C.M. Koller

Twin-atom beams

Twin photons — pairs of highly correlated photons — are one of the building blocks for quantum optics, and are used in both fundamental tests of quantum physics and technological applications. Now an efficient source for correlated atom pairs is demonstrated, promising to enable a wide range of experiments in the field of quantum matter-wave optics.

Hanbury Brown and Twiss correlations across the Bose-Einstein condensation threshold

Measurements of Hanbury Brown and Twiss correlations in atomic gases near the Bose–Einstein condensation threshold reveal strong signatures of interactions between the constituent atoms, and establish such correlation measurements as a sensitive probe for the quantum properties of matter-wave sources.

Microstructural modifications in powder-metallurgically produced Al0.675Cr0.275Fe0.05 targets during cathodic arc evaporation

Recently, the authors showed that metallic droplets, originating from the Al0.675Cr0.275Fe0.05 cathode surface, play an essential role in the nucleation process of hexagonal crystallites in mixed cubic and hexagonal-structured cathodic arc-evaporated (Al0.70Cr0.25Fe0.05)2O3 films. Here, the authors investigated in detail the corresponding powder-metallurgically produced Al0.7Cr0.3 and Al0.675Cr0.275Fe0.05 targets (after the arc evaporation process) and the ejected macroparticles, which were intentionally separated and collected from the plasma stream. The 15–200 μm thick melting zone (under given process conditions not entirely covering the target surface) of both target materials predominately consists of intermetallic Al80Cr20, Al9Cr4 and dominating Al8Cr5 phases. The selective melting process, induced by the cathodic arc spot size and particle size distribution of the targets, led to the formation of Al-enriched areas. Oxide islands, which form on the target surfaces, especially contain Cr- and Fe-enri...

Controlling microstructure, preferred orientation, and mechanical properties of Cr-Al-N by bombardment and alloying with Ta

Recent ab initio studies showed that the inherent ductility of cubic structured Cr1−xAlxN coatings (as compared with similar hard coatings) significantly increases when alloyed with Ta. As there is only little experimental and theoretical information available, we have performed a combined experimental and ab initio based study on the influence of Ta additions (0, 2, 6, 12, and 26 at. % on the metal sublattice) on structure and mechanical properties of arc evaporated Cr1−x-yAlxTayN coatings with Al/(Cr + Al) ratios >0.61. With increasing Ta-content, the droplet number density decreases and the coating surface smoothens, which is much more pronounced as with increasing the bias potential from −40 to −120 V. Simultaneously, the columnar structure observed for Ta-free Cr0.37Al0.63N significantly changes into a fine-grained structure (crystallite size ∼5 nm) with clearly reduced columnar character. Increasing the Ta content also favors the formation of a preferred 200 growth orientation resulting in a reducti...

First principles studies on the impact of point defects on the phase stability of (AlxCr1−x)2O3 solid solutions

Density Functional Theory applying the generalised gradient approximation is used to study the phase stability of (AlxCr1−x)2O3 solid solutions in the context of physical vapour deposition (PVD). Our results show that the energy of formation for the hexagonal α phase is lower than for the metastable cubic γ and B1-like phases–independent of the Al content x. Even though this suggests higher stability of the α phase, its synthesis by physical vapour deposition is difficult for temperatures below 800 °C. Aluminium oxide and Al-rich oxides typically exhibit a multi-phased, cubic-dominated structure. Using a model system of (Al0.69Cr0.31)2O3 which experimentally yields larger fractions of the desired hexagonal α phase, we show that point defects strongly influence the energetic relationships. Since defects and in particular point defects, are unavoidably present in PVD coatings, they are important factors and can strongly influence the stability regions. We explicitly show that defects with low formation ener...

Oxidation behavior and tribological properties of multilayered Ti-Al-N/Mo-Si-B thin films

The impact of highly oxidation resistant Mo1−x−ySixBy on the oxidation resistance and tribological properties of arc evaporated face-centered-cubic Ti0.57Al0.43N is investigated in detail through Mo1−x−ySixBy/Ti0.57Al0.43N and Mo1−x−ySixByN/Ti0.57Al0.43N compound coatings as well as Ti0.57Al0.43N/Mo1−x−ySixBy multilayer coatings. The Ti0.57Al0.43N/Mo0.54Si0.30B0.16 and Ti0.57Al0.43N/Mo0.68Si0.12B0.20 multilayers with bilayer periods of 35 and 70 nm (consisting of 6 and 15 nm thin sputtered Mo0.54Si0.30B0.16 and Mo0.68Si0.12B0.20 layers), respectively, significantly outperform all coatings investigated during 900 °C ambient-air-oxidation treatments. Consequently, the oxidation resistance of the multilayer coatings is higher as compared with their individual layers. Even after 1000 min exposure, only ∼1.0 μm of the Ti0.57Al0.43N/Mo0.68Si0.12B0.20 multilayer is consumed to form a dense adherent and protective oxide scale. The latter is composed of Si-, Al-, Ti-, and Al-rich layered outer oxides preventing th...

Oxidation behavior of intermetallic Al-Cr and Al-Cr-Fe macroparticles

The incorporation of macroparticles into arc evaporated thin films is generally unfavorable as they, in most cases, induce growth defects. However, in recent studies, the authors found that nucleation of hexagonal (α-phase) crystallites occurs on top of specific macroparticle types and is especially pronounced when using Fe-alloyed Al0.7Cr0.3 cathodes. In order to further investigate this interesting finding, intermetallic Al-Cr and Al-Cr-Fe macroparticles, generated from powder-metallurgically produced and nonreactively arc evaporated Al0.7Cr0.3 and Al0.675Cr0.275Fe0.05 cathodes, were collected and annealed in ambient air. Detailed structural and chemical analyses were carried out by transmission electron microscopy studies of focused ion beam-prepared samples. The predominately γ2-type Al8Cr5 and Al8(Cr,Fe)5 droplets oxidized at 900 °C for 3 and 6 h form a layered oxide structure consisting of a thin amorphous a-Al2O3 layer, on top of which transient γ-/θ-Al2O3 facets develop. The presence of Fe promote...

Effect of Mo on the thermal stability, oxidation resistance, and tribo-mechanical properties of arc evaporated Ti-Al-N coatings

The general tendency in industrial machining and forming operations toward further reduced operation time and increased efficiency requires continuous improvements of protective coatings regarding their tribo-mechanical properties. Alloying arc evaporated Ti1-xAlxN with small amounts of molybdenum (Mo) has shown very promising results to further improve the tribological properties of the industrially widely used Ti-Al-N. Therefore, the authors study in detail the effect of Mo on the thermo-mechanical properties of Ti1-xAlxN coatings, as well as their wear behavior at elevated temperatures. First principle density functional theory (DFT) calculations are conducted for face-centered cubic c-Al1-yMoyN, c-Ti1-yMoyN, and c-Ti1-x-yAlxMoyN showing increasing energy of formation with increasing Mo incorporation for all structures investigated. However, the thereby (DFT) obtained/calculated lattice parameters for comparable compositions perfectly matches with those of the single-phase face-centered cubic structure...