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.     

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.     

Deduction of generalised Kirchhoff's Laws from the basic principles of electromagnetism

The deduction of the so well-renowned and established Laws of Kirchhoff relating to the currents flowing in a network, which are considered to be almost axiomatic in electrical engineering sciences, seems to be preposterous and non-sensical at first sight. However, on a closer examination it will appear that these laws are based on two principles, viz, the steady-state condition under which accumulation of free electrical charge is precluded, and, the experimentalOhm's Law, which propounds a linear relation between the difference of potential across a conductor and the total current flowing through the same. But, there are two things, which are normally overlooked. The conductors inKirchhoff's Laws are “wires”, which are one-dimensional lines, and theOhm's Law is the macroscopicOhm's Law for total currents applied to these lumped resistors. Strictly speaking, the “Laws” are to be deduced from the more fundamental electromagnetic equations for continuous media and the microscopicOhm's Law. It is to be noted that theKirchhoff Nodal Law is but a consequence of the steady state condition derivable from these basic equations, viz, the current-density is divergence-free. For a continuous medium, what the form of theKirchhoff's Laws will be, is difficult to guess unless deduced from the basic equations. Once these are established from the basic equations of electromagnetism for a continuous medium, the usual form ofKirchhoff's Laws will follow as corollary of the general case, as has been shown here.     

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.     

Investigation of molar sound velocity in melts and solutions

Ultrasonic velocity studies are carried out in organic solutions of six low melting point organic solids and the melts of these solids. The constancy ofR is established over a wide range of temperature in the melt state. It is found thatR is strictly linear with concentration expressed in mole fraction of the solute and the average extrapolated value for 100% concentration of the solute is compared with that from the melt state and also with the theoretical computed values due toRao's atomic increment method andLagemann's bond increment method.     

Spezifische Wärme im Cr-Re-System Antiferromagnetismus und Supraleitung

The investigation of the specific heat at low temperatures, superconductivity and electrical resistivity in the Cr-Re system within the cubic phase shows a relatively sharp transition from an antiferromagnetic to a superconducting region. The results can fully be accounted for by theOverhauser spin-density wave (SDW) theory. On the other hand, the superconducting transition temperature varies in the manner predicted by the theory ofBardeen, Cooper andSchrieffer (BCS). Both phenomena show an evident d-character. A detailed analysis gives strong indication that, although possible, a coexistence of antiferromagnetism and superconductivity in Cr-Re is rather improbable.     

Paramagnetische Resonanz von Zentren mit den Symmetrienn,\bar n undn/mundn/m

The consequences of applying the spin-Hamiltonians ofAbragam andPryce to paramagnetic centers with the low symmetriesn,\(\bar n\) andn/m(C _n ,C _nh ,S _n ) are examined. The asymmetry of theg- and the hfs-tensors, described in an earlier paper, is considered in this context. The point is that for the symmetries under consideration the coordinate systems are not determined by the symmetry elements. For this reason it is possible to introduce separate coordinate systems for the magnetic field, the electron spin and the spins of the nuclei and this allows the symmetrisation of the tensors under certain conditions. This procedure also leeds to an understanding of the independent parameters found in the Hamiltonians. Ambiguities between the spin-Hamiltonians and the esr-spectra indicate the limits of theAbragam andPryce formalism. The application of the theory to paramagnetic centers under the influence of external, electrical fields is discussed and reveals interesting aspects.     

Die Polarisation der von den Betastrahlen des P32 erzeugten Čerenkov-Strahlung

The polarization of?erenkov radiation produced in Plexiglas by theβ-rays of P³² has been investigated by counting coincidences between fast electrons having traversed a thin Plexiglas plate and the light quanta emitted at an angle of 45° with respect to the track of the particles. The radiation was analyzed by a polarizing filter and was found to be linearly polarized, the degree of polarization being at least 87%. The remaining 13% of the observed intensity originate at least partly from weak scintillations which are excited by theβ-rays in Plexiglas and the intensity of which could not be measured exactly. With respect to this, the result is in agreement with the virtually complete linear polarization of?erenkov radiation predicted by theory.     

