Superconductivity and normal state resistivity of Ba0.6K0.4BiO3: an optical phonon approach

We discuss the nature of the pairing mechanism and the physical properties associated with the normal as well as the superconducting state of cubic perovskites Ba_0.6K_0.4BiO₃using the strong coupling theory. An interaction potential which includes the Coulomb, electron–optical phonon and electron–plasmon interactions is developed to elucidate the superconducting state. A model dielectric function is constructed with these interactions fulfilling thef-sum rule. The screening parameter (μ^* = 0.26) infers the poor screening of charge carriers. The electron–optical phonon strength (λ) estimated as 0.98 is consistent with an attractive electron–electron interaction and supports the moderate to strong coupling theory. The superconducting transition temperature of Ba_0.6K_0.4BiO₃is then estimated as 32 K. Ziman's formula of resistivity is employed to analyse and compare this with the temperature-dependent resistivity of a single crystal. The estimated contribution from the electron–optical phonon together with the residual resistivity clearly infers a difference when a comparison is made with experimental data. The subtracted data infer a quadratic temperature dependence in the temperature domain (30 ≤ T ≤ 200 K). The quadratic temperature dependence of ρ [ = ρ_exp − (ρ₀ + ρ_e–ph)] is understood in terms of 3D electron–electron inelastic scattering. The presence of these el–el and el–ph interactions allows a coherent interpretation of the physical properties. Analysis reveals that a moderate to strong coupling exists in the Ba_0.6K_0.4BiO₃system and the coupling of electrons with the high-energy optical phonons of the oxygen breathing mode will be a reason for superconductivity. The implications of the above analysis are discussed.     

Surface characterization of fluorinated polymers (PTFE, PVDF, PFA) for use in ultratrace analysis

Surface characterization of new unused PTFE, PFA and PVDF labware has been carried out by light microscopy, scanning electron microscopy, profilometry and atomic force microscopy. It has been found that in spite of higher micro-roughness, PFA exhibits the lowest nano-roughness and hence seems best suited as vessel material for relatively mild pressureless chemical operations for sample preparation of ultrapure substances, as container material for storage of ultrapure liquids, and for transport of such liquids, e.g., from the producer in the chemical to the end user in the microelectronics industry. This suitability refers only to the surface quality of the investigated materials. PTFE-surfaces, due to the sintering process of production, exhibit the most unfavourable surface quality of the investigated fluorinated polymers.     

Carbon thin films through laser ablation

Thin carbon films were deposited on silicon substrates at room temperature using a 0.355 μm Nd:YAG laser wavelength at low irradiance in the presence of argon gas. Various techniques including scanning electron microscopy, X-ray diffraction and Raman spectroscopy were used to analyze the film quality. The influence of the argon gas pressure on the properties of the films is demonstrated and a correlation with the optical emission data is presented.     

Ion Beam Driven Ion Cyclotron Double Layers

We present here a self consistent theory of small amplitude double layers associated with electrostatic ion cyclotron waves in a plasma containing hot electrons, cold ions and traversed by an ion beam. It has been shown that compressive type of double layers solution exists when θ_b (beam temperature) < α_b (beam concentration) < 1.     

Experimental study of regions of reversed magnetization in an amorphous layer of Co70Fe5Si10B15

The characteristics of the distribution of magnetization and remagnetization of microsections of the surface in the amorphous alloy Co₇₀Fe₅Si₁₀B₁₅, prepared in the form of thin ribbons, were studied by the magnetooptical method. It was found that the magnetization I on the surface of the amorphous ribbon studied was strongly nonuniform. It was established that heat treatment (HT) of the sample leads to the appearance of regions of reversed magnetization in the surface layer of the ribbon, while thermomagnetic annealing reduces the nonuniformity of I. The conservation of the transverse component in the HT sample in fields where the magnitude of the component of magnetization parallel to the length of the ribbon remains virtually unchanged suggested that a domain structure of the magnetization ripple type, owing to the strong microdispersion of the magnetic anisotropy, is realized in this sample.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 89–94, March, 1988.     

Verdampfen und Kondensieren binärer Gemische als Strömungsproblem

Zusammenfassung Die Grenze zwischen gasförmiger und flüssiger Phase eines binären Gemisches stellt beim Verdampfen oder Kondensieren eine Wärme- und Stoffquelle dar. Phasenumwandlungen von Gemischen führen folglich im allgemeinen auf Strömungsvorgänge mit Wärme- und Stoffübertragung. Die Grundgleichungen dafür werden sowohl in Massen- wie in Moldarstellung hergeleitet. Bei Filmkondensation werden im Falle des Gleichstromes von Dampf und Flüssigkeit neue Erkenntnisse gewonnen, womit ältere Versuchsergebnisse erklärt werden.

