Optical reflectance of blue bronze crystals near the Peierls transition

We report on the reflectance of blue bronze crystals in the visible frequency range as a function of temperature. The observed large oscillations in the reflectance at all wavelengths at temperatures near the Peierls transition are interpreted as interference effects stemming from a surface layer. It is found that the surface layer, which appears to be tied to fluctuations, can be as much as 1.6 microns thick.     

On the stability of bcc Co in Co/Fe superlattices an NMR and XRD study

While RHEED observations show that 10 to 11 As is the stability limit for an open bcc Co layer when grown on an Fe substrate, our XRD and NMR studies have shown that, in MBE grown Co/Fe superlattices, cobalt can be stabilised in a bcc structure up to a critical Co thickness of 21 Ås. In order to understand this apparent discrepancy, NMR experiments have been carried out in Co_x/Fe_y multilayers with thickness varying in the range 5 Å < x < 42 Å and 24 Å < y < 60 Å, grown on GaAs (1 1 0) as well as on MgO (1 0 0) substrates. The analysis of the chemical short range order by NMR concludes that the larger bcc Co thickness observed in superlattices results from the formation of a rather homogeneous CoFe_20% bcc alloy which contains the supplementary 10–11 As of Co and which coexists with pure Co grains. The concentration of about 20% Fe in the alloyed part of the Co layer happens to be close to the stability limit for a bcc structure in the equilibrium phase diagram of bulk CoFe alloys. However, while a mixture of bcc and fcc phases is observed in bulk alloys, the bcc structure is preserved in all phases under the MBE growth conditions and below the critical thickness. Above the critical thickness amixture of bcc Co, bcc CoFe and hcp Co is observed.     

Atomic scale properties of interior interfaces of semiconductor heterostructures as determined by quasi-digital highly selective etching and atomic force microscopy

We report on first experiments combining quasi-digital highly selective etching and atomic force microscopy (AFM) to examine the interior interfaces of semiconductor heterostructures. Lattice matched (GaIn)As/InP heterostructures grown by metalorganic vapour-phase epitaxy (MOVPE) are taken as a model system to check the capabilities of this new method. Standard selective etchants for different material systems have been optimized in selectivity and etch rate to achieve a quasi-digital etching behaviour. In this way, the real structure of interior interfaces can be determined by AFM. We find a significant difference between the surface of the heterostructure and the interior interfaces.     

Low-energy fission investigated in reactions of 750 AMeV 238U-ions on 208Pb. II: Isotopic distributions

Projectile fission of 750 AMeV ²³⁸U-ions interacting with a Pb target was studied by means of the spectrometer FRS, GSI-Darmstadt. One of the two fission fragments was detected with a transmission of few percent and identified in mass and charge. Low-energy fission (E^* < 25 MeV) events were selected by their magnetic rigidity. Whereas the production of asymmetric fission events is dominated by the GDR excitation, very asymmetric fission and symmetric fission take place after a GQR or DGDR excitation or after a nuclear interaction. Cross sections of more than 250 isotopes were measured. Isotopic distributions of low-energy fission were reconstructed for elements from Se to Te. The fission modes SI, SII and SL were clearly shown in these distributions and in the mass and TKE distributions. Charge polarization and mass dispersion were deduced for each fission mode. Finally, the characteristics of the low-energy fission process explain the production rates of neutron-rich species. Received: 14 July 1997 / Revised version: 6 October 1997     

Hund’s Case (a)-case (b) transition in the singlet-triplet absorption spectrum of pyrazine in a supersonic jet

An analytic expression is derived for calculating the intensities of individual spin-rovibronic lines in the fully resolved gas phase electronic spectrum of a polyatomic molecule, in which one of the zero-order electronic states is a triplet state. The expression is employed to calculate the effect of fine structure splitting on the singlet-triplet absorption spectrum of pyrazine using the parameters available from experiment. A transition from Hund’s coupling Case (a) to Case (b) on going from low J to high J rotational levels is predicted to occur at a moderate resolution of a few hundred MHz. The effect is more pronounced in pyrazine-d ₄ and the pyrazine-argon van der Waals complex owing to their larger mass. Zh. éksp. Teor. Fiz. 114, 63–90 (July 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Gradient- and Sensitivity-Enhanced TOCSY Experiments

A pulsed field gradient version of the sensitivity-enhanced 2D TOCSY experiment is proposed which yields high-quality spectra with improved sensitivity and a minimum of two scans pert₁increment. For rapid acquisition of 1D TOCSY spectra, the 1D DPFGSE–TOCSY experiment was modified to include phase-encoded multiple-selective excitation followed by a simple spectral editing. Combination of these two building blocks is used in a sensitivity-enhanced 2D analog of the 3D TOCSY–TOCSY experiment which provides an efficient tool for resolving severely overlapped signals of oligomers in short experimental time.     

Computational experience with general equilibrium problems

We report on computational experience with an implementation of three algorithms for the general economic equilibrium problem. As a result we get that the projection algorithm for variational inequalities increases the size of solvable models by a factor of 5–10 in comparison with the classical homotopy method. As a third approach we implemented a simulated annealing heuristic which might be suitable to estimate equilibria for very large models.Supported by the German Research Association (Deutsche Forschungsgemeinschaft, SFB 303).