Search

Eukaryontische Zellen besitzen ein Zytoskelett, ein zelluläres Netzwerk aus Biopolymeren. Unter diesen Biopolymeren sind die Mikrotubuli weitgehend konserviert. Diese aus Tubulin aufgebauten Filamente sind dynamisch und wechseln zwischen Phasen des Wachstums und der Schrumpfung. Die genauen Mechanismen, die die dynamische Instabilität der Mikrotubuli bestimmen, werden noch erforscht. Die Allgegenwart von Mikrotubuli wirft die Frage auf, wie sie in verschiedenen thermischen Umgebungen konservierte Funktionen ausführen können. Um dieser Fragestellung nachzugehen, habe ich verwandte Froscharten mit unterschiedlich temperierten Lebensräumen untersucht: Xenopus laevis (16-22 °C), Xenopus borealis (19-23 °C) und Xenopus tropicalis (22-30 °C). Um zu untersuchen, ob sich die biochemischen Eigenschaften von Tubulin und die Dynamik der Mikrotubuli bei den drei Arten an die Temperatur angepasst hat, habe ich die Methoden der Tubulin-Affinitätsreinigung und die temperaturgesteuerte TIRF-Mikroskopie zur Rekonstitution der Mikrotubuli-Dynamik kombiniert. Dabei habe ich festgestellt, dass bei einer Temperatur von 25°C die Wachstumsgeschwindigkeit der Mikrotubuli im Bezug zur thermischen Nische der einzelnen Arten negativ korreliert. Die Verwendung der Arrhenius-Gleichung zum Vergleich der Aktivierungsenergie der Mikrotubuli-Polymerisation für jede Spezies ergab, dass die freie Energie des Tubulins umso höher ist, je kälter die thermische Nische der Spezies ist. Die Mikrotubuli von X. laevis und X. borealis zeigten eine längere Lebensdauer und wurden häufiger zerstört als die von X. tropicalis. Die Tubuline von X. laevis und X. borealis sind phosphoryliert, im Gegensatz zu X. tropicalis. Die Ergebnisse zeigen, dass sich Xenopus Tubulin und die Dynamik der Mikrotubuli an die Temperatur angepasst haben. Kalt lebende Arten kommen mit der niedrigeren Energie des Milieus zurecht, durch verbessertes Wachstum und Stabilität., Eukaryotic cells hold a cytoskeleton, a cellular network of biopolymers. Among the filaments of the cytoskeleton, microtubules are widely conserved. Built from tubulin, those filaments are dynamic, alternating between phases of growth and shrinkage. The biochemical properties of tubulin shape the dynamic behavior of microtubules, which is crucial for many cellular processes. The precise mechanisms determining microtubule dynamic instability are still under investigation. The ubiquity of microtubules raises the question of how they can perform conserved functions within various thermal environments. To address this, I turned to closely related frog species living at different temperatures, Xenopus laevis (niche: 16-22°C), Xenopus borealis (19-23°C) and Xenopus tropicalis (22-30°C). To probe whether the biochemical properties of tubulin and microtubule dynamics adapted to temperature across those three species, I combined tubulin affinity purification and temperature-controlled TIRF microscopy of in vitro reconstitution of microtubule dynamics. I found that at 25°C, the microtubule growth velocity inversely correlates with the thermal niche of each species. Adjusting temperature to each species’ endogenous condition modulates the growth rate differences across species. Using the Arrhenius equation to compare the activation energy of microtubule polymerization for each species suggested that the colder the thermal niche of the species, the higher the free energy of its tubulin. Microtubules from the cold-adapted species X. laevis and X. borealis have longer lifetimes and rescue more often than those of X. tropicalis, both at 25°C and at each species’ endogenous condition. X. laevis and X. borealis tubulins are phosphorylated, contrary to X. tropicalis. My results show that Xenopus tubulin and microtubule dynamics have adapted to temperature. Cold-living species cope with the lower energy of the milieu by facilitating growth and stability.

We report a detailed analysis of atmospheric stabilities in the mesopause region (85–100 km) based on over 2,000 hr of high-resolution temperature and horizontal wind measurements made with a Na lidar at the Andes Lidar Observatory, located in Cerro Pachón, Chile (30.25°S, 70.74°W). The square of Brunt–Väisälä frequency and the Richardson number are calculated, and occurrence probabilities of convective and dynamic instabilities are derived. An approach to assess the biases due to measurement uncertainties is used to obtain more accurate occurrence probabilities. The overall occurrence probabilities of convective and dynamic instabilities are 2.7% and 6.7%, respectively. High-, medium-, and low- frequency gravity wave (GW) contributions to these probabilities are isolated, which show that the high-frequency GWs contribute most but simultaneous presence of high- and medium frequency GWs is much more effective in increasing the probabilities. Convective and dynamic instabilities are mainly generated because of the joint effect of different-scale GWs. Isolated parts of GWs have much less contribution to the generation of both convective and dynamic instabilities. The dynamic instability is mainly contributed from less stable stratification and large wind shear together. Either factor can lead to about 15% of dynamic instability.

The strengthening and hardening of metals increase with diminishing size due to plastic strain gradient accompanied by Geometrically Necessary Dislocations (GNDs). In contrast, the inertia forces under severe dynamic loading conditions decrease. To shed the light on the interaction between the two mechanisms, the present work focuses on the localization in notched bars subject to high strain-rate tensile loading and first lays out a new computational model where the Fleck and Willis strain gradient plasticity theory is implemented into a dynamic finite element framework. The new framework allows accounting for strain gradient strengthening through the non-local plasticity material model and models the “smaller is stronger” mechanism (controlled by a built-in length parameter), while material inertia governs the “larger is more dynamic” effect. The work by Needleman (2018) [Effect of size on necking of dynamically loaded notched bars, Mech. Materials, 116:180-188] is taken as a starting point, and the present study first conveys the message that the scale-dependent dynamic effect observed in visco-plastic tensile bars also exists for rate-independent materials across scales (questioned by Needleman, 2018). Subsequently, the notched bar's size and corresponding loading are scaled to the micrometer size range where plastic strain gradients influence the material response. Focusing attention on the shift between plastic localization in the notch and away from the notch (in the region subject to uni-axial tension) allows cementing this shift at the micron scale. The study shows that gradient strengthening owing the GNDs stabilizes the early localization in the notch and delays the shift between the localization mechanisms when the size of the notched bar diminishes while keeping the material length parameter constant. Moreover, severe gradient strengthening gives rise to a slow-growing neck-like (micron scale) localization mechanism where a neck-type deformation evolv