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Evaporation and condensation of binary mixtures as a flow problem

The boundary between the gaseous and liquid phases of a binary mixture is a source of both heat and mass, if evaporation or condensation takes place. Thus phase transitions of binary mixtures are related to flow processes with combined heat and mass transfer. The basic equations for this type of processes are derived in terms of mass and mole fractions, respectively. New results, which allow an interpretation of previous experimental data, are obtained for film condensation under cocurrent flow conditions.

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Formelzeichen a Bewegungsgeschwindigkeit vonF in Normalenrichtung - b Spaltbreite des Ringrohres - B Integrationsbereich - C Massenanteil - c _p spez. Wärme bei konst. Druck - D Diffusionskoeffizient - e spez. innere Energie - E Molenergie - F Begrenzungsfläche - g Schwerebeschleunigung - i spez. Enthalpie - I Molenthalpie - j Diffusionsstromdichte - K _Hy hydraulische Kennzahl - L Rohrlänge - Massenstromdichte - M Masse - Massenstrom - n Moldichte - Molstromdichte - n Einheitsvektor der Flächennormalen - N Mohlzahl - Molstrom - p Druck - q Wärmestrom - R Rohrradius - T Temperatur - t Zeit - mittlere Geschwindigkeit - u, v, w Geschwindigkeitskomponenten - v Rücklaufverhältnis - V Volumen - W Geschwindigkeitsvektor - x, y, z Koordinaten - X Molanteil - Filmdicke - Verdampfungswärme bei konst. Druck - Molmasse - kinematische Zähigkeit - Dichte - Stromfunktion Indizes a Ausströmung - d Dampfphase - e Einströmung - f flüssige Phase - i Phasengrenze - i=1, 2 Komponente - n Normalenrichtung     

Untersuchung von Einzelblasen an ölbeschichteten Oberflächen mit künstlichen Keimstellen

Zusammenfassung An feinpolierten, ölbeschichteten V2A-Oberflächen mit jeweils einer gebohrten (d=140 µm) und einer geätzten (d=47 µm) Vertiefung als Keimstelle wurde für Wasserblasen das Wachstum, der Abreißdurchmesser und die Abreißfrequenz gemessen. Für die wärmetransportgesteuerte Blasenbildung liefert der Vergleich mit Beziehungen, die für reine Metallflächen gewonnen wurden, gute Übereinstimmung, so daß keine Einfluß der Ölschicht erkennbar wurde.

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Investigation of single vapor bubbles on surfaces covered with a thin oil film with artificial nuclei

For single vapor bubbles, the growth rates, the detachment diameter, and the frequency were measured on single artificial nuclei, either drilled (d=140 µm) or etched (d=47 µm) into a highly polished stainless steel plate which was covered with a thin oil film. For evaporation controlled by heat transport, good agreement was observed with relations from literature which were obtained for clean surfaces; thus, an effect of the oil film could not be discovered.

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Formelzeichen a Temperaturleitfähigkeit - D _a Blasenabreißdurchmesser - f Blasenfrequenz - Ja Jakob-Zahl - Pr Prandtl-Zahl /a - q Wärmestromdichte - R Blasenradius - T Temperatur - dynamische Zähigkeit - Wärmeleitfähigkeit - Dichte - Oberflächenspannung - Zeit Indizes g Gas, Dampf - l flüssig - s Sättigung - w Wand Herrn Prof. Dr.-Ing. Karl Stephan zum 60. Geburtstag gewidmet     

Taylor expansion of effective Hamiltonians by Monte Carlo simulations with fixed block spins

Monte Carlo simulations with fixed block spins allow the computation of Taylor expansion kernels for effective Hamiltonians. The expansion can be performed around arbitrary block spin configurations \(\bar \phi\) and does not suffer from truncation errors. Monte Carlo calculation of the Taylor kernels offers a ready possibility to check whether the effective Hamiltonian has good locality properties in the neighborhood of a given configuration \(\bar \phi\) . The method is applied in a renormalization group study of the 2-dimensional critical Ising model. The results show that one has to deal with a “large field problem”, as had been expected from rigorous renormalization group studies.