Prozeßverbesserung der Projektorganisation

Im Mittelpunkt steht die organisatorische sowie die informations- und kommunikationstechnische Gestaltung von (Teil-)Projekten. Es werden die Grundideen des Fertigungsinsel- und Planungsinselkozeptes aufgegriffen und ein Modell zur Prozeßverbesserung von Projekten, zum sogenannten ""Projektinselkonzept"" (PIK), zusammengefügt. Ein weiteres tragendes Elemente im PIK stellt eine Lernende Projektorganisation dar und gewährleistet damit Größenvorteile, Lern- bzw. Erfahrungskurven und damit eine Beschleunigung der Projektprozesse. Unterstützt wird das PIK durch eine Auswahl individueller Informations- und Kommunikationssysteme, wie bspw. Groupware- und Worflowanwendungen

Precursor phase with full phonon softening above the charge-density-wave phase transition in 2H2H-TaSe2_2

Research on charge-density-wave (CDW) ordered transition-metal dichalcogenides continues to unravel new states of quantum matter correlated to the intertwined lattice and electronic degrees of freedom. Here, we report an inelastic x-ray scattering investigation of the lattice dynamics of the canonical CDW compound $2H$-TaSe$_2$ complemented by angle-resolved photoemission spectroscopy. Our results rule out the central-peak scenario for the CDW transition in $2H$-TaSe$_2$ and provide evidence for a novel precursor phase above the CDW transition temperature $T_{CDW}$. The phase at temperatures between $T^{*}\,(= 128.7\,,\rm{K})$ and $T_{CDW}\,(= 121.3\,\rm{K})$ is characterized by a fully softened phonon mode and medium-range ordered ($\xi_{corr} = 100\,\rm{\mathring{A}}- 200\,\rm{\mathring{A}})$ static CDW domains. Only $T_{CDW}$ is detectable in our photoemission experiments. Thus, $2H$-TaSe$_2$ exhibits structural before electronic static order and emphasizes the important lattice contribution to CDW transitions

Precursor region with full phonon softening above the charge-density-wave phase transition in 2H-TaSe2

Research on charge-density-wave (CDW) ordered transition-metal dichalcogenides continues to unravel new states of quantum matter correlated to the intertwined lattice and electronic degrees of freedom. Here, we report an inelastic x-ray scattering investigation of the lattice dynamics of the canonical CDW compound 2H-TaSe2 complemented by angle-resolved photoemission spectroscopy and density functional perturbation theory. Our results rule out the formation of a central-peak without full phonon softening for the CDW transition in 2H-TaSe2 and provide evidence for a novel precursor region above the CDW transition temperature TCDW, which is characterized by an overdamped phonon mode and not detectable in our photoemission experiments. Thus, 2H-TaSe2 exhibits structural before electronic static order and emphasizes the important lattice contribution to CDW transitions. Our ab-initio calculations explain the interplay of electron-phonon coupling and Fermi surface topology triggering the CDW phase transition and predict that the CDW soft phonon mode promotes emergent superconductivity near the pressure-driven CDW quantum critical point

Reconciling Apparent Conflicts between Mitochondrial and Nuclear Phylogenies in African Elephants

Conservation strategies for African elephants would be advanced by resolution of conflicting claims that they comprise one, two, three or four taxonomic groups, and by development of genetic markers that establish more incisively the provenance of confiscated ivory. We addressed these related issues by genotyping 555 elephants from across Africa with microsatellite markers, developing a method to identify those loci most effective at geographic assignment of elephants (or their ivory), and conducting novel analyses of continent-wide datasets of mitochondrial DNA. Results showed that nuclear genetic diversity was partitioned into two clusters, corresponding to African forest elephants (99.5% Cluster-1) and African savanna elephants (99.4% Cluster-2). Hybrid individuals were rare. In a comparison of basal forest “F” and savanna “S” mtDNA clade distributions to nuclear DNA partitions, forest elephant nuclear genotypes occurred only in populations in which S clade mtDNA was absent, suggesting that nuclear partitioning corresponds to the presence or absence of S clade mtDNA. We reanalyzed African elephant mtDNA sequences from 81 locales spanning the continent and discovered that S clade mtDNA was completely absent among elephants at all 30 sampled tropical forest locales. The distribution of savanna nuclear DNA and S clade mtDNA corresponded closely to range boundaries traditionally ascribed to the savanna elephant species based on habitat and morphology. Further, a reanalysis of nuclear genetic assignment results suggested that West African elephants do not comprise a distinct third species. Finally, we show that some DNA markers will be more useful than others for determining the geographic origins of illegal ivory. These findings resolve the apparent incongruence between mtDNA and nuclear genetic patterns that has confounded the taxonomy of African elephants, affirm the limitations of using mtDNA patterns to infer elephant systematics or population structure, and strongly support the existence of two elephant species in Africa

