Amorphous carbon films deposited by electron beam evaporation onto glass and oxidized Si(111) substrates have been investigated by local probe methods, spectroscopic techniques and electrical measurements. Clusters ∼ 15 Å in diameter are visible in the topography imaged by scanning tunneling microscopy. The microscopic potential distribution mapped by scanning tunneling polcntiomciry is characterized by a disturbed lateral potential drop in the submicrometcr scale. X-ray diffraction analysis reveals the amorphous structure of the evaporated film. In the Raman spectrum two broad lines at 1585 cm−1 and at 1375 cm−1 are observed. We obtained an optical absorption coefficient α greater than 105 cm−1, and a small optical gap of about 0.2 eV. The electrical resistance at low temperatures can be fitted to the form R α exp(T0/T)½. Our experimental results indicate a structure consisting of very small graphite (sp2) clusters with an incomplete trigonally atomic arrangement. The changed bonding configuration between the clusters may include sp3 sites.

The relation between σ and dσ/dT is studied at room-temperature. The experimental results confirm a phenomenological model developed mainly on the basis of low temperature measurements in two preceding papers. In particular, all samples investigated, which show activated conduction at low temperatures, fulfil σ(293 K) < 360Ω−1 cm−1, whereas all samples with metallic conduction at low temperatures fulfil σ(293 K) > 440Ω−1 cm−1. Within the regions 100,..., 360Ω−1 cm−1 and 440,..., 1400Ω−1 cm−1 nearly linear relations between σ and dσ/dT are observed. It seems likely that dσ/dT as a function of σ exhibits a knee at about 420Ω−1 cm−1 indicating the presence of sharp metal-semiconductor transition at room-temperature.

AbstractThe relation between σ and dσ/dT is studied at room‐temperature. All samples investigated, which show activated conduction at low temperatures, fulfil σ(293 K) < 360 Ω−1 cm−1, whereas all samples with metallic conduction at low temperatures fulfil σ(293 K) > 440 Ω−1 cm−1. Within the regions 100 to 360 Ω−1 cm−1 and 440 to 1400 Ω−1 cm−1 nearly linear relations between σ and dσ/dT are observed. It seems likely that dσ/dT as a function of σ exhibits a knee at about 420 Ω−1 cm−1 indicating the presence of a sharp metal‐semiconductor transition at room‐temperature. The experimental results confirm quantitatively a phenomenological model of σ(T, x) in the semiconducting region, which is developed mainly on the basis of low temperature measurements in two preceding papers. On the other hand, doubts arise on the physical meaning of the ansatz α + βT1/2 + γT often used to analyse the metallic region at low T.

AbstractThe theoretical part of this paper shows that, by means of a suitable chosen predictor, only one Newton iteration step is needed in order to get the desired consistency and stability properties of an implicit numerical integration method. Furthermore, an algorithm for this Newton step is suggested, whose effort depends only on the number of the stiff solution components. In the practical considerations the introduced principles are applied to a mixed extrapolation method based on the trapezoidal rule. Special cheap methods for the error estimation and stepsize control are developed.

A computer program for testing coil and protection design of a composed magnet system is presented. Small high field magnets consisting of two uniaxial cylindrical coils of different superconducting materials (eg NbTi and V3Ga or Nb3Sn) are considered, each coil may be subdivided into several sections which are protected by parallel resistors. Quench propagation due to thermal conduction and also due to rapid current increase (important in inductively coupled systems) is taken into account by means of solving a one-dimensional thermodiffusion equation. Field and temperature dependence of the critical current for every layer of the coils, is taken into account. The program calculates the time dependence of currents, external and internal voltages, resistances of the sections and of the radial temperature distribution. Calculations are compared with experimental investigations of three different systems, the results agree with experimental values concerning current decay and propagation velocity.

In recent years the manufacturing of Nb3Sn or V3Ga composite wires by the bronze process has been mastered. Nb3Sn was investigated more intensively than V3Ga [1,2] although in high fields V3Ga is expected to be superior [3]. Accordingly V3Ga tape was used in a 17.5 Tesla magnet [4] and multifilamentary V3Ga conductor in a 10 Tesla magnet [5]. In this paper we describe an 11 Tesla V3Ga-NbTi magnet ...

A computer program for testing coil and protection design of a composed magnet system is presented. We consider small high field magnets composed of two uniaxial cylindrical coils of different superconducting materials (e.g. NbTi and V 3 Ga or Nb 3 Sn). Each coil consists of several sections protected by shunts. The whole system is driven by one power supply. In order to take into account quench propagation due to thermal conduction and also that due to rapid current increase which is important in inductively coupled systems a one-dimensional thermodiffusion equation is solved. Field and temperature dependence of the critical current of every layer of the magnet system are regarded. Current sharing as well as the temperature dependence of specific heat, of resistivity and of thermal conductivity are taken into account. The program calculates the time dependence of currents, of external and internal voltages, of resistances of the sections and of the radial temperature distribution. Calculations for an 11 T V 3 Ga-NbTi magnet system agree well with experimental results. The influence of a subdivision of a magnet into several sections protected by shunts is discussed.

The d.c. electrical resistivity is considered for a general solid state model characterized by local interactions of the charge carriers with a system of scatterers which may be e.g. quasiparticles, spins or impurities. A basic relationship - valid over the full temperature range - between the electrical resistivity of the charge carrier system (without magnetic field) and the generalized dynamical susceptibility of the scattering system is derived in the weak scattering limit. The equivalence of the following methods is shown: Boltzmann equation approach (variation procedure), Mori formalism, force-force correlation function method (for the total forces on the charge carriers), and the method of the transformed Liouville equation; herewith the equivalence with the Kubo or Green function method, the Zubarev formalism, the information-theoretical method and the Zwanzig formalism is also given. As a specific example, the s-d exchange model for the scattering of conduction electrons by localized spins is discussed including the corresponding electron-magnon system as a low-temperature approximation.

AbstractThe electrical resistivity of a disordered ferromagnetic alloy is estimated in terms of the two‐current model. Besides the residual resistivity due to spin dependent potential scattering a spin‐disorder residual resistivity is obtained. A significantT3/2term associated with incoherent electron magnon collisions and representing large deviations from Matthiessen's rule dominates the temperature dependence of the resistivity below about 10 K. Deviations from Matthiessen's rule arising from the two‐current conduction are discussed.

AbstractThe electrical resistivity of iron is measured between 2 and 30 K. The results can be extremely well explained by fitting the data to a simple two‐current model including electron–magnon, electron–phonon, and electron–impurity scattering. Below 10 K large deviations from Matthiessen's rule are observed arising from the two‐current character of the conduction.

Within the framework of general statistical mechanics of irreversible processes the electrical resistivity of an open electron-phonon system is calculated. By means of the projection operator technique an evolution equation for coupled subsystems in a heat bath is derived and specialized for electrons and longitudinal phonons, the latter being coupled to a bath of transverse phonons. The influence of heating of the electron-phonon system is investigated and the question of validity of the linear response theory with or without inclusion of a dissipative mechanism is discussed. In the balance equation for the total electron momentum, terms of only third (and higher) order in the electrical field strength and the current density appear; consequently, the transverse phonons act only as a “momentum bath”. A general resistivity formula is derived containing the Bloch-Grüneisen law as a special case and including corrections due to phonon drag up to infinite order without a partial summation.