Phase diagram of a model for diluted magnetic semiconductors beyond mean-field approximations

A lattice spin-fermion model for diluted magnetic semiconductors (DMS) is investigated numerically, improving on previously used mean-field approximations. Curie temperatures are obtained varying the Mn spin x and hole n densities, and the impurity-hole exchange J in units of the hop-ping t. Optimal values are found in the subtle intermediate regime between itinerant and localized carriers. At intermediate and large J/t, a "clustered" state is observed above the Curie temperature and ferromagnetism is suppressed. Formal analogies between DMS and manganites are discussed.     

Non-ohmic behaviour of iodine-doped polyacetylene

We report on the temperature and electric field strength dependence of the conductivity of iodine-doped polyacetylene in the metallic regime. The experimental results are discussed in terms of Sheng model of fluctuation induced tunneling and characteristic dimensions of the tunneling zones between fibres are deduced from a fit between this theory and our experiments.     

Analysis of nitrogen-influenced surface near zones of ferrous materials by glow discharge spectroscopy (GDOS)

Glow discharge spectroscopy (GDOS) will be shown to be a quick, informative and simple method for quantitative depth profile analysis of elements of nitrided layers well suited for their quality control. By systematic variation of all glow discharge determining parameters it is possible to get an excellent depth resolution in the order of sub-m corresponding to a comparatively large analytical activated area (50 mm²). In this paper the behaviour of a number of important parameters related to sputtering of the activated area will be discussed. Some quantitative GDOS depth profiles of carbon and nitrogen of pure iron samples nitrided by different procedures will be shown as examples for application.     

Wigner crystallization in Na3Cu2O4 and Na8Cu5O10 chain compounds

We report the synthesis of novel edge-sharing chain systems Na(3)Cu(2)O(4) and Na(8)Cu(5)O(10), which form insulating states with commensurate charge order. We identify these systems as one-dimensional Wigner lattices, where the charge order is determined by long-range Coulomb interaction and the number of holes in the d shell of Cu. Our interpretation is supported by x-ray structure data as well as by an analysis of magnetic susceptibility and specific heat data. Remarkably, due to large second neighbor Cu-Cu hopping, these systems allow for a distinction between the (classical) Wigner lattice and the 4k(F) charge-density wave of quantum mechanical origin.     

Kinetic roughening of laser deposited polymer films: crossover from single particle character to continuous growth

The crossover in kinetic roughening of thin films from a particle-character-dominated regime to continuous growth behavior has been observed in this work. This has been accomplished by atomic force microscopy investigations of pulsed laser deposited amorphous organic films with thicknesses ranging from several nanometers to more than 4 microm. The early-stage random-deposition-like processes end once a closed layer is formed, which grows without saturation on the characteristic length scales. In addition, the influence of oblique film deposition has been examined and interpreted.     

Resistivity of Mixed-Phase Manganites

The resistivity rho(dc) of manganites is studied using a random resistor-network, based on phase separation between metallic and insulating domains. When percolation occurs, both as chemical composition or temperature vary, results in good agreement with experiments are obtained. Similar conclusions are reached using quantum calculations and microscopic considerations. Above the Curie temperature, it is argued that ferromagnetic clusters should exist in Mn oxides. Small magnetic fields induce large rho(dc) changes and a bad-metal state with (disconnected) insulating domains.