Theory of nonlinear optical response of ensembles of double quantum dots

We study theoretically the time-resolved four-wave mixing (FWM) response of an ensemble of pairs of quantum dots undergoing radiative recombination. At short (picosecond) delay times, the response signal shows beats that may be dominated by the subensemble of resonant pairs, which gives access to the information on the interdot coupling. At longer delay times, the decay of the FWM signal is governed by two rates which result from the collective interaction between the two dots and the radiation modes. The two rates correspond to the subradiant and super-radiant components in the radiative decay. Coupling between the dots enhances the collective effects and makes them observable even when the average energy mismatch between the dots is relatively large.Comment: 8 pages, 3 figures; moderately modifie

Exploring Living Labs through Transition Management - Challenges and Opportunities for Sustainable Urban Transitions

The transition towards a sustainable society has become more and more an urban challenge due to a dramatic shift from rural to urban living over the past century. Cities cause environmental and social problems and offer solutions to many sustainability challenges. Living Labs are research infrastructures embedded in a real-life environment, which have recently emerged as a tool for urban governance and sustainability research to drive innovation towards sustainable urban development. This thesis explores Living Labs through Transition Management (TM) and concludes that their strategic, operational, tactical and reflexive activities and structures potentially contribute to sustainable urban transitions. TM was useful as an analytical framework to better understand how Living Labs work in practice. An important finding of this exploratory research is that the Living Lab approach has been further developed in two complementary research streams, which this thesis categorises as Sustainable Living Lab (SLL) and Urban Living Lab (ULL). SLLs focus on the product and service system development and mainly target the generation of knowledge for future up-scaling activities, while ULLs focus on the implementation of socio-technical innovations on an urban territory. National and international networks are vital for Living Labs to share and spread knowledge, to jointly develop methodologies and evaluation indicators, increase their visibility as well as the probability of getting funded. The significant partnership between researchers, citizens, companies and local governments within a Living Lab creates beneficial preconditions to connect sustainable innovations with the market and society, and thus potentially advance sustainable urban transitions. Major challenges are collaborative alignment work, divergent stakeholder interests as well as the motivation of users during the experimentation phase. The most important success indicator for SLL and ULL is what has been learned within a project. This contrasts them from conventional Living Labs targeting innovation service delivery. Providing space for innovative experimentation, that would not have taken place outside a SLL or ULL, is one of the key contributions to sustainable urban transitions

Kinesin Kar3 and Vik1 Go Head to Head

The yeast kinesin motor protein Kar3 forms a heterodimer with a nonmotor protein Vik1. A study in this issue by Allingham et al. (2007) reveals that Vik1 unexpectedly has a structure similar to a kinesin motor domain yet lacks a nucleotide-binding site and is thus catalytically inactive. However, this does not hinder movement of the heterodimer because other features of the remarkably divergent Vik1 motor domain are retained, including the ability to bind microtubules

Diffusion of myosin V on microtubules

Organelle transport in eukaryotes employs both microtubule and actin tracks to deliver cargo effectively to their destinations, but the question of how the two systems cooperate is still largely unanswered. Recently, in vitro studies revealed that the actin-based processive motor myosin V also binds to, and diffuses along microtubules. This biophysical trick enables cells to exploit both tracks for the same transport process without switching motors. The detailed mechanisms underlying this behavior remain to be solved. By means of single molecule Total Internal Reflection Microscopy (TIRFM), we show here that electrostatic tethering between the positively charged loop 2 and the negatively charged C-terminal E-hooks of microtubules is dispensable. Furthermore, our data indicate that in addition to charge-charge interactions, other interaction forces such as non-ionic attraction might account for myosin V diffusion. These findings provide evidence for a novel way of myosin tethering to microtubules that does not interfere with other E-hook-dependent processes

General no-go condition for stochastic pumping

The control of chemical dynamics requires understanding the effect of time-dependent transition rates between states of chemo-mechanical molecular configurations. Pumping refers to generating a net current, e.g. per period in the time-dependence, through a cycle of consecutive states. The working of artificial machines or synthesized molecular motors depends on it. In this paper we give short and simple proofs of no-go theorems, some of which appeared before but here with essential extensions to non-Markovian dynamics, including the study of the diffusion limit. It allows to exclude certain protocols in the working of chemical motors where only the depth of the energy well is changed in time and not the barrier height between pairs of states. We also show how pre-existing steady state currents are in general modified with a multiplicative factor when this time-dependence is turned on.Comment: 8 pages; v2: minor changes, 1 reference adde

Size-dependent fine-structure splitting in self-organized InAs/GaAs quantum dots

A systematic variation of the exciton fine-structure splitting with quantum dot size in single InAs/GaAs quantum dots grown by metal-organic chemical vapor deposition is observed. The splitting increases from -80 to as much as 520 $\mu$eV with quantum dot size. A change of sign is reported for small quantum dots. Model calculations within the framework of eight-band k.p theory and the configuration interaction method were performed. Different sources for the fine-structure splitting are discussed, and piezoelectricity is pinpointed as the only effect reproducing the observed trend.Comment: 5 pages, 5 figure