Stimulated emission through an electron-hole plasma in colloidal CdSe quantum rings

Colloidal CdSe quantum rings (QRs) are a new class of nanomaterials synthetized via thermo-chemical edge reconfiguration of thinner CdSe nanoplatelets [1],[2]. In the latter, the photo-physics is consistently dominated by strongly bound electron-hole pairs, so-called excitons, that can merge to form excitonic molecules (biexcitons), giving rise to net stimulated emission along the molecule-to-exciton recombination pathway.[3] On the other hand, little is known on the nature of elementary excitations in thicker CdSe QRs - whether they are excitons or free electron-hole pairs- and their behavior at high density regime. Here, we show that charge carriers in QRs condense into a hot uncorrelated electron-plasma at high density opposed to the stable exciton gas found in thinner nanoplatelets. Through strong band gap renormalization, this plasma state is able to produce sizable optical gain with a broadband spectrum. Next, we show that the typical signatures of excitonic transitions are indeed absent in QRs. The gain is limited by a second order radiative recombination process and the buildup is counteracted by a typical charge cooling bottleneck. Overall, our results show that weakly confined QRs are a unique system to study uncorrelated electron-hole dynamics in nanoscale materials. [1] Fedin, I.; Talapin, D. V. Colloidal CdSe Quantum Rings. J. Am. Chem. Soc. 2016, 138, 9771-9774. [2] Salzmann, B. B. V.; Vliem, J. F.; Maaskant, D. N.; Post, L. C.; Li, C.; Bals, S.; Vanmaekelbergh, D. From CdSe Nanoplatelets to Quantum Rings by Thermochemical Edge Reconfiguration. Chem. Mater 2021. [3] Geiregat, P.; Tomar, R.; Chen, K.; Singh, S.; Hodgkiss, J. M.; Hens, Z. Thermodynamic Equilibrium between Excitons and Excitonic Molecules Dictates Optical Gain in Colloidal CdSe Quantum Wells. J. Phys. Chem. Lett 2019, 10, 3637-3644

Stimulated Emission through an Electron–Hole Plasma in Colloidal CdSe Quantum Rings

Colloidal CdSe quantum rings (QRs) are a recently developed class of nanomaterials with a unique topology. In nanocrystals with more common shapes, such as dots and platelets, the photophysics is consistently dominated by strongly bound electron-hole pairs, so-called excitons, regardless of the charge carrier density. Here, we show that charge carriers in QRs condense into a hot uncorrelated plasma state at high density. Through strong band gap renormalization, this plasma state is able to produce broadband and sizable optical gain. The gain is limited by a second-order, yet radiative, recombination process, and the buildup is counteracted by a charge-cooling bottleneck. Our results show that weakly confined QRs offer a unique system to study uncorrelated electron-hole dynamics in nanoscale materials

Stimulated emission through an electron–hole plasma in colloidal CdSe quantum rings

Colloidal CdSe quantum rings (QRs) are a recently developed class of nanomaterials with a unique topology. In nanocrystals with more common shapes, such as dots and platelets, the photophysics is consistently dominated by strongly bound electron–hole pairs, so-called excitons, regardless of the charge carrier density. Here, we show that charge carriers in QRs condense into a hot uncorrelated plasma state at high density. Through strong band gap renormalization, this plasma state is able to produce broadband and sizable optical gain. The gain is limited by a second-order, yet radiative, recombination process, and the buildup is counteracted by a charge-cooling bottleneck. Our results show that weakly confined QRs offer a unique system to study uncorrelated electron–hole dynamics in nanoscale materials