Photoassisted growth and nitrogen doping of ZnSe

ZnSe was grown on GaAs(100) substrates by photoassisted MOVPE using the precursors dimethylzinc-triethylamine (DMZn-TEN) and ditertiarybutylselenide (DtBSe). The optimal growth temperature was T_g = 300°C. Above 300°C no enhancement of growth rate was observed under illumination. Furthermore, nitrogen doping experiments were performed using phenylhydrazine, allylamine and tert-butylamine as nitrogen precursors. Nitrogen concentrations up to 10¹⁹ cm⁻³ (SIMS) were achieved with input flow ratios as low as [PhHz]/[Se] = 4 × 10⁻⁴. All as-grown samples were highly compensated. Successful nitrogen incorporation was also observed with allylamine. However, the use of tert-butylamine together with the adduct compound DMZn-TEN showed no incorporation of nitrogen.     

In situ reflectance difference spectroscopy of ZnSe-based semiconductor surfaces

We will give a brief report on applications of reflectance difference spectroscopy (RDS) for in situ growth monitoring during MOVPE. The experiments were made on GaAs(001) substrates using dimethylzinc-triethylamine, ditbutylselenide and as metalorganic precursors in a laminar flow reactor. We analyzed the influence of the in situ GaAs-substrate preparation on the optical response of the surface. The growth of ZnSe was investigated and compared to data obtained in an MBE process. Spectra at various stages of growth and time-dependent kinetic RDS records were measured during deposition and fitted with a three-layer growth model. Furthermore we utilized the surface sensitivity of the RDS technique to demonstrate surface kinetic processes during p-doping with tbutylamine.     

Phase transitions from saddles of the potential energy landscape

The relation between saddle points of the potential of a classical many-particle system and the analyticity properties of its thermodynamic functions is studied. For finite systems, each saddle point is found to cause a nonanalyticity in the Boltzmann entropy, and the functional form of this nonanalytic term is derived. For large systems, the order of the nonanalytic term increases unboundedly, leading to an increasing differentiability of the entropy. Analyzing the contribution of the saddle points to the density of states in the thermodynamic limit, our results provide an explanation of how, and under which circumstances, saddle points of the potential energy landscape may (or may not) be at the origin of a phase transition in the thermodynamic limit. As an application, the puzzling observations by Risau-Gusman et al. [Phys. Rev. Lett. 95, 145702 (2005)] on topological signatures of the spherical model are elucidated.     

Atomic parameters for Fe(XV) and Ni(XVII)

Transition probabilities and collision strengths are tabulated, between the fourteen lowest levels of the magnesium-like ions Fe(XV) and Ni(XVII), for use in calculations of line intensities radiated from laboratory and astrophysical plasmas.     

Mesoscale kinetics produces martensitic microstructure

We present molecular dynamics (MD) simulations of a martensitic phase transformation studying post-transformation microstructure and moving austenite-martensite interfaces. Unlike in energy-minimisation theories, the transformation dynamics dominate the martensite morphology. We use a binary Lennard-Jones potential to describe a square-to-hexagonal transformation by shear-and-shuffle. The high-T stable square lattice and low-T hexagonal lattice represent austenite and martensite, giving four martensitic variants. Compatible twin variants have no lattice misfit and zero interfacial energies which makes our model directly comparable with the crystallographic theory of martensite. Although our dynamical interpretation is different to previous work, our MD simulations exhibit very similar martensitic morphologies to real materials. We observe the nucleation of wedge-shaped, twinned martensite plates, plate growth at narrow, travelling transformation zones, subsonic transformation waves, elastic precursors inducing secondary nucleations and the formation of martensitic domains. Martensite is produced within narrow transformation zones where atoms change their lattice sites in a co-operative manner so as to form crystallographic layers. These motions produce inertia forces on the mesoscopic length-scale which induce the formation of twin variants in the subsequent layers to transform.     

Hexaamminecobalt(III) aquabis(nitrilotriacetato‐κ4N,O,O′,O′′)gadolinate(III) octahydrate

Crystals of the title compound, [Co(NH₃)₆][Gd(C₆H₆NO₆)₂(H₂O)]·8H₂O, were synthesized in and collected from aqueous solution. The hexaamminecobalt(III) cation has the expected octahedral geometry, while the Gd coordination sphere has the geometry of a tricapped trigonal prism, with the two nitrilo­tri­acetate N atoms and one water mol­ecule occupying the capping positions.     

Enantiomeric Excess Sensitivity to Below One Percent by Using Femtosecond Photoelectron Circular Dichroism

Photoelectron circular dichroism (PECD) is experimentally investigated with chiral specimens with varying amounts of enantiomeric excess (ee). As a prototype, we measure and analyze the photoelectron angular distribution from randomly oriented fenchone molecules in the gas phase that result from ionization with circularly polarized femtosecond laser pulses. The quantification of these measurements shows a linear dependence with respect to the ee values. In addition, differences in the ee values (denoted as detection limit) of below one percent can be distinguished for nearly enantiopure samples, as well as for almost racemates. In combination with the use of a reference, the assignment of absolute ee values is possible. The present measurement time is a few minutes, but this could be reduced. This table‐top laser‐based approach should facilitate widespread implementation in chiral analysis.     

Der Wärmeübergang im Post-Dryout-Gebiet während transienter Vorgänge

Zusammenfassung Werden in einem beheizten Verdampferrohr die Randbedingungen so geändert, daß sich der Ort der Siedekrise in Strömungsrichtung verschiebt, so kann dies vorübergehend zu einer starken Reduktion des Wärmeübergangskoeffizienten im Post-Dryout-Gebiet führen. Es werden die an einem Hochdruckkreislauf gewonnenen zeitlichen Verläufe der Wandtemperaturen analysiert, wie sie bei Absenkung des System-druckes bzw. der Enthalpie der Strömung am Eintritt in das Versuchsrohr auftreten. Anhand der Siedekurve werden die beobachteten Wandtemperaturverläufe interpretiert. Durch Nachrechnung der Experimente mit einem dynamischen Rechen-programm lassen sich die unterschiedlichen Effekte, die während der Transiente auftreten, separieren.

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Heat transfer in post-dryout region during transients

A change in the boundary conditions in a heated evaporator tube (moving the location of the boiling crisis downstream) could lead to a strong reduction in the heat transfer coefficient in the Post-Dryout region. The analyzed wall temperature data were obtained in a high pressure test loop by reducing the system pressure, or the enthalpy at the test section inlet. The wall temperature time histories were interpreted using the boiling curve. The various effects occurring during the transient could be separated by recalculating the tests with a dynamic simulation computer program.

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Formelzeichen h Sättigungsenthalpie des Wassers - p Druck - p _c kritischer Druck - Q Wärmestrom - q Wärmestromdichte - q₁, q₂ Wärmestromdichte für Rohrabschnitt l bzw. 2 - t Zeit - x wirklicher Dampfmassenanteil - ¯x bilanzmäßiger Dampfmassenanteil Herrn Prof. Dr.-Ing. U. Grigull zum 70. Geburtstag gewidmet