-Sn sintered between 700 and 1500°C with 20 and 25 percent Sn the inductive and resistive transition to superconductivity has been measured. Especially the influence of magnetic fields on the transition has been examined. The inductive transition is deduced from shielding currents, the anomalous rise of the transition temperature for the 4Nb∶1Sn-samples first found byJansen turns out to be a consequence of higher critical currents of these samples. At low sintering temperatures a rise of the critical fields has been observed.     

Konversion der 238,6 keV-γ-Linie des ThB und Vergleich mit den Tabellen von Sliv und von Rose

The conversion ratiosK/L _I andK/L _II of the 238.6 keVγ transition of ThB have been measured with the new Heidelbergπ/2 √13Β-ray spectrometer. For this transition there are great discrepancies between the values as given bySliv andBand, and byRose. The experimental resultsK/L _I=5.96±0.26 andK/L _II=60.3±7.1 are in excellent agreement with the values ofSliv andBand.     

Direct determination of the transition energy of CsCl at 470°C

The energy of the transition I?II of CsCl at 470°C was measured by using a direct differential calorimetric method. In the present work this method was applied for the first time to investigate a transition above 220°C. Several modifications in the construction of the original calorimeter were necessary. The result 707±20 cal/mole was obtained. This is in good agreement with the earlier result 690±28 cal/mole ofPöyhönen andMansikka [3], determined by an indirect method.     

Die Temperaturabhängigkeit des Bandabstandes von Bleiselenid,gemessen an photoleitenden Schichten

The temperature dependence of spectral distribution of photoconductivity was measured on evaporated polycrystalline layers of lead-selenide in the range from 80 to 300 °K. The method ofBardeen, Blatt andHall was used, to calculate the band gap for direct and indirect transitions. A linear positive temperature coefficient was obtained for both transitions. The values areβ _dir=+(4.5±0.2) · 10^?4 eV/°K andβ _ind=+(3.0±0.2)· 10^?4eV/°K.     

Sur les propriétés magnétiques de l'adénosine triphosphate (ATP) et de ses constituants

A detailed thermomagnetic study (80°–380°K) has been made on polycrystalline adenosine triphosphate (disodium salt), hydrated, dried, compressed and deeply cooled. The susceptibility-temperature curves exhibit around 180°K an abrupt transition from the Curie paramagnetismχ?χ ₀=CT ^?1,C=1.37·10^?5 cm³ g^?1°K, to a state, which most frequently is a temperature independent paramagnetism of the order of 10^?7 cm³ g^?1 (extremely weak ESR absorption at room temperature according toIsenberg andSzent-Györgyi). The Curie paramagnetism might be related to the acceptor-donor process of charge transfer imagined bySzent-Györgyi to explain the energetic comportment of ATP. Extensive thermomagnetic study of adenosine, KH₂PO₄ and KD₂PO₄ was made. Using a simple model based on these data we have constructed susceptibility-temperature curves for ATP di-Na which approximate the real behaviour fairly well.     

Erzeugung und Ausheilung von Gitterstörungen und ihr Einfluß auf die Supraleitung von Indium und Thallium

The Variation of the transition temperature with residual resistance of coldworked In and Tl shows a similar behaviour as in dilute alloy experiments. Subsequent annealing removes lattice defects in distinct steps and restores the properties of pure samples at temperatures below 250 °K. Some of the results can be interpreted in accordance with the model developed byMarkowitz andKadanoff on alloy experiments. Values for the anisotropy parameter 〈a ²〉_In=0.02 and 〈a ²〉_Tl=0.04 were obtained. The interpretation of the “valence” effect and the influence of stress is discussed.     

Eine Temperaturabhängigkeit der Feldelektronenemission bei sehr geringen Stromdichten

Low current density field electron emission between 1 and 10^?6 A/cm² was measured for clean tungsten as a function of temperature between 300 and 1350° K. A strong increase in temperature dependence was found with decreasing current density. Measurements are interpreted according to calculations ofMurphy, Good andMüller. Agreement is better if the work function temperature dependence is also considered.     

Calculated rates for the reactionμ −+He3→H3+ν

The effective Hamiltonian ofMorita andFujii is used to calculate the muon capture rate\(\mathfrak{W}_\mu \) in He³. The calculation is done including and omitting the “weak magnetic interaction” and for positive and negative sign of pseudoscalar coupling. The results are related to the known value offt for the beta decay of the triton. The r.m.s. radius of the charge distribution in He³ is taken to be 1·55 fm rsp. 1·85fm. (The first value follows from the results of the variational calculation ofKikuta, Morita, andYamada, the last value is based on a variational calculation for a potential with a long tailed attractive part.) The capture rates differ by 10% for the two r.m.s. radii. Our result forv _r.m.s.= 1·55 fm deviates slightly from the corresponding result ofWerntz.     

