Exciton-Exciton Annihilation Is Coherently Suppressed in H-Aggregates, but Not in J-Aggregates

We theoretically demonstrate a strong dependence of the annihilation rate between (singlet) excitons on the sign of dipole-dipole couplings between molecules. For molecular H-aggregates, where this sign is positive, the phase relation of the delocalized two-exciton wavefunctions causes a destructive interference in the annihilation probability. For J-aggregates, where this sign is negative, the interference is constructive instead, as a result of which no such coherent suppression of the annihilation rate occurs. As a consequence, room temperature annihilation rates of typical H- and J-aggregates differ by a factor of ~3, while an order of magnitude difference is found for low-temperature aggregates with a low degree of disorder. These findings, which explain experimental observations, reveal a fundamental principle underlying exciton-exciton annihilation, with major implications for technological devices and experimental studies involving high excitation densities

Island formation without attractive interactions

We show that adsorbates on surfaces can form islands even if there are no attractive interactions. Instead strong repulsion between adsorbates at short distances can lead to islands, because such islands increase the entropy of the adsorbates that are not part of the islands. We suggest that this mechanism cause the observed island formation in O/Pt(111), but it may be important for many other systems as well.Comment: 11 pages, 4 figure

Fast and Simple Relational Processing of Uncertain Data

This paper introduces U-relations, a succinct and purely relational representation system for uncertain databases. U-relations support attribute-level uncertainty using vertical partitioning. If we consider positive relational algebra extended by an operation for computing possible answers, a query on the logical level can be translated into, and evaluated as, a single relational algebra query on the U-relation representation. The translation scheme essentially preserves the size of the query in terms of number of operations and, in particular, number of joins. Standard techniques employed in off-the-shelf relational database management systems are effective for optimizing and processing queries on U-relations. In our experiments we show that query evaluation on U-relations scales to large amounts of data with high degrees of uncertainty.Comment: 12 pages, 14 figure

Two-Dimensional Spectroscopy of Extended Molecular Systems: Applications to Energy Transport and Relaxation in an α-Helix

A simulation study of the coupled dynamics of amide I and amide II vibrations in an α-helix dissolved in water shows that two-dimensional (2D) infrared spectroscopy may be used to disentangle the energy transport along the helix through each of these modes from the energy relaxation between them. Time scales for both types of processes are obtained. Using polarization-dependent 2D spectroscopy is an important ingredient in the method we propose. The method may also be applied to other two-band systems, both in the infrared (collective vibrations) and the visible (excitons) parts of the spectrum.

Leadership for Emergence: Exploring organisations through a living system lens

In this article, we outline a research project with adolescent-focused NGOs (non-government organisations) in Christchurch, New Zealand. This project involved 25 managers who used appreciative inquiry process methodology to explore their leadership practices, beliefs, and values. Throughout the article, we construct a conceptual leadership frame for fostering the emergence of adaptive, innovative and responsive organisational capacity that allows organisations to more readily adapt to the complex and changing conditions in which they operate. We describe this frame as a living system lens that is based on viewing organisations as complex adaptive systems of the kind readily found in the natural world. We go on to outline the leaders’ reflections as they drew strong connections between the dynamics found in complex adaptive systems and their own organisations. Proactive mentoring, fostering interaction and shared learning, strategies for distributing power and decentralising control, and exploration and articulation of deeply held values emerged as the key leadership enactments that these leaders implemented in their roles

Quantum-Classical Simulation of Molecular Motors Driven Only by Light

[Image: see text] Molecular motors that exhibit controlled unidirectional rotation provide great prospects for many types of applications, including nanorobotics. Existing rotational motors have two key components: photoisomerization around a π-bond followed by a thermally activated helical inversion, the latter being the rate-determining step. We propose an alternative molecular system in which the rotation is caused by the electric coupling of chromophores. This is used to engineer the excited state energy surface and achieve unidirectional rotation using light as the only input and avoid the slow thermally activated step, potentially leading to much faster operational speeds. To test the working principle, we employ quantum-classical calculations to study the dynamics of such a system. We estimate that motors built on this principle should be able to work on a subnanosecond time scale for such a full rotation. We explore the parameter space of our model to guide the design of a molecule that can act as such a motor

Finite Element Flow Simulations of the EUROLIFT DLR-F11 High Lift Configuration

This paper presents flow simulation results of the EUROLIFT DLR-F11 multi-element wing configuration, obtained with a highly scalable finite element solver, PHASTA. This work was accomplished as a part of the 2nd high lift prediction workshop. In-house meshes were constructed with increasing mesh density for analysis. A solution adaptive approach was used as an alternative and its effectiveness was studied by comparing its results with the ones obtained with other meshes. Comparisons between the numerical solution obtained with unsteady RANS turbulence model and available experimental results are provided for verification and discussion. Based on the observations, future direction for adaptive research and simulations with higher fidelity turbulence models is outlined.Comment: 52nd Aerospace Sciences Meetin