La présente thèse se situe dans un contexte théorique traitant de la modélisation et de la simulation. Son objectif est de fournir un outil numérique d'aide à la décision et à la prédiction des vibrations induites par le frottement afin d'adresser les éventuelles problématiques de crissement et ce dans les phases de conception. Afin de développer l'outil prédictif, l'aspect aléatoire est utilisé comme un moyen pour décrire plusieurs phénomènes physiques de nature tribologique (le frottement, le contact normal, la rugosité des interfaces de contact) mais aussi la dispersion de quelques propriétés intrinsèques suite aux aléas des procédés de fabrication. La description aléatoire de ces derniers éléments a été mise en œuvre par plusieurs algorithmes stochastiques et approches microscopiques, ce qui a permis le développement de $4$ solveurs éléments finis stochastiques et une procédure des éléments finis multiéchelle. Les méthodologies de calcul ont fait l'objet de plusieurs tests numériques que ce soit sur des modèles analytiques ou sur des modèles éléments finis d'un frein à disque ayant comme objectif l'étude de leur efficacité et robustesse. Les résultats démontrent que les nouveaux développements offrent une alternative très intéressante par rapport aux méthodes classiques. Pour avoir des niveaux de précision similaires, les méthodes introduites dans la thèse nécessitent un temps de calcul largement inférieur à celui requis par la méthode de référence couplant l'algorithme de Monte-Carlo avec la méthode des éléments finis., With the progress of the automotive industry, manufacturers are increasingly sensitive to the design of mechanical systems: mechanical strength, vibroacoustic performance... With the development of a new hybrid and electric technologies, it is even more crucial and very important that all aspects of noise are addressed; in particular the braking system which is considered the noisiest and the most polluting element. Currently, engineers model friction-induced vibrations using a deterministic approach whose results overestimate or underestimate reality. To fill this gap, they use aprocedure that combines deterministic modeling and experimental techniques, which increases the time and the cost of designing braking systems. This thesis is set in a theoretical context dealing with modeling and simulation. Its objective is to provide a numerical tool to assist in the decision-making and prediction of friction induced vibrations at the design stage. In order to develop the prediction tool, randomness has been used to describe several physical phenomena of tribological nature (friction, normal contact, roughness of contact interfaces) but also the dispersion of some intrinsic properties due to the randomness of manufacturing processes. The random description of the latter has been implemented by several stochastic algorithms and microscopic approaches, which allowed the development of 4 stochastic finite element solvers and a multiscale finite element procedure. The computational methodologies have been tested numerically on analytical functions and finite element model of a disc brake in order to investigate their efficiency and robustness. The results show that the new developments offer avery interesting alternative to classical methods. In order to obtain similar levels of accuracy, the methods introduced in this thesis require a computational time much lower than the reference method coupling the Monte-Carlo algorithm with the finite element method.

Boundary layer rolls are a rather frequent phenomena, where regions of alternating up- and downdraft motion causes clouds to form in elongated, parallel rows oriented with the mean wind direction. The clouds can be seen during certain atmospheric conditions and are often called ”cloud streets” because of their characteristic appearance. By performing a linear instability analysis, the underlying mechanisms causing the onset of boundary layer rolls has been analysed in this study. There are two governing mech- anisms that cause the boundary layer rolls to form, the thermal instability and the dynamic instability. The thermal instability is caused by convection in an unstable airmass, while the dynamic instability usually is associated with neutral or stable conditions. The dynamic instability arise due to an inflection point in the wind profile, around which eddies develop. In a previous study by Svensson et al. (2017), rolls were observed over the Swedish east-coast, stretching out over sea during four days; 2 of May 1997, 3 of May 1997, 17 of May 2011 and 25 of May 2011. The aim of this study is to simulate the rolls on these four dates, analyse the underlying mechanisms and establish what type of instability that primarily causes the rolls to form. The linear stability analysis performed in this study indicate that the dynamic instability is the main mechanism giving rise to the rolls on all four studied dates. The rolls are found to arise over the Swedish mainland and are advected out over the sea. Both the orientation of the rolls and the modeled wind direction are in accordance with the observations. A qualitative agreement is found for the wavelength, the amplitude and the altitude of the rolls, when comparing the results of this study with the observations.

Boundary layer rolls are a rather frequent phenomena, where regions of alternating up- and downdraft motion causes clouds to form in elongated, parallel rows oriented with the mean wind direction. The clouds can be seen during certain atmospheric conditions and are often called ”cloud streets” because of their characteristic appearance. By performing a linear instability analysis, the underlying mechanisms causing the onset of boundary layer rolls has been analysed in this study. There are two governing mech- anisms that cause the boundary layer rolls to form, the thermal instability and the dynamic instability. The thermal instability is caused by convection in an unstable airmass, while the dynamic instability usually is associated with neutral or stable conditions. The dynamic instability arise due to an inflection point in the wind profile, around which eddies develop. In a previous study by Svensson et al. (2017), rolls were observed over the Swedish east-coast, stretching out over sea during four days; 2 of May 1997, 3 of May 1997, 17 of May 2011 and 25 of May 2011. The aim of this study is to simulate the rolls on these four dates, analyse the underlying mechanisms and establish what type of instability that primarily causes the rolls to form. The linear stability analysis performed in this study indicate that the dynamic instability is the main mechanism giving rise to the rolls on all four studied dates. The rolls are found to arise over the Swedish mainland and are advected out over the sea. Both the orientation of the rolls and the modeled wind direction are in accordance with the observations. A qualitative agreement is found for the wavelength, the amplitude and the altitude of the rolls, when comparing the results of this study with the observations.

Microtubules (MTs) are hollow cylinders made of tubulin, a GTPase responsible for essential functions during cell growth and division, and thus, key target for anti-tumor drugs. In MTs, GTP hydrolysis triggers structural changes in the lattice, which are responsible for interaction with regulatory factors. The stabilizing GTP-cap is a hallmark of MTs and the mechanism of the chemical-structural link between the GTP hydrolysis site and the MT lattice is a matter of debate. We have analyzed the structure of tubulin and MTs assembled in the presence of fluoride salts that mimic the GTP-bound and GDP•Pi transition states. Our results challenge current models because tubulin does not change axial length upon GTP hydrolysis. Moreover, analysis of the structure of MTs assembled in the presence of several nucleotide analogues and of taxol allows us to propose that previously described lattice expansion could be a post-hydrolysis stage involved in Pi release.