Der Einfluß von Gitterfehlern auf den elektrischen Widerstand und die Thermokraft von Metallen

Starting from theGreen's function formulation of the scattering of crystal electrons by a localized perturbation, the transition probability is calculated. It is shown that the kernel of the transport equation degenerates if the scattering potential is characterized by its Wannier function matrix elements. In this way a solution in closed form of the transport problem can be obtained for arbitrary anisotropic Fermi surfaces and anisotropic scatterers. The general solution is applied to the calculation of the effect of point defects on the electrical resistivity and the thermoelectric power of metals at low and high temperatures.     

The Hall coefficient of the small polaron

The Hall coefficient of the small polaron in theHolstein model suitably generalized to three dimensions and to the presence of a magnetic field is calculated by means of the Kubo formula. A perturbation expansion of the correlation function appearing in the Kubo formula is used to introduce higher order jumps of the polaron which are the first to be affected by the magnetic field. A general expression for the Hall coefficient is obtained which essentially coincides with the result ofFriedman andHolstein obtained for special cases. Moreover, it is shown that interference processes of thermal activation and free motion of the small polaron change the Hall effect, and their contribution to the value of the Hall coefficient is calculated.     

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.     

Einfluß der Pion-Pion-Wechselwirkung auf Pion-Nukleon-Streuphasen

The isospin dependent part of theS-wave pion-nucleon amplitude is calculated according to the CGLN approach, but recoil and projection on the partial waves are treated exactly. Then the difference against the experimental data is approximately a constant up to 100 MeV and amounts to 1/3 of the total value. It is shown, that at low energies the main corrections of the CGLN equations are taken into account by a subtraction of theπ+π→?+\(\overline \Re \),J=I=1 amplitudesf _± ¹ (t) as proposed byMandelstam andFrazer. Calculations have been done forS, P andD pion-nucleon amplitudes. The main result is a prediction of theS-wave amplitude obtained without adjusting free parameters up to 100 MeV. The relations of this method to the equations derived byBowcock, Cottingham andLurié according to the Cini-Fubini approach are discussed.     

L/K-Einfangverhältnis von65Zn

A multiwire proportional counter was constructed to measureL/K-capture ratios. The length of the counter is 90 cm, the diameter 14 cm, and it works with pressures from 1 to 6 atm. — The counter was proved by measuring theL/K-capture ratio of³⁷Ar. The result obtained is 0,098±0,003, in agreement with the calculated ratio 0,100 ofBahcall and the results of many other measurements with multiwire proportional counters. — Gaseous radioactiv Zincdiethyl was the substance to determine theL/K-capture ratio of⁶⁵Zn. The result obtained is 0,111±0,006, in agreement with the calculated ratio 0,109 ofBahcall, while 0,119±0,007 measured with a multiwire proportional counter bySantos Ocampo andConway is somewhat above the calculated ratio.     

Messung der komplexen Dielektrizitätskonstanten von Seignette-Salz bei 10 GHz in Abhängigkeit von der Temperatur

We determined the complex dielectric constant of Rochelle Salt at 9.61·10⁹ Hz and its temperature dependence using a microwave interferometer. In thex-direction of the crystal which becomes its ferroelectric axis between ?18°C and +24°C, the real part of the dielectric constant shows distinct minima at the transition points according to our measurements. We find agreement with results ofJäckle but discrepancies to older measurements. Neither the humidity of the surrounding atmosphere nor the position of the sample in the growing crystal proved to have an effect. In they- andz-directions, we observed no anomaly of the dielectric constant.     

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.     

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\(\overline {\Delta v}\).     