Ein neuer Variationsansatz für das Polaron

A new trial function for the self-energy of the polaron is presented which is a combination of the weak and strong coupling methods ofLee,Low andPines resp.Pekar. The result for the ground-state energy is a smooth interpolation between both methods which is better than either one but not as good asFeynman's result.     

Supraleitende Bleischichten mit Zusatz von Übergangsmetallen und ihren Oxiden

The influence of small additions of 3d-metals (Cr, Fe, Co) on the superconducting transition temperature of lead has been studied. Both components are condensed simultaneously on a quartz substrate, held at 10 °K. With this “Quench evaporation technique” we get a statistical distribution of the impurity atoms in the lead matrix. As in early experiments on In and Sn the superconducting transition temperature of Pb decreases linearly with increasing 3d-metal content. This is in agreement with the theoretical results ofAbrikosov-Gorkov andSkalski et al. For the first time an influence of 3d-metal oxides (Cr-, Mn- and Co-oxide) on the superconductivity of Pb has been found. The transition temperatureT _c decreases linearly with increasing oxide content (c) as in the case of the pure metals. For Mn and Co the slopesdT _c /dc are nearly equal for the pure metal and its oxide. In the case of Cr the influence of the oxide is about seven times greater than that of the pure metal. Furtheron it is shown by annealing experiments that the degree of precipitation has also an influence on the transition temperature in the system of lead with Fe, Co and CO₂O₃.     

Zur Supraleitung von Doppelschichten aus Ag/Sn

Superimposed films of Ag and Sn are produced on suitable conditions of condensation, so that diffusion between the layers is prevented (the condensation temperatures of Ag and Sn are 320 ?K and 150 ?K, respectively). Immediately after condensation the films are cooled to liquid helium temperature, and the transition temperatures of superconductivity are measured. These values are in agreement with a phenomenological theory of P. and R.Hilsch for sufficient thick films. Resistance measurements indicate that an alloy of Ag and Sn is formed above 280 ?K. There is only a small shift of transition temperature caused by the alloying process.     

Direct determination of the transition energy of NH4I at the transition I⇆II

The energy and temperature of the transition I?II of NH₄I were measured by using a direct differential calorimetric method. This method was now for the first time applied to a transition below room temperature. The results 809 cal/mole for the transition energy and the temperature interval from ?17.4°C to ?14.4°C for the transition temperature were obtained. The value of the transition energy is somewhat greater than the earlier result ofBridgman determined by means of an indirect method.     

The dielectric behaviour of acetaldehyde vapour at 9000 MC/sec.

Complex dielectric constant of the acetaldehyde vapour and its dependence on pressure and temperature has been studied in the microwave region at 9000 MC/sec. The electric susceptibility is found to vary linearly with pressure in the moderate pressure region. The absorption coefficient varies approximately as P² showing convexity towards pressure axis. The temperature dependence law as obtained by absorption coefficient measurements at temperatures between ?8° to 50° C follows T^?3 law as expected onAnderson's orMargenau's expressions for\(\overline {\Delta v}\).     

Optische Bestimmung der Nichtparabolizität des dritten Valenzbandes von Germanium

Measurements of the optical absorption of free holes in germanium produced by direct intraband transitions in the valence band have been made at temperatures of 300–390 °K. The experimental method of the photoabsorption was used which enables the absorption spectra to be determined at intrinsic material without an influence of the absorption edge due to interband transitions between the valence band and the conduction band. An analysis of the spectra gives an experimental determination of the shape of the non parabolic split-off valence band. By comparison withKane's theory a marked difference is found but good agreement with calculations fromFawcett is achieved. The temperature dependence of the shape of the split-off band is discussed concerning the optical absorption measurements on “hot holes” in high electric fields with respect of the non-equilibrium energy distribution of the carriers.     

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.     

Note on the significance of the Aharonov-Bohm effect

The fringe shift first predicted byEhrenberg andSiday and subsequently more fully discussed byAharonov andBohm has been consistently explained in terms of a classical interaction energy arising from overlap of the electron's and solenoid's magnetic fields.Kasper criticizes this theory on the ground that “skin effect” would prevent penetration of the electron's magnetic field into the interior of the solenoid. This criticism is shown to be invalid.