Die Anlage des zentralen Nervensystems wird während der frühen Embryonalentwicklung gebildet. Aus dem Neuroektoderm entwickelt sich die Neuralplatte, welche sich durch seitliche Auffaltungen und umfängliche Zellumordnungen zum Neuralrohr schließt. Aus dem Neuralrohr entsteht in der weiteren Embryogenese anterior das Gehirn und posterior das Rückenmark. Störungen beim Neuralrohrschluss führen zu schwerwiegenden Fehlentwicklungen. Das Auftreten differenzieller Neuralrohrdefekte, welche spezifische Abschnitte des Neuralrohrs betreffen, deutet darauf hin, dass der Neuralrohrschluss entlang der anterior-posterioren Achse verschieden reguliert wird. So liegt z.B. bei der schwerwiegenden Fehlbildung Kraniorachischisis die gesamte Länge des Neuralrohrs ab dem Rautenhirn offen, wobei die Region des Vorderhirns jedoch normal geschlossen wird. Im Wildtyp zeigt sich diese unterschiedliche Regulierung auch in einer zeitlichen Verzögerung zwischen dem Schluss des kranialen und des kaudalen Neuralrohrs. Während die Mechanismen, die zum posterioren Neuralrohrschluss führen, recht gut erforscht sind, sind die anterioren morphologischen Prozesse im Bereich des zukünftigen Vorderhirns weitgehend unverstanden. Der Fokus dieser Arbeit lag darauf, zell- und gewebemorphologische Vorgänge zu identifizieren, die für die Entstehung und Entwicklung des Neuralrohrs im Bereich des Vorderhirns wichtig sind. Für Zellformänderungen sowie Zellwanderung ist die Regulation des Cytoskeletts von großer Bedeutung. Aus diesem Grund wurde in dieser Arbeit die Rolle des mit Mikrotubuli assoziierten Proteins Hmmr im embryonalen Modellorganismus Xenopus laevis untersucht. Im Menschen erhöhen Mutationen in HMMR die Wahrscheinlichkeit an Brustkrebs zu erkranken. In diesem Zusammenhang wurde Hmmr bisher vor allem eine Rolle bei der Regulierung von Zellmotilität sowie bei der Aufrechterhaltung der Spindel-Integrität zugeschrieben. In Xenopus verlangsamte eine gestörte Modulation von hmmr durch Funktionsverlust, The anlage of the central nervous system is formed during early embryonic development. The neuroectoderm establishes the neural plate which folds up to form the neural tube, a process that requires extensive cell rearrangements. During further embryogenesis the anterior part of the neural tube develops into the brain while the posterior part forms the spinal cord. Disturbances during neural tube closure (NTC) lead to severe developmental aberrations. Occurrence of specific neural tube defects indicates a distinct regulation of NTC along the anterior-posterior axis. For example, the severe malformation craniorachischisis is characterized by a failure to close the neural tube from hindbrain levels onwards, while the forebrain region develops normally. This distinct regulation presents itself in the wildtype in a delay between cranial and caudal NTC. While the mechanisms leading to posterior NTC are quite well understood, the morphological processes at the future forebrain level are largely unknown. The aim of this dissertation was to identify cell and tissue morphogenetic processes which are required for the formation and development of the anterior neural tube. As the underlying changes in cell shape as well as cell migration depend on the regulation of the cytoskeleton, the role of the microtubule-associated protein Hmmr was analyzed in the model organism Xenopus laevis. HMMR is a breast cancer susceptibility gene with described roles mainly in the tumor context, regulating cell motility and maintenance of mitotic spindle integrity. In Xenopus, gain as well as loss of function of hmmr delayed NTC and led to defects during further forebrain development. Loss of hmmr impaired separation of telencephalic hemispheres, resembling the human malformation “middle interhemispheric variant of holoprosencephaly”. Failure of ventricle separation could be traced back to disturbed roof plate formation. This was due to impaired NTC resulting from a lack of neural cell convergence.

We consider the competition among quantity setting players in a linear evolutionary environment. To set their outputs, players adopt, alternatively, the best response rule having perfect foresight or an imitative rule. Players are allowed to change their behavior through an evolutionary mechanism according to which the rule with better performance will attract more followers. The relevant stationary state of the model describes a scenario where players produce at the Cournot-Nash level. Due to the presence of imitative behavior, we find that the number of players and implementation costs, needed to the best response exploitation, have an ambiguous role in determining the stability properties of the equilibrium and double stability thresholds can be observed. Differently, the role of the intensity of choice, representing the evolutionary propensity to switch to the most profitable rule, has a destabilizing role, in line with the common occurrence in evolutionary models. The global analysis of the model reveals that increasing values of the intensity of choice parameter determine increasing dynamic complexities for the internal attractor representing a population where both decision mechanisms coexist.

In the rank of behavioral rules, imitation-based heuristics have received special attention in economics (see Vega-Redondo, 1997; Schlag, 1998). In particular, imitative behavior explains the evidence from experimental oligopolies where the Cournot–Nash equilibrium does not emerge as unique outcome and outputs are placed, for the most part, on fairly competitive levels (see e.g. Apesteguia et al., 2006 and Oechssler et al., 2016). Here, we derive a dynamic model of a linear Cournot oligopoly with a deterministic structure where rational agents having perfect foresight and imitators are present. The Cournot–Nash equilibrium is a stationary state of our models together with a further production level that can be interpreted as the competitive equilibrium in case only imitators are present. We found that the heterogeneities among players influence the stability properties of the Cournot–Nash equilibrium and the arising dynamical scenarios are characterized by similar features as those from experimental outcomes.

In the rank of behavioral rules, imitation-based heuristics has received special attention in economics (see [14] and [12]). In particular, imitative behavior is considered in order to understand the evidences arising in experimental oligopolies which reveal that the Cournot-Nash equilibrium does not emerge as unique outcome and show that an important component of the production at the competitive level is observed (see e.g.[1,3,9] or [7,10]). By considering the pioneering groundbreaking approach of [2], we build a dynamical model of linear oligopolies where heterogeneous decision mechanisms of players are made explicit. In particular, we consider two different types of quantity setting players characterized by different decision mechanisms that coexist and operate simultaneously: agents that adaptively adjust their choices towards the direction that increases their profit are embedded with imitator agents. The latter ones use a particular form of proportional imitation rule that considers the awareness about the presence of strategic interactions. It is noteworthy that the Cournot-Nash outcome is a stationary state of our models. Our thesis is that the chaotic dynamics arousing from a dynamical model, where heterogeneous players are considered, are capable to qualitatively reproduce the outcomes of experimental oligopolies.

In the rank of behavioral rules, imitation-based heuristics has received special attention in economics (see [14] and [12]). In particular, imitative behavior is considered in order to understand the evidences arising in experimental oligopolies which reveal that the Cournot-Nash equilibrium does not emerge as unique outcome and show that an important component of the production at the competitive level is observed (see e.g.[1,3,9] or [7,10]). By considering the pioneering groundbreaking approach of [2], we build a dynamical model of linear oligopolies where heterogeneous decision mechanisms of players are made explicit. In particular, we consider two different types of quantity setting players characterized by different decision mechanisms that coexist and operate simultaneously: agents that adaptively adjust their choices towards the direction that increases their profit are embedded with imitator agents. The latter ones use a particular form of proportional imitation rule that considers the awareness about the presence of strategic interactions. It is noteworthy that the Cournot-Nash outcome is a stationary state of our models. Our thesis is that the chaotic dynamics arousing from a dynamical model, where heterogeneous players are considered, are capable to qualitatively reproduce the outcomes of experimental oligopolies.

We formulate an evolutionary oligopoly model where quantity setting players produce following either the static expectation best response or a performance-proportional imitation rule. The choice on how to behave is driven by an evolutionary selection mechanism according to which the rule that brought the highest performance attracts more followers. The model has a stationary state that represents a heterogeneous population where rational and imitative rules coexist and where players produce at the Cournot–Nash level. We find that the intensity of choice, a parameter representing the evolutionary propensity to switch to the most profitable rule, the cost of the best response implementation as well as the number of players have ambiguous roles in determining the stability property of the Cournot–Nash equilibrium. This marks important differences with most of the results from evolutionary models and oligopoly competitions. Such differences should be referred to the particular imitative behavior we consider in the present modeling setup. Moreover, the global analysis of the model reveals that the above-mentioned parameters introduce further elements of complexity, conditioning the convergence toward an inner attractor. In particular, even when the Cournot–Nash equilibrium loses its stability, outputs of players little differ from the Cournot–Nash level and most of the dynamics is due to wide variations of imitators’ relative fraction. This describes dynamic scenarios where shares of players produce more or less at the same level alternating their decision mechanisms.