Zur Thermolumineszenz von NaCl/Ag

Angular and energy dependence of the polarizationP of slow electrons scattered elastically by a mercury-atom beam have been measured. The angular resolution being comparatively high (about ±1°), the results could be compared directly with uncorrected theoretical values and high polarizations (up to ¦P¦=0.85±0.05) could be observed. The polarization was measured by a Mott-detector, the electrons scattered by the mercury atoms having been accelerated to 120 keV. The high yield of this detector made up for the rather low intensity of the observed beam caused by the high angular resolution and resulted in low statistical errors. The experimental results, which cover the angular range from 30° to 150° and energies from 180eV to 1700eV are in good agreement with the theoretical values ofHolzwarth andMeister as well as with those ofBunyan andSchonfelder.     

Messung der Fehlordnungsenergie an abschreckend kondensierten Metallschichten bei tiefer Temperatur

Metal films are produced with a high degree of disorder by quenching condensation. The stored energy release and the resistivity decrease are measured during annealing of Au-, Pb-, and Sn-films from 20 to 60° K and during crystallization of amorphous Sn-films with a 10 atomic-% Cu-content. The results are compared with earlier measurements on Bi-, Ga-, and Cu-films produced by quenching condensation. The energy released during the crystallization of the amorphous films (Bi, Ga, Sn+10 at.-% Cu) amounts to half of the heat of melting. This is in good agreement with calculations ofOriani andLumsden for the disorder part of the entropy of melting. The recovery of the crystalline Cu-, Au-, Pb-, and Sn-films is described by a crystallization of highly disordered atoms to crystallites formed during condensation. About 30% of all atoms of these films are in this highly disordered state.     

Theory of superconducting contacts

Metallic contacts between superconducting and normal films are known to have a pair-breaking effect on the electron pairs. By limiting our considerations to the gapless regime and to contacts between superconducting and paramagnetic metals the features of this pair-breaking mechanism are investigated. We compute the normalization of the order parameter and with it such quantities as the tunneling conductance, electrical and thermal conductivity and the critical current parallel to the contact. Furthermore we investigate the influence of a magnetic field. By calculating the? ₂(t)-parameter we demonstrate the interplay between the different pair-breaking mechanisms in perpendicular and parallel fields. In the latter case we discuss the influence of the contact on the superconducting sheath proposed bySaint-James andde Gennes. Experiments to test the present theory are suggested.     

Dispersion of the stress optic coefficient of the alkali halides

The dispersion of the stress optic coefficient C=n³/2 (q₁₁?q₁₂) of the alkali halides, NaCl, KCl, KBr and KI have been measured from the visible to the ultraviolet region. In general the value of “C” decreases with wavelength for all crystals. While the dispersion is only a few per cent in the visible region of wavelengths, it is enormous in the ultraviolet. NaCl shows a dispersion of about 100% from 5800 to 2400 Å; KCl about 200% from 5000 to 2400 Å; KBr about 300% from 5000 to 2400 Å; and KI about 400% from 5000 to 2800 Å. Also the potassium halides exhibit a change in sign of their “C” values in the ultraviolet. In KCl the sign reversal occurs at about 2550 Å; in KBr at 2760 Å and in KI at 3380 Å. Below these wavelengths, the potassium halides belong to the same class inMueller's classification as sodium chloride. The theory ofRamaseshan andSivaramakrishnan based on the assumption that a stress causes a change in the frequencies and oscillator strengths of atoms is unable to explain the observed behaviour of the alkali halides. On the other hand, the mere variation of the ionic refractivities with wavelength is also unable to explain the observed dispersion onMueller's theory. One is forced to assume that the strain polarisability constantK inMueller's theory varies with wavelength. When “K” is calculated from the experimentally observed values of “C”, it is found to increase with decreasing wavelength for all alkali halides. The variation with wavelength of “K” for all the alkali halides can be fitted up well by a formula of the type given byRamaseshan andSivaramakrishnan. Hence it appears that the total dispersion ofC can be explained only when we take into account the variation with wavelength of 1. theLorentz andCoulomb contributions fromMueller's theory and 2. the strain polarisability constant fromRamaseshan andSivaramakrishnan's theory.