Metal - Insulator transition driven by vacancy ordering in GeSbTe phase change materials

Phase Change Materials (PCMs) are unique compounds employed in non-volatile random access memory thanks to the rapid and reversible transformation between the amorphous and crystalline state that display large differences in electrical and optical properties. In addition to the amorphous-to-crystalline transition, experimental results on polycrystalline GeSbTe alloys (GST) films evidenced a Metal-Insulator Transition (MIT) attributed to disorder in the crystalline phase. Here we report on a fundamental advance in the fabrication of GST with out-of-plane stacking of ordered vacancy layers by means of three distinct methods: Molecular Beam Epitaxy, thermal annealing and application of femtosecond laser pulses. We assess the degree of vacancy ordering and explicitly correlate it with the MIT. We further tune the ordering in a controlled fashion attaining a large range of resistivity. Employing ordered GST might allow the realization of cells with larger programming windows

Metal - Insulator transition driven by vacancy ordering in GeSbTe phase change materials

Phase Change Materials (PCMs) are unique compounds employed in non-volatile random access memory thanks to the rapid and reversible transformation between the amorphous and crystalline state that display large differences in electrical and optical properties. In addition to the amorphousto-crystalline transition, experimental results on polycrystalline GeSbTe alloys (GST) films evidenced a Metal-Insulator Transition (MIT) attributed to disorder in the crystalline phase. Here we report on a fundamental advance in the fabrication of GST with out-of-plane stacking of ordered vacancy layers by means of three distinct methods: Molecular Beam Epitaxy, thermal annealing and application of femtosecond laser pulses. We assess the degree of vacancy ordering and explicitly correlate it with the MIT. We further tune the ordering in a controlled fashion attaining a large range of resistivity. Employing ordered GST might allow the realization of cells with larger programming windows

Anisotropic exchange interaction of localized conduction-band electrons in semiconductor structures

The spin-orbit interaction in semiconductors is shown to result in an anisotropic contribution into the exchange Hamiltonian of a pair of localized conduction-band electrons. The anisotropic exchange interaction exists in semiconductor structures which are not symmetric with respect to spatial inversion, for instance in bulk zinc-blend semiconductors. The interaction has both symmetric and antisymmetric parts with respect to permutation of spin components. The antisymmetric (Dzyaloshinskii-Moriya) interaction is the strongest one. It contributes significantly into spin relaxation of localized electrons; in particular, it governs low-temperature spin relaxation in n-GaAs with the donor concentration near 10^16cm-3. The interaction must be allowed for in designing spintronic devices, especially spin-based quantum computers, where it may be a major source of decoherence and errors

Ein mehrkanaliges Biosensormesssystem zur Überwachung der Nitrifikation in Abwasserreinigungsanlagen

Bei der biologischen Abwasserreinigung treten häufig Störungen der Nitrifikationsstufe (biochemische Oxidation von Ammonium über Nitrit zu Nitrat) auf, die durch Hemmstoffe sowie durch Stossbelastungen hoher Stickstofffrachten (N-BSB) verursacht werden. Dadurch gelangen erhöhte Mengen an sauerstoffzehrenden reduzierten Stickstoffverbindungen in die Oberflächengewässer. Dies ist besonders kritisch bei Vorflutern mit einem bereits niedrigem Sauerstoffgehalt, da durch diese reduzierten Stickstoffverbindungen die Konzentration an gelöstem Sauerstoff in Folge mikrobieller Oxidation stark absinken kann und somit die Biocönose im Gewässer nachhaltig gestört wird. Da es sich bei Nitrit und Ammoniak zudem um starke Fischgifte handelt, ist die Elimination dieser Stickstoffverbindungen aus dem Abwasser nicht zuletzt auch gesetzlich vorgeschrieben. Zur Erfassung von Störungen der Nitrifikationsstufe wurde deswegen in den vergangenen Jahren am ISWA ein Nitrifikanten-Einzelbiosensor entwickelt. Dabei wird über den Sauerstoffverbrauch des Immobilisates die bakterielle Stoffwechselaktivität überwacht, wobei dies ein Maß für das Vorhandensein von Hemmstoffen bzw. Nitrifikationssubstraten in einer Probe ist. Der Vorteil dieses Systems ist vor allem darin zu sehen, dass die damit durchgeführten Messungen sowohl schnell als auch reproduzierbar durchführbar sind. So ist mit diesem Geräteprototyp über die Durchführung von ca. 10 – 15 Einzelmessungen verschiedener Probenverdünnungen die Quantifizierung der Hemmwirkung von einer Probe pro Messtag möglich. Schlussfolgerung und Ausblick Die Untersuchungen zur Bestimmung der nitrifikationshemmenden Wirkung von Standardhemmstoffen, Abwasserproben und des N-BSB ergaben, dass die neu entwickelten Nitrifikanten-Biosensoren schnell und mit hoher Signalstabilität auf sich ändernde Hemmstoff- und Substratkonzentrationen reagieren und somit für die Klärwerksüberwachung prinzipiell einsetzbar sind

The diet of the black widow spider Latrodectus mirabilis (Theridiidae) in two cereal crops of central Argentina

The spider Latrodectus mirabilis (Holmberg, 1876) is commonly found in cereals crops of central Argentina. We studied its diet composition at the field and capture rate on leaf-cutting ants based on laboratory experiments. This study comprises the first approach that documents the diet of L. mirabilis in wheat and oat fields of central Argentina. We identified 1,004 prey items collected from its webs during the last phenological stages of both cereal crops. The prey composition was variable but the spiders prey mainly on ants (Formicidae, Hymenoptera), who represented more than 86% of the total. Meanwhile, in the capture rate experiences we registered a high proportion of ants captured by spiders at the beginning of experiences, capturing the half of the ants from total in the first four hours. Summarizing, we reported a polyphagous diet of this spider species in wheat and oat fields. Ants were the most important prey item of this spider, as found in other Latrodectus spiders around the world

Self Assessment in Insects: Honeybee Queens Know Their Own Strength

Contests mediate access to reproductive opportunities in almost all species of animals. An important aspect of the evolution of contests is the reduction of the costs incurred during intra-specific encounters to a minimum. However, escalated fights are commonly lethal in some species like the honeybee, Apis mellifera. By experimentally reducing honeybee queens' fighting abilities, we demonstrate that they refrain from engaging in lethal contests that typically characterize their reproductive dominance behavior and coexist peacefully within a colony. This suggests that weak queens exploit an alternative reproductive strategy and provides an explanation for rare occurrences of queen cohabitation in nature. Our results further indicate that self-assessment, but not mutual assessment of fighting ability occurs prior to and during the agonistic encounters