During mitosis and meiosis, microtubule (MT) assembly is locally upregulated by the chromatin-dependent Ran-GTP pathway. One of its key targets is the MT-associated spindle assembly factor TPX2. The molecular mechanism of how TPX2 stimulates MT assembly remains unknown because structural information about the interaction of TPX2 with MTs is lacking. Here, we determine the cryo-electron microscopy structure of a central region of TPX2 bound to the MT surface. TPX2 uses two flexibly linked elements ('ridge' and 'wedge') in a novel interaction mode to simultaneously bind across longitudinal and lateral tubulin interfaces. These MT-interacting elements overlap with the binding site of importins on TPX2. Fluorescence microscopy-based in vitro reconstitution assays reveal that this interaction mode is critical for MT binding and facilitates MT nucleation. Together, our results suggest a molecular mechanism of how the Ran-GTP gradient can regulate TPX2-dependent MT formation.

During mitosis and meiosis, microtubule (MT) assembly is locally upregulated by the chromatin-dependent Ran-GTP pathway. One of its key targets is the MT-associated spindle assembly factor TPX2. The molecular mechanism of how TPX2 stimulates MT assembly remains unknown because structural information about the interaction of TPX2 with MTs is lacking. Here, we determine the cryo-electron microscopy structure of a central region of TPX2 bound to the MT surface. TPX2 uses two flexibly linked elements ('ridge' and 'wedge') in a novel interaction mode to simultaneously bind across longitudinal and lateral tubulin interfaces. These MT-interacting elements overlap with the binding site of importins on TPX2. Fluorescence microscopy-based in vitro reconstitution assays reveal that this interaction mode is critical for MT binding and facilitates MT nucleation. Together, our results suggest a molecular mechanism of how the Ran-GTP gradient can regulate TPX2-dependent MT formation.

Microtubules switch between growing and shrinking states, a feature known as dynamic instability. The biochemical parameters underlying dynamic instability are modulated by a wide variety of microtubule-associated proteins that enable the strict control of microtubule dynamics in cells. The forces generated by controlled growth and shrinkage of microtubules drive a large range of processes, including organelle positioning, mitotic spindle assembly, and chromosome segregation. In the past decade, our understanding of microtubule dynamics and microtubule force generation has progressed significantly. Here, we review the microtubule-intrinsic process of dynamic instability, the effect of external factors on this process, and how the resulting forces act on various biological systems. Recently, reconstitution-based approaches have strongly benefited from extensive biochemical and biophysical characterization of individual components that are involved in regulating or transmitting microtubule-driven forces. We will focus on the current state of reconstituting increasingly complex biological systems and provide new directions for future developments., BN/Marileen Dogterom Lab, BN/Bionanoscience

Microtubules switch between growing and shrinking states, a feature known as dynamic instability. The biochemical parameters underlying dynamic instability are modulated by a wide variety of microtubule-associated proteins that enable the strict control of microtubule dynamics in cells. The forces generated by controlled growth and shrinkage of microtubules drive a large range of processes, including organelle positioning, mitotic spindle assembly, and chromosome segregation. In the past decade, our understanding of microtubule dynamics and microtubule force generation has progressed significantly. Here, we review the microtubule-intrinsic process of dynamic instability, the effect of external factors on this process, and how the resulting forces act on various biological systems. Recently, reconstitution-based approaches have strongly benefited from extensive biochemical and biophysical characterization of individual components that are involved in regulating or transmitting microtubule-driven forces. We will focus on the current state of reconstituting increasingly complex biological systems and provide new directions for future developments., BN/Marileen Dogterom Lab, BN/Bionanoscience

Structural characterization of microtubules has been the realm of three-dimensional electron microscopy and thus has evolved hand in hand with the progress of this technique, from the initial 3D reconstructions of stained tubulin assemblies, and the first atomic model of tubulin by electron crystallography of 2D sheets of protofilaments, to the ever more detailed cryoelectron microscopy structures of frozen-hydrated microtubules. Most recently, hybrid helical and single particle image processing techniques, and the latest detector technology, have lead to atomic models built directly into the density maps of microtubules in different functional states, shading new light into the critical process of microtubule dynamic instability.

Structural characterization of microtubules has been the realm of three-dimensional electron microscopy and thus has evolved hand in hand with the progress of this technique, from the initial 3D reconstructions of stained tubulin assemblies, and the first atomic model of tubulin by electron crystallography of 2D sheets of protofilaments, to the ever more detailed cryoelectron microscopy structures of frozen-hydrated microtubules. Most recently, hybrid helical and single particle image processing techniques, and the latest detector technology, have lead to atomic models built directly into the density maps of microtubules in different functional states, shading new light into the critical process of microtubule dynamic instability.

The commonly used two-parameter Mooney-Rivlin incompressible hyper-elastic material model can show non-intuitive responses under certain conditions. This paper shows that critical states with non-unique responses occur at least at very specific bi-axial stress states. This can happen for cases where the constant related to the second invariant of strain is positive, but not for the case with this constant equal to zero (the Neo-Hookean case). The dependence of the instabilities on the ratio between the two constitutive constants is shown by evaluated fold lines. The instability is shown to be related to the imposed boundary conditions. An analytical treatment of the problem shows that dynamic edge effects correspond to the static instability., QC 20141204

Dynamic instability, polarity, and spatiotemporal organization are hallmarks of the microtubule cytoskeleton that allow formation of complex structures such as the eukaryotic spindle. No similar structure has been identified in prokaryotes. The bacteriophage-encoded tubulin PhuZ is required to position DNA at mid-cell, without which infectivity is compromised. Here, we show that PhuZ filaments, like microtubules, stochastically switch from growing in a distinctly polar manner to catastrophic depolymerization (dynamic instability) both in vitro and in vivo. One end of each PhuZ filament is stably anchored near the cell pole to form a spindle-like array that orients the growing ends toward the phage nucleoid so as to position it near mid-cell. Our results demonstrate how a bacteriophage can harness the properties of a tubulin-like cytoskeleton for efficient propagation. This represents the first identification of a prokaryotic tubulin with the dynamic instability of microtubules and the ability to form a simplified bipolar spindle.

Dynamic instability, polarity, and spatiotemporal organization are hallmarks of the microtubule cytoskeleton that allow formation of complex structures such as the eukaryotic spindle. No similar structure has been identified in prokaryotes. The bacteriophage-encoded tubulin PhuZ is required to position DNA at mid-cell, without which infectivity is compromised. Here, we show that PhuZ filaments, like microtubules, stochastically switch from growing in a distinctly polar manner to catastrophic depolymerization (dynamic instability) both in vitro and in vivo. One end of each PhuZ filament is stably anchored near the cell pole to form a spindle-like array that orients the growing ends toward the phage nucleoid so as to position it near mid-cell. Our results demonstrate how a bacteriophage can harness the properties of a tubulin-like cytoskeleton for efficient propagation. This represents the first identification of a prokaryotic tubulin with the dynamic instability of microtubules and the ability to form a simplified bipolar spindle.

