Upper Bounds for a Ramsey Theorem for Trees

For natural numbers n, k, r and s there exists a complete tree ? of height and arity $r^{\left( {\mathop {K + 1}\limits_s } \right)} (n - 1) + 1$ such that for an arbitrary r-colouring of the s-chains of T a level-preserving monochromatically embedded copy of a complete tree of height k and arity n can be found in T. Moreover, a best possible upper bound for K is given.     

AES-depth-profiling of thin annealed Pt-films on Si(100)

Thin PtSi films can be grown by evaporating Pt on Si(100) at RT and subsequent annealing of the system at 600–700 K. Contaminants like oxygen are known to have a strong influence on this reaction. In the present study we concentrate on the effect of oxygen partial pressure during the annealing on the silicide growth process. Under proper vacuum conditions annealing at 500 K leads to a homogeneous Pt₂Si film which reacts around 600 K completely to PtSi. A substantial oxygen partial pressure ( 0.1 mbar) in contrast results in an incomplete reaction in the same temperature range: unreacted platinum remains at the surface separated from the silicide by an oxygen enriched layer.Presented at the Seminar on Secondary Electrons in Electron Spectroscopy, Microscopy, and Microanalysis, Chlum (The Czech Republic), 21–24 September, 1993.This work was supported by Deutsche Forschungsgemeinschaft (DFG) through Sonderforschungsbereich 292.We thank Dr. W. Platz (Deutsche Aerospace AG, Ottobrunn) for providing us with Pt evaporated Si-wafers and Th. Hierl (Lehrstuhl für Angewandte Physik, University of Erlangen-Nürnberg), who performed the RBS measurements for AES calibration.     

μSR study of UTr2Si2, Tr=Co,Rh OR Pt

We have studied bySR spectroscopy the intermetallicsUTr ₂Si₂ where Tr=Co, Rh or Pt. Whereas for the Co compound we only get information on the muon localisation site, from the other two materials we obtain information on their electronic properties. We compare the characteristics of theirSR response.     

Spin correlations in amorphous Bi2Sr2CaCu2Ox

Zero and longitudinal field SR has been used to probe the magnetic state of amorphous Bi₂Sr₂CaCu₂O_x prepared by rapid quenching from the melt. The results suggest quasi-static spin correlations between extremely dilute Cu²⁺ ions.Work performed while at the J J Thomson Physical Laboratory, University of Reading.     

Observation of quasi-static spin dynamics in the giant anisotropic ferromagnet PrRu2Si2 by μSR spectroscopy

We report zero field and longitudinal fieldSR measurements on single crystals of the ferromagnet PrRu₂Si₂ (T _c15K). BetweenT=T _c and 250K we observe a quasi-static spin dynamics of the Pr³⁺ ion sub-lattice. BelowT _c, depending on the orientation of the initial muon beam polarisation relative to thec axis of the compound, we find either an oscillating signal or an exponential depolarisation function. These latter data allow us to get information related to the muon in PrRu₂Si₂.     

Mössbauer studies on the series La1-xSrxFeO3

Mössbauer spectra have been recorded at 4.2 and 300 K on the series La_1–x Sr _x FeO₃, wherex varies from 0 to 1.0 in steps of 0.1. Neutron diffraction experiments have shown that the crystal structure is orthorhombic for 0x<0.3, rhombohedral for 0.4x0.7, and cubic for 0.8<x1.0. Mössbauer spectra at 4.2 K are composed of magnetic sextet components arising from different charge states of iron ions. In the orthorhombic and rhombohedral phases, the charge states Fe³⁺ and Fe⁵⁺ coexist. In the cubic phase, iron is present as Fe³⁺ and Fe⁴⁺ states. At 300 K, the samples are magnetically ordered in the range 0 x0.3 and the coexistence of Fe³⁺ and Fe⁵⁺ remains. For samples 0.4x1.0, the samples are paramagnetic. Fits to these spectra require two components, one corresponding to an Fe⁴⁺ state, the other being best described as an Fe³⁺ ion forx0.7 but forx>0.7 having a mean charge state which increases to 3.5 forx=1.0.     

Evaluation of electric hyperfine interactions inAuFe andCrFe

A detailed analysis of room temperature⁵⁷Fe Mössbauer spectra ofAuFe (5 at.% Fe) andCrFe (5, 10 at.% Fe) samples is presented. The deviation from Lorentzian line shape observed in the spectra indicates both thickness effects and structural disorder in these systems. The thickness effect has been taken into account using a new fitting technique that solves the transmission integral numerically rather than relying on the thin absorber approximation. The present results and analyses using both the thin absorber approximation and the full transmission integral methods on these alloy systems are compared with earlier results obtained on these alloy systems using only the thin absorber approximation.     

High-resolution laser spectroscopy on H2 at 97–98 nm

A narrowband tunable eXtreme UltraViolet (XUV) laser source is used for a high resolution study of the Lyman (B ¹ _u ⁺ X ¹ _g ⁺ ) band system of molecular hydrogen. Seven rotational transitions of two vibrational bands, (10,0) and (11,0), in the wavelength range from 97.2–98.3 nm have been investigated for the first time under sub-Doppler molecular beam conditions. A calibration procedure using the I₂ standard in the visible yielded an absolute frequency accuracy of 0.02 cm^–1. The obtained H₂ transition frequencies provide a calibration standard in the extreme ultraviolet wavelength region.     

From oscillations to excitability: A case study in spatially extended systems

This volume is devoted to the presentation of the main contributions to the workshop "From oscillations to excitability: A case study in spatially extended systems," organized by the authors in Nice in June 1993. It gives an overview of the current research on spatiotemporal patterns in a wide range of systems that display self-oscillatory or excitable behavior. It tries to give a better understanding of the transition from the oscillatory to the excitable regime and of its effect on the properties of spiral waves, and to fill the gap between the theories and concepts used to describe both regimes in the so-called "active media."