The dynamic instability and related problems when turning high chromium cast iron (HCCI) with two polycrystalline cubic boron nitride (PCBN) tools in wide range of cutting speeds and feed rates were studied in the article. The tool wear mechanisms and specifics for both tools as a main criterion of appearance of process instability were investigated. In spite of the chatter-like surface of machined workpieces it was shown the dynamic stability of process by two method ‘0-1 test’ and determination of maximum Lyapunov exponent. Taking into account the strong periodicity of process with several clear harmonics it was concluded that such process is related to self-resonant phenomena.

The dynamic assembly and disassembly of microtubules and the mechanical properties of these polymers are essential for many key cellular processes. Mathematical and computational modeling, especially coupled mechanochemical modeling, has contributed significantly to our understanding of microtubule dynamics. However, critical discrepancies exist between experimental observations and modeling results that need to be resolved before further progress toward a complete model can be made. Open sheet structures ranging in length from several hundred nanometers to one micron have often been observed at the growing ends of microtubules in in vitro studies. Existing modeling studies predict these sheet structures to be short and rare intermediates of microtubule disassembly rather than important components of the assembly process. Atomic force microscopy (AFM) studies also reveal interesting step-like gaps of the force-indentation curve that cannot yet be explained by existing theoretical models. We have carried out computational studies to compare the mechanical properties of two alternative models: a more conventional model where tubulin dimers are added directly into a microtubule lattice, and one that considers an additional type of tubulin lateral interaction proposed to exist in intermediate sheet structures during the microtubule assembly process. The first model involves a single type of lateral interactions between tubulin subunits, whereas the latter considers a second type that can convert to the canonical lateral contact during microtubule closure into a cylinder. Our analysis shows that only the second model can reproduce the AFM results over a broad parameter range. We propose additional studies using different sizes of AFM tips that would allow to unambiguously distinguish the relative validity of the two models.

The dynamic assembly and disassembly of microtubules and the mechanical properties of these polymers are essential for many key cellular processes. Mathematical and computational modeling, especially coupled mechanochemical modeling, has contributed significantly to our understanding of microtubule dynamics. However, critical discrepancies exist between experimental observations and modeling results that need to be resolved before further progress toward a complete model can be made. Open sheet structures ranging in length from several hundred nanometers to one micron have often been observed at the growing ends of microtubules in in vitro studies. Existing modeling studies predict these sheet structures to be short and rare intermediates of microtubule disassembly rather than important components of the assembly process. Atomic force microscopy (AFM) studies also reveal interesting step-like gaps of the force-indentation curve that cannot yet be explained by existing theoretical models. We have carried out computational studies to compare the mechanical properties of two alternative models: a more conventional model where tubulin dimers are added directly into a microtubule lattice, and one that considers an additional type of tubulin lateral interaction proposed to exist in intermediate sheet structures during the microtubule assembly process. The first model involves a single type of lateral interactions between tubulin subunits, whereas the latter considers a second type that can convert to the canonical lateral contact during microtubule closure into a cylinder. Our analysis shows that only the second model can reproduce the AFM results over a broad parameter range. We propose additional studies using different sizes of AFM tips that would allow to unambiguously distinguish the relative validity of the two models.

Syntaxin resides in the plasma membrane, where it helps to catalyze membrane fusion during exocytosis. The protein also forms clusters in cell-free and granule-free plasma-membrane sheets. We imaged the interaction between syntaxin and single secretory granules by two-color total internal reflection microscopy in PC12 cells. Syntaxin-GFP assembled in clusters at sites where single granules had docked at the plasma membrane. Clusters were intermittently present at granule sites, as syntaxin molecules assembled and disassembled in a coordinated fashion. Recruitment to granules required the N-terminal domain of syntaxin, but not the entry of syntaxin into SNARE complexes. Clusters facilitated exocytosis and disassembled once exocytosis was complete. Syntaxin cluster formation defines an intermediate step in exocytosis.

Proper regulation of MT (microtubule) dynamics is essential for various vital processes, including the segregation of chromosomes, directional cell migration and differentiation. MT assembly and disassembly is modulated by a complex network of intracellular factors that co-operate or antagonize each other, are highly regulated in space and time and are thus attuned to the cell cycle and differentiation processes. While we only begin to appreciate how the concerted action of MT stabilizers and destabilizers shapes different MT patterns, a clear picture of how individual factors affect the MT structure is emerging. In this paper, we review the current knowledge about proteins that modulate MT dynamic instability.

The first purpose of this research is to investigate the effect of added viscous fluid dampers on a nine story special steel moment frame designed for strength in Seattle. At the initial stages of the work, knowing the fact that moment frames are almost always controlled by drift, it was thought that two different moment frames, controlled by strength or controlled by drift (stiffness), could be designed in Seattle and the effect of additional dampers on the structural behavior of the strength controlled design could be studied. However, since ASCE 7 permits determining the elastic drifts by using the seismic design forces based on the computed fundamental period of the structure, without the upper limit (CuTa), the strength controlled design satisfied the drift limit requirements of ASCE 7. Although the strength controlled design meets the drift requirements, the stability checks of both ASCE 7 and the AISC Seismic Design Manual were not satisfied. Thus, the strength controlled frame was redesigned to meet the stability requirements, and the process is called stability controlled design. By adding supplemental dampers to the strength controlled design, it was expected that the seismic drift would be controlled and a better structural behavior would be obtained in terms of dynamic stability. Incremental Dynamic Analysis (IDA) was implemented to investigate the benefits of the dampers on the structural behavior. Using ten different earthquakes scaled up to a maximum target multiplier two, with ten increments, damage measures such as interstory drift, residual displacement, IDA dispersion, base shear, and roof displacement were studied. Using IDA dispersion, the effect of dampers on dynamic instability was also investigated in this study. As a result, it was found that as the damping of the structure increases with the help of added dampers, the structural response gets better. Maximum and residual roof displacements, interstory drifts, and IDA dispersion decreases wi

The first purpose of this research is to investigate the effect of added viscous fluid dampers on a nine story special steel moment frame designed for strength in Seattle. At the initial stages of the work, knowing the fact that moment frames are almost always controlled by drift, it was thought that two different moment frames, controlled by strength or controlled by drift (stiffness), could be designed in Seattle and the effect of additional dampers on the structural behavior of the strength controlled design could be studied. However, since ASCE 7 permits determining the elastic drifts by using the seismic design forces based on the computed fundamental period of the structure, without the upper limit (CuTa), the strength controlled design satisfied the drift limit requirements of ASCE 7. Although the strength controlled design meets the drift requirements, the stability checks of both ASCE 7 and the AISC Seismic Design Manual were not satisfied. Thus, the strength controlled frame was redesigned to meet the stability requirements, and the process is called stability controlled design. By adding supplemental dampers to the strength controlled design, it was expected that the seismic drift would be controlled and a better structural behavior would be obtained in terms of dynamic stability. Incremental Dynamic Analysis (IDA) was implemented to investigate the benefits of the dampers on the structural behavior. Using ten different earthquakes scaled up to a maximum target multiplier two, with ten increments, damage measures such as interstory drift, residual displacement, IDA dispersion, base shear, and roof displacement were studied. Using IDA dispersion, the effect of dampers on dynamic instability was also investigated in this study. As a result, it was found that as the damping of the structure increases with the help of added dampers, the structural response gets better. Maximum and residual roof displacements, interstory drifts, and IDA dispersion decreases wi

The par operon of the R1 plasmid constructs a DNA segregating spindle from three components, parC, ParR, and ParM. parC is a centromeric stretch of DNA which is bound by the protein ParR. ParM is an actin homologue that polymerizes into two stranded filaments that can interact with the ParR/parC complex. These three components are both necessary and sufficient to segregate parC containing plasmids, implying that the par operon is a biochemically self-contained DNA segregation apparatus. To elucidate how this system functions to segregate plasmid DNA, we wished to determine the intrinsic kinetic properties of ParM assembly, and how this assembly is regulated by the interaction with its cargo, the ParR/parC complex.We find that ParM displays three properties that distinguish its assembly from that of eukaryotic actins. 1) ParM displays hydrolysis dependent dynamic instability, the stochastic switching between states of growth and rapid depolymerization. 2) ParM filaments display a very rapid rate of nucleation, occurring at a rate 200-fold faster than actin. 3) ParM filaments exhibit no kinetic polarity, with equal rates of growth at each filament end.By combining ParM with bead immobilized ParR/parC we demonstrate that these three components can create bipolar spindles in vitro similar to those observed in vivo. ParR/parC coated microspheres are connected by bundles of ParM filaments that elongate over time, pushing the microspheres apart over long distances. Speckle microscopy and photobleaching experiments demonstrate that this motility is driven by insertional polymerization at the bead surface. We find that the ParR/parC complex prevents ParM filament ends from catastrophe by stabilizing filaments down to the ATP-ParM critical concentration. Futhermore, we show that the dynamic instability of ParM is critical to the spatial regulation of the R1 spindle, as filament ends that are not bound at each end by ParR/parC are destined to undergo catastrophe. The dynamic i

Microtubules are an essential component of the eukaryotic cytoskeleton. Proper functioning of the microtubular network depends highly on its dynamic properties, with frequent transitions from phases of prolonged growth to rapid shrinkage (catastrophe) and vice versa (rescue). Several proteins have been shown to bind to microtubules, some of them, the so-called plus-end-tracking proteins (+TIPs), binding specifically to the growing ends of microtubules. This thesis describes the functional analysis of two +TIPs, the Cytoplasmic Linker Proteins CLIP-115 and CLIP-170. These proteins are structurally highly similar resulting in a high level of redundancy. Although single CLIP-115 or CLIP-170 knockout mice both have characteristic features, no large abnormalities were found in fibroblasts derived from them. To learn more about the CLIPs and their redundant functions, CLIP double knockout (DKO) mice were generated. The dynamic properties of the microtubules in ce! lls derived from these mice were profoundly altered with a five-fold decrease in rescue-frequency. In addition, the microtubule network was changed notably, with large microtubule-free areas in which proteins aggregates were found. These aggregates contained the microtubule motor-protein cytoplasmic dynein and several components of the dynactin complex. Furthermore DKO fibroblasts spread abnormally, resulting in a two-fold increase of the surface area in attached cells compared to wild type cells. The mice deficient for both CLIP-115 and CLIP-170 have an abnormal posture and show hind limb clasping. Although they perform poorly on certain behavioral and strength-related tasks, they show a surprising age-related improvement of motor coordination. At older ages, certain tasks start to deteriorate again. Using magnetic resonance imaging (MRI) and histological analysis a progressive form of hydrocephalus is detected. In younger mice, an accumulation of cerebellar neurons is see! n in other parts of the brain, indica

The par operon of the R1 plasmid constructs a DNA segregating spindle from three components, parC, ParR, and ParM. parC is a centromeric stretch of DNA which is bound by the protein ParR. ParM is an actin homologue that polymerizes into two stranded filaments that can interact with the ParR/parC complex. These three components are both necessary and sufficient to segregate parC containing plasmids, implying that the par operon is a biochemically self-contained DNA segregation apparatus. To elucidate how this system functions to segregate plasmid DNA, we wished to determine the intrinsic kinetic properties of ParM assembly, and how this assembly is regulated by the interaction with its cargo, the ParR/parC complex.We find that ParM displays three properties that distinguish its assembly from that of eukaryotic actins. 1) ParM displays hydrolysis dependent dynamic instability, the stochastic switching between states of growth and rapid depolymerization. 2) ParM filaments display a very rapid rate of nucleation, occurring at a rate 200-fold faster than actin. 3) ParM filaments exhibit no kinetic polarity, with equal rates of growth at each filament end.By combining ParM with bead immobilized ParR/parC we demonstrate that these three components can create bipolar spindles in vitro similar to those observed in vivo. ParR/parC coated microspheres are connected by bundles of ParM filaments that elongate over time, pushing the microspheres apart over long distances. Speckle microscopy and photobleaching experiments demonstrate that this motility is driven by insertional polymerization at the bead surface. We find that the ParR/parC complex prevents ParM filament ends from catastrophe by stabilizing filaments down to the ATP-ParM critical concentration. Futhermore, we show that the dynamic instability of ParM is critical to the spatial regulation of the R1 spindle, as filament ends that are not bound at each end by ParR/parC are destined to undergo catastrophe. The dynamic i

A mathematical approach to the concepts of copolymerization of microtubules in the presence of anti-mitotic drugs is presented in this work. A general feature of the mathematical equations is presented. The possibility of having analytical steady state solutions of dynamic equations is investigated. The structure of equations is narrowed down for the specific brand of anti-mitotic drug, colchicine. The behavior of total T-tubulin concentration in the steady state in a regeneration system is investigated and analyzed through the numerical calculations.

This thesis presents the results of basically two separate studies. The first study involved identifying structural and earthquake parameters that influenced seismic structural collapse. The parameter study involved nonlinear dynamic analyses using single-degree-of-freedom (SDOF) bilinear models. Four parameters were associated with the SDOF models — the lateral stiffness, the post-yield stiffness ratio, the yield strength, and the stability ratio (P-Delta effects). Then, three parameters were associated with the ground motions — the records themselves, the lateral ground motion scales, and the vertical ground motion scales. From the parameter study, it was found that the post-yield stiffness ratio augmented by P-Delta effects (rp) in conjunction with the ductility demand was the best predictor of collapse. These two quantities include all four structural parameters and the seismic displacement demands. It was also discovered in the parameter study that vertical accelerations did not significantly influence lateral displacements unless a given combination of model and earthquake parameters was altered such that the model was on the verge of collapsing. The second study involved Incremental Dynamic Analysis (IDA) using bilinear SDOF models representative of low rise buildings in both the Western United States (WUS) and the Central and Eastern United States (CEUS). Models were created that represented three, five, seven, and nine story buildings. Five sites from both the WUS and CEUS were used. Four different damage measures were used to assess the performance of the buildings. The IDA study was primarily interested in the response of the structures between the earthquake intensities that have a 10 percent probability of occurring in 50 years (10/50) and 2 percent probability of occurring in 50 years (2/50). The results showed that all structures could be in danger of severe damage and possible collapse, depending on which damage measure and which earthquake was used.

The structure and seasonal variations of static (convective) and dynamic (shear) instabilities in the mesopause region (80–105 km) are examined using high-resolution wind and temperature data obtained with a Na lidar at the Starfire Optical Range, NM. The probabilities of static and dynamic instability are sensitive functions of N2/S2, where N is the buoyancy frequency and S is the total vertical shear in the horizontal winds. The mesopause region is most stable in summer when the mesopause is low, N is large and S is small. Monthly mean N2/S2 varies from a maximum value of about 1.06 in mid-summer to a minimum of 0.68 in January. The annual mean values of N and S are, respectively, 0.021 s−1 and 23 ms−1 km−1. The probabilities of static and dynamic instabilities are maximum in mid-winter when they average about 10% and 12%, respectively, and are minimum in summer when they average about 7% and 5%, respectively. The observations are generally consistent with theoretical predictions based on Gaussian models for the temperature and wind fluctuations induced by gravity waves. They also show that statically unstable conditions are generally preceded by dynamically unstable conditions. The instability probabilities vary considerably from night to night and the structure of the unstable regions are significantly influenced by atmospheric tides. Tides alone are usually not strong enough to induce instability but they can establish the environment for instabilities to develop. As the tidal temperature perturbations propagate downward, they reduce the stability on the topside of the positive temperature perturbation. Instabilities are then induced as gravity waves propagate through this layer of reduced static stability.

The Hopf bifurcation is the dynamical instability which occurs in a feedback amplifier as the positive feedback is increased to the point where the system starts to oscillate spontaneously. The howl heard in a public address system when the presenter moves so the microphone gets too close to the loudspeaker is an example of increased positive feedback leading to oscillatory behaviour through a Hopf bifurcation. Hair cells are the sensory cells responsible for hearing and balance; they contain mechanosensitive transducer channels that convert mechanical vibration into an oscillation of their membrane potential. In many hair cells the membrane potential sinusoidally oscillates at small amplitude without input; their input-output transfer function has a large gain for small input and a reduced gain for larger inputs. These and other features are easily explained if hair cells are poised at a Hopf bifurcation. An amplifier poised at the Hopf bifurcation will have a compressively-nonlinear transfer function and also infinitely sharp tuning for vanishingly small input. The cube-root shape of its transfer function provides for an extraordinarily large gain for a small input signal at the natural frequency and a reduced gain for larger inputs. Moreover, any biosensor for detecting periodic signals of any sort would enjoy these great advantages by employing this commonly occurring instability.

A 4-year (1993-1996) temperature and wind data set obtained from over 2000 high-resolution balloon soundings at South Pole is used to study gravity wave characteristics in the atmosphere and lower stratosphere. Extensive analyses of energy density, spectra, and static stability are performed to present a comprehensive view of the gravity waves are ubiquitous and often fairly strong at the South Pole, even though the generation mechanisms are not clear. Gravity wave characteristics are, in general, similar to those obtained at other high-latitude southern hemisphere stations. Potential energies vary between about 0.5 J/kg and 5 J/kg with season and altitude. Variations in kinetic energies are not well correlated with potential energy variations and range from 1 J/kg to 11 J/kg. We observe significant seasonal variations of the slope and magnitude of the vertical wavenumber spectrum of temperature fluctuations, especially in the stratosphere. In general, the gravity waves in the stratosphere are stronger (weaker) in austral spring (fall). Stability analysis shows that instabilities are more likely to occur in the troposphere than in the stratosphere. The probability of wave instability is 13.7% in the troposphere and 5.4 % in the stratosphere. This is due to the less stratification of the troposphere, where the buoyancy period averages 8.3 min compared to 4.9 min in the stratosphere. Dynamic (shear) instability is more likely to occur than convective instability (11% versus 2.6% in the troposphere and 4.7% versus 0.7% in the stratosphere), due to the prevailing strong wind shear. The instability probabilities vary seasonally with the austral winter exhibiting the probability of instabilities (dynamic and convective instabilities combined) in both the troposphere and stratosphere.

Dynamic instability of a laminated composite panel subjected to a transverse load is studied. Up to cubic variations in the thickness coordinate are included in the inplane displacement field, and only the constant component is kept in the transverse displacement. The transverse shear strains retain only linear displacement terms and vary parabolically through the thickness. The complete quadratic displacement functions are included in the inplane strains. A 36 degree of freedom shell element is used to obtain numerical results. The static snap through load vs displacement curve, as well as the critical collapse load, is examined by invoking the Riks technique along with the Newton-Raphson iteration scheme. The beta-m time marching integration method is employed to evaluate a dynamic response. Two step loads, with the step magnitude slightly below and above the critical collapse load, are introduced in the dynamic analysis. The response resulting from the dynamic analysis matches the displacement on the static load vs displacement curve.

Flexible Mechanisms such as slider crank and four-bar mechanisms are modeled and their dynamic instability and optimum design analyzed. The primary aim of the project was a thorough understanding and analysis of conditions of dynamic instability in flexible components of mechanisms and robots. Dynamic instability characterizes the behavior when amplitude of vibrations have a tendency to become unbounded with the passage of time. Other aims of the study included the optimal design of mechanisms on the basis of flexibility and control of stresses and deflections.

A discussion is given on the importance of damping in fiber-reinforced composites and is followed by concise definitions of the various measures of damping. The current state of the theory of damping in fiber-reinforced composites is reviewed for perfectly-bonded viscoelastic composites, yielding-matrix composites, and those in which slip takes place at the fiber-matrix interface. General trends in stiffness and damping of composites with polymeric matrices are reviewed. (Author)

Partial contents: Shock in Solids--Army materials research and applications; Cross-flow-induced instabilities of circular cylinders; and Abstract from the Current Literature., See also Volume 12, Number 4, AD-A084 108.

Dynamic behavior of flexible components of mechanisms is prone to instabilities which create resonant speed barriers. By considering small deformations superimposed on the steady dynamic state equations governing evolution of disturbances can be obtained. For the case of mechanisms driven by periodic inputs these equations reduce to a system of coupled Mathieu-Hill equations for the amplitudes of modes of vibrations. Application of the Floquet theory determines the critical conditions of speed, geometry and material properties causing dynamic instability. (Author), This article is from 'Transactions of the Army Conference on Applied Mathematics and Computing (2nd) Held at Washington, DC on 22-25 May 1984,' AD-A154 047, p105-116.

Ultrakalte Moleküle mit elektrischen Dipolmomenten erfahren derzeit großes Interesse seitens theoretischer und experimenteller Quantenphysiker. Auf Grund der lange Lebensdauer stabiler Grundzustandsmoleküle in Kombination mit der starken, langreichweitigen Wechselwirkung der elektrischen Dipolmomente sind dipolare Moleküle ideal geeignet zur Erforschung von exotischen Quanten-Zuständen in optischen Gittern, wie zum Beispiel der Suprasolidität eines wechselwirkenden Vielteilchen-Systems. Weitere potentielle Anwendungen finden dipolare Moleküle in der Simulation von Spin-Systemen oder in der Realisierung von Quantencomputern. Die Umsetzung der meisten dieser Vorschläge basiert auf Ensembles von dipolaren Molekülen mit geringer Entropie in Gittern. Solche ultrakalten Molekül-Ensembles in optischen Gittern zu präparieren, stellt dabei jedoch eine große experimentelle Herausforderung dar. In der Vergangenheit gelang es nicht, Ensembles aus dipolaren Molekülen mit geringer Entropie in Gittern zu realisieren. Die Molekül-Ensembles wurden üblicherweise zunächst ohne Gitter erzeugt und in manchen Fällen im Anschluss in ein optisches Gitter geladen. Die erreichten Phasenraumdichten der Molekülensembles waren zu gering, um Systeme mit geringer Entropie zu erzeugen. In dieser Dissertation wird eine neue Methode vorgestellt, bei der Ensembles aus schwachgebundenen, heteronuklearen Molekülen direkt in einem optischen Gitter erzeugt werden. Dadurch lassen sich deutlich niedrigere Entropien erreichen, was einen idealen Ausgangspunkt für weiterführende Experimente mit dipolaren Molekülen in Gittern darstellt. Diese Arbeit ist wie folgt strukturiert: Zunächst wird die Erzeugung von 87Rb133Cs-Molkülen, den dipolaren Molekülen unserer Wahl, allgemein beschrieben. Ausgangspunkt sind hierfür ultrakalte Ensembles aus Rb- und Cs-Atomen. Die Atome werden mit Hilfe einer Feshbach-Resonanz zu schwachgebundenen RbCs-Molekülen zusammengeführt. Anschließend werden die schwachgebundenen Moleküle, Ultracold molecules with electric dipole moments are currently of large interest to the community of experimental and theoretical quantum physicists. Due to the long lifetime of stable ground-state molecules in combination with the strong long-range interaction of the electronic dipole moments, dipolar molecules are ideal candidates to study exotic quantum phases in optical lattices, such as the many-body lattice super solid. Further potential applications of dipolar molecules are the simulation of spin systems, or to use them as a platform for quantum computation. Most of these proposals require a low-entropy sample of dipolar molecules in a lattice. However, it is a great experimental challenge to prepare such ultracold molecular samples in an optical lattice. In the past the realization of low-entropy samples of dipolar molecules in lattices was not feasible. The molecular samples were typically created in the absence of a lattice and in some cases the samples were loaded into an optical lattice subsequently. The resulting phase-space densities of the molecular samples were too low to form low-entropy samples. In this thesis a novel method is introduced that is based on the formation of weakly bound heteronuclear molecules directly in an optical lattice. This way, a significantly lower entropy can be achieved, which presents an ideal starting point to study dipolar molecules in a lattice. This thesis is structured as follows: Initially, the preparation of 87Rb133Cs ground-state molecules, the dipolar molecules of our choice, is introduced. We start from ultracold samples of Rb and Cs atoms. The atoms are associated to weakly bound RbCs molecules by means of a magnetic Feshbach resonance. Subsequently the weakly bound molecules are transferred to their rovibrational and hyperfine ground-state via stimulated Raman adiabatic passage (STIRAP). Next, we demonstrate our method to form the weakly bound RbCs molecules directly in a three-dimensional (3D) optical lattice., by Andreas Schindewolf, Kumulative Dissertation aus zwei Artikeln, Zusammenfassung in deutscher Sprache, Dissertation University of Innsbruck 2018

Ultrakalte Moleküle mit elektrischen Dipolmomenten erfahren derzeit großes Interesse seitens theoretischer und experimenteller Quantenphysiker. Auf Grund der lange Lebensdauer stabiler Grundzustandsmoleküle in Kombination mit der starken, langreichweitigen Wechselwirkung der elektrischen Dipolmomente sind dipolare Moleküle ideal geeignet zur Erforschung von exotischen Quanten-Zuständen in optischen Gittern, wie zum Beispiel der Suprasolidität eines wechselwirkenden Vielteilchen-Systems. Weitere potentielle Anwendungen finden dipolare Moleküle in der Simulation von Spin-Systemen oder in der Realisierung von Quantencomputern. Die Umsetzung der meisten dieser Vorschläge basiert auf Ensembles von dipolaren Molekülen mit geringer Entropie in Gittern. Solche ultrakalten Molekül-Ensembles in optischen Gittern zu präparieren, stellt dabei jedoch eine große experimentelle Herausforderung dar. In der Vergangenheit gelang es nicht, Ensembles aus dipolaren Molekülen mit geringer Entropie in Gittern zu realisieren. Die Molekül-Ensembles wurden üblicherweise zunächst ohne Gitter erzeugt und in manchen Fällen im Anschluss in ein optisches Gitter geladen. Die erreichten Phasenraumdichten der Molekülensembles waren zu gering, um Systeme mit geringer Entropie zu erzeugen. In dieser Dissertation wird eine neue Methode vorgestellt, bei der Ensembles aus schwachgebundenen, heteronuklearen Molekülen direkt in einem optischen Gitter erzeugt werden. Dadurch lassen sich deutlich niedrigere Entropien erreichen, was einen idealen Ausgangspunkt für weiterführende Experimente mit dipolaren Molekülen in Gittern darstellt. Diese Arbeit ist wie folgt strukturiert: Zunächst wird die Erzeugung von 87Rb133Cs-Molkülen, den dipolaren Molekülen unserer Wahl, allgemein beschrieben. Ausgangspunkt sind hierfür ultrakalte Ensembles aus Rb- und Cs-Atomen. Die Atome werden mit Hilfe einer Feshbach-Resonanz zu schwachgebundenen RbCs-Molekülen zusammengeführt. Anschließend werden die schwachgebundenen Moleküle, Ultracold molecules with electric dipole moments are currently of large interest to the community of experimental and theoretical quantum physicists. Due to the long lifetime of stable ground-state molecules in combination with the strong long-range interaction of the electronic dipole moments, dipolar molecules are ideal candidates to study exotic quantum phases in optical lattices, such as the many-body lattice super solid. Further potential applications of dipolar molecules are the simulation of spin systems, or to use them as a platform for quantum computation. Most of these proposals require a low-entropy sample of dipolar molecules in a lattice. However, it is a great experimental challenge to prepare such ultracold molecular samples in an optical lattice. In the past the realization of low-entropy samples of dipolar molecules in lattices was not feasible. The molecular samples were typically created in the absence of a lattice and in some cases the samples were loaded into an optical lattice subsequently. The resulting phase-space densities of the molecular samples were too low to form low-entropy samples. In this thesis a novel method is introduced that is based on the formation of weakly bound heteronuclear molecules directly in an optical lattice. This way, a significantly lower entropy can be achieved, which presents an ideal starting point to study dipolar molecules in a lattice. This thesis is structured as follows: Initially, the preparation of 87Rb133Cs ground-state molecules, the dipolar molecules of our choice, is introduced. We start from ultracold samples of Rb and Cs atoms. The atoms are associated to weakly bound RbCs molecules by means of a magnetic Feshbach resonance. Subsequently the weakly bound molecules are transferred to their rovibrational and hyperfine ground-state via stimulated Raman adiabatic passage (STIRAP). Next, we demonstrate our method to form the weakly bound RbCs molecules directly in a three-dimensional (3D) optical lattice., by Andreas Schindewolf, Kumulative Dissertation aus zwei Artikeln, Zusammenfassung in deutscher Sprache, Dissertation University of Innsbruck 2018